Chapter 4
Morphology of Different Parts of Medicinal Plant
4.1 Introduction
Arrangements of plants into groups and subgroups are commonly spoken as classification. Various systems of classifying plants have gradually developed during past few centuries which have emerged as a discipline of botanical science known as Taxonomy or Systematic botany. The Taxonomy word is derived from two Greek words ‘Taxis’ meaning an arrangement and ‘nomos’ meaning laws. Therefore, the systemization of our knowledge about plants in an orderly manner becomes subject matter of systematic botany.
The aim and objective of taxonomy is to discover the similarities and differences in the plants, indicating their closure relationship with their descents from common ancestry. It is a scientific way of naming, describing and arranging the plants in an orderly manner.
The classification of plants may be based upon variety of characters possessed by them. Features like specific morphological characters, environmental conditions, geographical distribution, colours of flowers and types of adaptations or reproductive characteristics can be used as a base for taxonomical character.
4.2 History
Many attempts were made in the earlier days to name and distinguish the plants as well as animals. Earliest mentions of classifications are credited to the Greek scientist Aristotle (384–322 B.C.) who is also called as the father of natural history. Aristotle attempted a simple artificial system for classifying number of plants and animals on the basis of their morphological and anatomical resemblances. It worked with great success for more than two thousand years.
Theophrastus (370–285 B.C.), the first taxonomist who wrote a systematic classification in a logical form was a student of Aristotle. He attempted to extend the botanical knowledge beyond the scope of medicinal plants. Theophrastus classified the plants in about 480 taxa, using primarily the most obvious morphological characteristics, i.e. trees, shrubs, under-shrubs, herbs, annuals, biennials and perennials. He recognized differences based upon superior and inferior ovary, fused and separate petals and so on. He is called father of botany. Several of the names mentioned by him in his treatise, ‘De Historia Plantarum’ was later taken up by Linnaeus in his system of classification.
A. P. de Tournfort (1658–1708) carried further the promotional work on genus. He had a clear idea of genera and many of the names used by him in his Institutions Rei Herbariae (1700) were adopted by Linnaeus. Tournfort’s system classified about 9000 species into 698 genera and 22 classes. This system although artificial in nature was extremely practical in its approach.
Most of the taxonomists after Tournfort used the relative taxonomic characterization as a basis for classification. This natural base helped to ascertain the nomenclature and also showed its relative affinities with one another. All the modern systems of classification are thus natural systems.
John Ray (1682), an English Botanist used a natural system based on the embryo characteristics. Most important of his works were Methodus Plantarum Nova (1682), Historia Plantarum (1686) and Synopsis Methodica Stirpium Britanicarum (1698). He classified the plants into two main groups: Herbae, with herbaceous stem and Arborae, with woody stem.
The main groups of flowerless and flowering plants were subdivided distinctly into 33 smaller groups. He divided flowering plants in monocotyledonae and dicotyledonae, which later worked as a great foundation for the further developments of systematic botany
Carrolus Linnaeus (1707–1778), a Swedish botanist, introduced the system of binomial nomenclature. His artificial system was based oh particular names of a substantive and adjective, nature. It is best known as binomial system of nomenclature in which the first general name indicates the genus and the second specific name denotes the species. Linnaeus characterized and listed about 4378 different species of plants and animals in his works Species Plantarum and Genera Plantarum (1753). He classified plants on the basis of reproductive organs, i.e. stamens and carpels—and hence this system is also known as the sexual system of classification. According to this system, plants are divided into 24 classes having 23 phanerogams and one cryptogam. Phanerogams were classified on the basis of unisexual and bisexual flowers. Further classification is based on the number and types of stamens and carpels.
A French Botanist A. P. de Candolle (1819) extensively worked and improved the natural system of classification. Along with the recognition of cotyledons, corolla and stamen characteristics, Candolle introduced the arrangement of fibrovascular bundles as a major character. He also provided a classification system for lower plants; Candolle mainly divided plants into vascular and cellular groups, i.e. plants with cotyledons and without cotyledons. There groups were further divided and subdivided on the basis of cotyledons and floral characteristics.
Bentham and Hooker’s System
George Bentham (1830–1884) and Joseph Hooker (1817–1911) two British Botanists, adopted a very comprehensive, natural system of classification in their published work Genera Plantarum (1862–1883), which dominated the botanical science for many years. It is an extension of Candolle’s work.
According to this system, the plant kingdom comprises about 97,205 species of seed plants which are distributed in 202 orders and were further divided in families. Dicotyledons have been divided in three divisions on the basis of floral characteristics namely: polypetalae, gamopetalae and mono-chlamydeae—all the three divisions consisting of total 163 families. Polypetalae have both calyx and corolla with free petals and indefinite number of stamens along with carpels. Gamopetalae have both calyx and corolla, but the latter is always gamopetalous or fused. Stamens are definite and epipetalous along with carpels. In monochlamydeae flowers are incomplete because of the absence of either calyx or corolla, or both the whorls. It generally includes the families which do not come under the above two subclasses.
Following the above scheme of classification Indian senna, Cassia angustifolia and Ginger, Zingiber officinalis may be referred to its systematic position as mentioned in Table 4.1.
Table 4.1 Scheme of systematic classification of drugs |
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| Division | Phanerogam | Phanerogam |
| Subdivision | Angiosperm | Angiosperm |
| Class | Dicotyledonae | Monocotyledonae |
| Subclass | Polypetalae | – |
| Series | Calyciflorae | Epigynae |
| Order | Resales | Scitamineae |
| Family | Leguminosae | Zingiberaceae |
| Subfamily | Caesalpinieae | – |
| Genus | Cassia | Zingiber |
| Species | angustifolia | officinalis |
Bentham and Hookers system of classification was accepted throughout the British Empire and in the United States, and was adapted to lesser extent by Continental botanists. It was regarded as the most convenient and suitable for practical utility.
Adolf Engler (1844–1930), a German Botanist published his system of classification in Die Naturlichen Pflanzenfamilien in 23 volumes, covering the whole plant kingdom. The increasing complexity of the flowers is considered for classification. Engler believed that woody plants with unisexual and apetalous flowers are most primitive in origin. This is a natural system which is based on the relationships and is compatible with evolutionary principles.
Hutchinson’s System of Classification
A British systematic Botanist J. Hutchinson published his work, The Families of Flowering Plants in 1926 on Dicotyledons and in 1934 on monocotyledons. Hutchinson made it clear that the plants with sepals and petals are more primitive than the plants without petals and sepals on the assumption that free parts are more primitive than fused ones. He also believed that spiral arrangement of floral parts, numerous free stamens and hermaphrodite flowers are more primitive than unisexual flowers with fused stamens. He considered monochlamydous plants as more advanced than dicotyledons. Hutchinson’s system indicates the concept of phylogenetic classification and seems to be an advanced step over the Bentham and Hooker system of classification. Hutchinson accepted the older view of woody and herbaceous plants and fundamentally called them as Lignosae and Herbaceae. He revised the scheme of classification in 1959. Hutchinson placed the gymnosperms first, then the dicotyledons and lastly the monocotyledons.
H. H. Rusby (1931) worked on phylogenic classification. His work is the scathing criticism on the phylogenic system attempted by M. C. Nair, ‘Angiosperm Phylogeny on a Chemical basis.’ While criticizing M. C. Nair, he indicated that the taxonomists need to study and use all the criteria including chemical nature while working on phylogenic system. He stubbornly criticized a publication on Cinchona that when the whole genus has been thoroughly investigated for its morphology; chemistry, reproduction, embryology, horticulture, ecology and geography, all the information is ignored in the chemotaxonomical study which is a great misfortune to Cinchona literature.
M.P. Morris (1954) worked on chemotaxonomy of toxic cyanogenetic glycosides of Indigofera endecaphylla and pointed out that p-nitropropionic acid, a hydrolysis product of Hiptagenic acid, occurs in a free state in the plants. His work provided the direction to chemotaxonomy of cyanogenetic principles.
4.3 Study of Different Tissue Systems
The flowering plants have highly evolved organizations which indicate the structural and functional specialization. Externally these organizations may be regarded as the morphological parts, but internally it can be categorized in cells, tissues and tissue systems. The morphologically most easily and clearly recognizable units of the plant body are the cells. The united masses of cells are distinct from one another structurally as well as functionally. Such groupings of cells may be referred to as tissues which further may develop into a simpler or complex cellular organization.
The arrangement of various tissues or tissue systems in the plant indicates its specialized nature. For example, vascular tissues are mainly concerned with the conduction of food and water, and for the efficient functioning; a complex network is developed with the places of water intake, sites of food synthesis and with areas of growth, development and storage. In the same way nonvascular tissues are also continually arranged which indicates the specific interrelationship of vascular tissues, storage tissues and supportive tissues. Plant tissues are generally categorized in to two categories
Difference between Merismetic and Permanent Tissues |
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| Sr. No. | Merismetic Tissue | Permanent Tissue |
| 1. | Comprises of young cells which have the power to redivide and multiply. | These cells are living or dead having attained their definite form and size. |
| 2. | These cells are present at growing points, i.e. tips of roots, shoots and epidermis. | Usually present in the ground tissue and make the fundamental tissue system. |
| 3. | These cells are closely packed without intracellular spaces. | Intracellular spaces are present. |
In the plant body, the following three tissue systems can be distinguished.
(A) Dermal tissue system: It represents the outer most part of the plant which forms a protective covering line. It includes epidermis, periderm, etc.
(B) Vascular tissue system: It is concerned with transmission of material in the plant and represents stelar structures like xylem and phloem.
(C) Ground tissue system: It consists of simple cells which may be strengthened by addition of thickened cells. It represents ground tissue made up of parenchyma, collenchyma and sclerenchyma.
Dermal Tissue System
Epidermis
The epidermal tissue system is derived from the dermatogen of the apical meristem and forms the epidermis (epi - upon, derma - skin) or outermost skin layer, which extends over the entire surface of the plant body. Epidermis is the outermost layer of the plant consisting normally of a single layer of flattened cells. The walls may be straight, wavy or beaded and often covered with a layer of cuticle made up of cutin.
Fig. 4.1 Different type of cell walls of epidermis
Functions
1. The primary function of the epidermis is protection of the internal tissues against mechanical injury, excessive heat or cold, fluctuations of temperature, attacks of parasitic fungi and bacteria, and against the leaching effect of rain. This is possible due to the presence of cuticle, hairs, tannin, gum, etc.
2. Prevention of excessive evaporation of water from the internal tissues by the development of thick cuticles, wax and other deposition, cutinized hairs, scales, multiple epidermis, etc., is another important function of the epidermis.
3. Strong cuticles and cutinized hairs, particularly a dense coating of hairs, protect the plant against intense illumination (i.e. strong sunlight) and excessive radiation of heat.
5. The epidermis sometimes has some minor functions like photosynthesis, secretion, etc.
Stomata
Stomata are minute openings usually found in the epidermis of the leaves as in Digitalis, Senna, etc., or in young green stems as in Ephedra, in flower as in clove and in fruit as in fennel, orange peel. These openings are surrounded with a pair of kidney-shaped cells called guard cells. The term ‘stoma’ is often applied to the stomatal arrangement, which consists of slit like opening along with the guard cells. The epidermal cells surrounding the guard cells are called neighbouring cells or subsidiary cells. These, in many cases, as in Digitalis resemble the other epidermal cells, but in large number of plants they differ in size, arrangement and shape from the other epidermal cells.
Fig. 4.2 Stomata
Types of stomatal arrangement: According to the arrangement of the epidermal cells surrounding the stomata, they have been grouped as follows:
1. Diacytic or Caryophyllaceous (cross celled): The stoma is accompanied by two subsidiary cells, the long axis of which is at right angles to that of the stoma. This type of stoma is also, called the Labiatae type as it is found in many plants of the family Labiatae such as vasaka, tulsi, spearmint and peppermint.
2. Anisocytic or Cruciferous (unequal celled): The stoma is surrounded by usually three subsidiary cells of which one is markedly smaller than the others. This type of stoma is also called the Solanaceous type as it is found in many plants of the family Solanaceae, such as Belladonna, Datura, Hyoscyamus, Stramonium, Tobacco; it is also found in many plants of the family Compositae.
3. Anomocytic or Ranunculaceous (irregular celled): The stoma is surrounded by a varying number of cells in no way differing from those of the epidermal cells as in Digitalis, eucalyptus, henna, lobelia, neem, etc.
4. Paracytic or Rubiaceous (parallel celled): The stoma is surrounded usually by two subsidiary cells, the long axis of which are parallel to that of stoma as in Senna and many Rubiaceous plants.
5. Actinocytic (radiate celled): The stoma is surrounded by circle of radiating cells, as in Uva ursi.
Functions and distributions of stomata: Stomata perform the function of gaseous exchange and transpiration in the plant body. They are most abundant in the lower epidermis of a dorsiventral leaf and less abundant on the upper epidermis. In isobilateral leaves, stomata remain confined to the upper epidermis alone; in submerged leaves no stoma is present. In Buchu and Neem, stomata are present only on lower surface, while in case of Belladonna, Datura, Senna, etc., stomata are present on the both surfaces. The distribution of stoma shows great variation between upper and lower epidermis. In desert plants and in those showing xerophytic adaptations, e.g. Ephedra, Agave, Oleander, etc., stomata are situated in grooves or pits in the stem or leaf. This is a special adaptation to reduce excessive evaporation, as the stomata sunken in pits are protected from gusts of wind.
Trichomes
Trichomes are more elongated outgrowths of one or more epidermal cells, and consist of two carts, a foot or root embedded in the epidermis and a free projecting portion termed as body. Trichomes usually occur in leaves but are also found to be present on some other parts of the plant as in Kurchi, Nux vomica and Strophanthus seeds, Andrographis and Belladonna stem, Cummin, and Lady’s finger fruits, etc. Trichomes are rarely present on the leaves of Bearberry, Buchu, Henna, etc., and are absent in glabrous leaves like Coca, Hemlock, Savin, etc.
Fig. 4.3 Different types of stomata
Functions of trichomes: Trichomes or hairs are adapted to many different purposes. A dense covering of trichomes prevents the damage by insects and the clogging of stomata due to accumulation of dust. Trichomes also aid the dispersion of seeds of Milkweed (Asclepias) and Madar (Calotropis), which are readily scattered by wind. In Peppermint, Rosemary, Tulsi, etc., trichomes perform the function of secreting volatile oil.
Types of trichomes: Broadly, the trichomes are classified as:
Depending upon the structure, shape and number of cells, they are further classified as follows:
[A] Covering Trichomes
(a) Unicellular trichomes
1. Linear, strongly waved, thick walled trichomes—Yerba santa
2. Linear, thick walled and warty trichomes—Damiana
3. Short. conical trichomes—Tea
4. Short, conical, warty trichomes—Senna
5. Large, conical, longitudinally striated trichomes—Lobelia
6. Long, tubular, flattened and twisted trichomes—Cotton
7. Lignified trichomes—Nux vomica, strophanthus
8. Short, sharp, pointed, curved, conical trichomes—Cannabis
9. Unicellular, stellate trichomes—Deutezia scabra
(b) Multicellular unbranched trichornes
[B] Glandular Trichomes
(b) Multicellular glandular trichomes
1. Unicellular stalk with single spherical secreting cell at the apex—Digitalis purpurea
2. Uniseriate, multicellular stalk with single spherical cell at the apex—Digitalis thapsi
3. Uniseriate stalk and bicellular head—Digitalis purpurea
4. Multicellular, uniseriate stalk and multicellular head—Hyoscyamus
5. Biseriate stalk and biseriate secreting head—Santonica
6. Short, unicellular stalk and head formed by a rosette of two to eight club-shaped cells—Mentha
7. Multiseriate, multicellular cylindrical stalk and a secreting head of about eight radiating club-shaped cells—Cannabis
Fig. 4.4 Covering trichomes
Fig. 4.5 Glandular trichomes
Periderm
In the stem and root of mature plant, the layers immediately below the epidermis (phellogen) divide and redivide. On the outside they form cork or phellem and on the inner side they form phelloderm.
Fig. 4.6 Periderm
The cork cells are rectangular brick shaped or polygonal; phelloderm cells are mostly parenchymatous in nature. Lenticels are present in the periderm, especially in the bark of old plants which are similar in function to stomata. These are open pores with absence of guard cells. The cork cells are impregnated with a layer of suberin. The various types of cork cells are shown bellow.
Fig. 4.7 Various types of cork cells
Vascular Tissue System
This system consists of a number of vascular bundles which are distributed in the stele. The stele is the central cylinder of the stem and the root surrounded by the endodermis. It consists of vascular bundles, pericycle, pith and medullary rays. Each bundle is made up of xylem and phloem, with a cambium in dicotyledonous stems, or without a cambium in monocotyledonous stems, or only one kind of tissue xylem or phloem, as in roots.
Function
The function of this system is to conduct water and raw food material from the roots to the leaves, and prepared food material from leaves to the storage organs and the growing regions.
The vascular bundle of a dicotyledonous stem, when fully formed, consists of three well-defined tissues:
[1] Xylem
Xylem or wood is a conducting tissue and is composed of elements of different kinds, viz. (a) tracheids, (b) vessels or tracheae, (c) wood fibres and (d) wood parenchyma. Xylem, as a whole, is meant to conduct water and mineral salts upwards from the root to the leaf to give mechanical strength to the plant body.
(a) Tracheids: These are elongated, tube-like cells with hard, thick and lignified walls and large cell cavities. Their ends are tapering, either rounded or chisel-like and less frequently, pointed. They are dead, empty cells and their walls are provided with one or more rows of bordered pits. Tracheids may also be annular, spiral, scalariform or pitted (with simple pits). In transverse section, they are angular—either polygonal or rectangular. Tracheids (and not vessels) occur alone in the wood of ferns and gymnosperms, whereas in the wood of angiosperms, they are associated with the vessels. Their walls being lignified and hard, their function is conduction of water from the root to the leaf.
Fig. 4.8 (a) Tracheids with bordered pits (b) Scalariform tracheid
(b) Vessels or tracheae: Vessels are cylindrical, tube-like structures. They are formed from a row of cells placed end to end, from which the transverse partition walls break down. A vessel or trachea is, thus, a tube-like series of cells, very much like a series of water pipes forming a pipeline. Their walls are thickened in various ways, and vessels can be annular, spiral, scalariform, reticulate, or pitted, according to the mode of thickening. Associated with the vessels are often some tracheids. Vessels and tracheids form the main elements of the wood or xylem of the vascular bundle. They serve to conduct water and mineral salts from the roots to the leaves. They are dead, thick-walled and lignified, and as such, they also serve the mechanical function of strengthening the plant body.
Fig. 4.9 Different kinds of vessels
(c) Xylem (wood) fibres: Sclerenchymatous cells associated with wood or xylem are known as wood fibres. They occur abundantly in woody dicotyledons and add to the mechanical strength of the xylem and of the plant body as a whole.
(d) Xylem (wood) parenchyma: Parenchymatous cells are of frequent occurrence in the xylem, and are known as wood parenchyma. The cells are alive and generally thin walled. The wood parenchyma assists, directly or indirectly, in the conduction of water, upwards, through the vessels and the tracheids. It also serves to store food.
[2] Phloem
The phloem or bast is another conducting tissue, and is composed of the following elements: (a) sieve tubes, (b) Companion cells, (c) phloem parenchyma and (d) bast fibres (rarely). Phloem, as a whole, is meant to conduct prepared food materials from the leaf to the storage organs and growing regions.
(a) Sieve tubes: Sieve tubes are slender, tube-like structures, composed of elongated cells which are placed end to end. Their walls are thin and made of cellulose. The transverse partition walls are, however, perforated by a number of pores. The transverse wall then looks very much like a sieve, and is called the sieve plate. The sieve plate may sometimes be formed in the side (longitudinal) wall. In some cases, the sieve plate is not transverse (horizontal), but inclined obliquely, and then different areas of it become perforated. A sieve plate of this nature is called a compound plate. At the close of the growing season, the sieve plate is covered by a deposit of colourless, shining substance in the form of a pad, called the callus or callus pad. This consists of carbohydrate, called callose. In winter, the callus completely clogs the pores, but in spring, when the active season begins, it gets dissolved. In old sieve tubes, the callus forms a permanent deposit. The sieve tube contains no nucleus, but has a lining layer of cytoplasm, which is continuous through the pores. Sieve tubes are used for the longitudinal transmission of prepared food materials—proteins and carbohydrates—downward from the leaves to the storage organs, and later upward from the storage organs to the growing regions. A heavy deposit of food material is found on either side of the sieve plate with a narrow median portion.
Fig. 4.10 A sieve tube in longitudinal section
(b) Companion cells: Associated with each sieve lube and connected with it by pores is a thin-walled, elongated cell known as the companion cell. It is living and contains protoplasm and an elongated nucleus. The companion cell is present only in angiosperms (both dicotyledons and monocotyledons). It assists the sieve tube in the conduction of food.
(c) Phloem parenchyma: There are always some parenchymatous cells forming a part of the phloem in all dicotyledons, gymnosperms and ferns. The cells are living, and often cylindrical. They store up food material and help to conduct it. Phloem parenchyma is, however, absent in most monocotyledons.
(d) Bast fibres: Sclerenchymatous cells occurring in the phloem or bast are known as bast fibres. These are generally absent in the primary but occur frequently in the secondary phloem.
[3] Cambium
This is a thin strip of primary meristem lying between the xylem and phloem. It consists of one or a few layers of thin-walled and roughly rectangular cells. Although cambial cells look rectangular in transverse section, they are very elongated, often with oblique ends. They become flattened tangentially, i.e. at right angles to the radius of the stem.
Types of Vascular Bundles
According to the arrangement of xylem and phloem, the vascular bundles are of the following types:
(A) Radial vascular bundle: When the xylem and phloem form separate bundles which lie on different radii, alternating with each other, as in roots. The radial vascular bundle is the most primitive type of vascular bundles.
(B) Conjoint vascular bundle: When the xylem and phloem combine into one bundle, it is called as conjoint vascular bundle. There are different types of conjoint vascular bundles.
(1) Collateral: When the xylem and phloem lie together on the same radius, the xylem being internal and the phloem external is called collateral. When cambium is present in collateral as in all dicotyledonous stems, the bundle is said to be open collateral, and when the cambium is absent, it is said to be closed collateral, as in monocotyledonous stems.
(2) Bicollateral:
When the both phloem and cambium occur twice in a collateral bundle—once on the outer side of the xylem and again on the inner side of it, is called as bicollateral. The sequence is outer phloem, outer cambium, xylem, inner cambium and inner phloem. Bicollateral bundles are characteristics of Cucurbitaceae. They are also often found in Solanaceae, Apocynaceae, Convolvulaceae, Myrtaceae, etc. A bicollateral bundle is always open.
(C) Concentric vascular bundle: When one kind of vascular tissue (xylem or phloem) is surrounded by the other is called as concentric vascular bundle. Evidently, there are two types, according to whether one is central or the other one is so. When the phloem lies in the centre and is surrounded by xylem, as in some monocotyledonous, the concentric bundle is said to be amphivasal (leptocentric). When, on the other hand, the xylem lies in the centre and is surrounded by phloem, the concentric bundle is said to be amphicribral (Hadrocentric). A concentric bundle is always closed.
Ground Tissue System
Ground tissue system is represented by the cortex, hypodermis, pith, mesophyll and portion of midrib of leaves and comprises of the following tissues.
(a) Parenchyma
The parenchyma consists of a collection of cells which are more or less isodiametric, that is, equally expanded on all sides. Typical parenchymatous cells are oval, spherical or polygonal. Their walls are thin and made of cellulose. They are usually living. Parenchymatous tissue is of universal occurrence in all the soft parts of plants. Its main function is storage of food material. When parenchymatous tissue contains chloroplasts, it is called chlorenchyma. Its function is to manufacture food material. A special type of parenchyma develops in many aquatic plants and in the petiole of banana. The wall of each such cell grows out in several places, like rays radiating from a star and is, therefore, stellate or star-like in general appearance. These cells leave a lot of air cavities between them, where air is stored up. Such a tissue is often called aerenchyma.
Fig. 4.11 (a) Parenchyma, (b) Chlorenchyma and (c) Aerenchyma
(b) Collenchyma
This tissue consists of somewhat elongated, parenchymatous cells with oblique, slightly rounded or tapering ends. The cells are much thickened at the corners against the intercellular spaces. They look circular, oval or polygonal in a transverse section of the stem. The thickening is due to a deposit of cellulose, hemicellulose and protopectin. Although thickened, the cells are never lignified. Simple pits can be found here and there in their walls. Their thickened walls have a high refractive index and, therefore, this tissue in section is very conspicuous under the microscope. Collenchyma is found under the skin (epidermis) of herbaceous dicotyledons, e.g. sunflower, gourd, etc., occurring there in a few layers with special development at the ridges, as in gourd stem. It is absent from the root and the monocotyledon, except in special cases. The cells are living and often contain a few chloroplasts. Being flexible in nature, collenchyma gives tensile strength to the growing organs, and being extensible, it readily adapts itself to rapid elongation of the stem. Since it contains chloroplasts, it also manufactures sugar and starch. Its function is, therefore, both mechanical and vital.
Fig. 4.12 (a) Collenchyma in transaction and (b) Collenchyma in longitudinal section
(c) Sclerenchyma
Sclerenchyma (scleros means hard) consists of very long, narrow, thick and lignified cells, usually pointed at both ends. They are fibre-like in appearance and hence, they are also called sclerenchymatous fibres, or simply fibres. Their walls often become so greatly thickened that the cell cavity is nearly obliterated. They have simple, often oblique, pits in their walls. The middle lamella is conspicuous in sclerenchyma. They are dead cells and serve a purely mechanical function, i.e. they give the requisite strength, rigidity, flexibility and elasticity to the plant body and thus enable it to withstand various strains.
Sclereids: Sometimes, special types of sclerenchyma develop in various parts of the plant body to meet local mechanical needs. They are known as Sclereids or Stone cells. They may occur in the cortex, pith, phloem, hard seeds, nuts, stony fruits, and in the leaves and stems of many dicotyledons and also gymnosperms. The cells, though very thick-walled, hard and strongly lignified (sometimes cutinized or suberized), are not long and pointed like sclerenchyma, but are mostly isodiametric, polyhedral, short-cylindrical, slightly elongated, or irregular in shape. Usually, they have no definite shape. They are dead cells, and have very narrow cell cavities, which may be almost obliterated, owing to excessive thickness of the cell wall. They may be somewhat loosely arranged or closely packed. They may also occur singly. They contribute to the firmness and hardness of the part concerned.
Fig. 4.13 (a) Sclerenchymatous fibres and (b) Sclereids (Stone cells)
4.4 Cell Contents
In pharmacognosy, we are concerned with the cell contents which can be identified in plant drugs by microscopical and physical tests. These are either food storage products or the by-products of plant metabolism and include carbohydrates, proteins, lipids, calcium oxalate, calcium carbonate, tannins, resins, etc. Some of these cell contents of diagnostic importance can be briefly described as follows.
Starch
Starch is present in different parts of the plant in the form of granules of varying size. Starch is found abundantly in fruit, seed, root, rhizome and as smaller grains in chlorophyll containing tissue of the plant such as leaf. Starches of different origins can be identified by studying their size, shape and structure, as well as, position of the hilum and striations. Chemically, starches are polysaccharides containing amylopectin and β-amylose. Starch turns blue to violet when treated with iodine solution.
Starches of pharmaceutical interest are obtained from maize, rice, wheat and potato. These starches can be differentiated from each other by microscopical examination. A comparative account of their macroscopical, microscopical and physical characteristics is given in the Table 4.2. For purpose of microscopical studies, the powder should be mounted in Smiths starch reagent containing equal parts of glycerin, water and 50% acetic acid.
Table 4.2 Characteristics of some starch grains
A systematic description of starch grains should include:
1. Shape—Ovoid, spherical, sub-spherical, ellipsoidal, polyhedral, etc.
2. Size—Dimensions in μm.
3. Position of hilum—Central, eccentric, pointed, radiate, linear, etc.
4. Aggregation—Simple, compound; number of components present in a compound grain.
5. Appearance between crossed polaroids.
6. Location—Loose, present in type of cell and tissue.
7. Frequency—Occasional, frequent, abundant.
Fig. 4.14 Starch grains obtained from the different sources
Aleurone Grain
Protein is stored in the form of aleurone grain by plants. Aleurone grain consists of a mass of protein surrounded by a thin membrane, and is found abundantly in the endosperm of the seed. The ground mass of protein, however, often encloses an angular body (crystalloid) arid one or more rounded bodies (globoids).
Defat thin sections containing aleurone grains and treat with the following reagents.
Calcium Oxalate Crystals
Calcium oxalate crystals are considered as excretory products of plant metabolism. They occur in different forms and provide valuable information for identification of crude drugs in entire and powdered forms.
The sections to be examined for calcium oxalate should be cleared with caustic alkali or chloral hydrate. These reagents very slowly dissolve the crystals, so the observation should be made immediately after clearing the section. The polarizing microscope is useful in the detection of small crystals.
Calcium Carbonate
Aggregates of crystals of calcium carbonate are called ‘cystoliths’, which appear like small bunches of grapes in the tissue. Calcium carbonate dissolves with effervescence in acetic, hydrochloric or sulphuric acid. When treated with 60% w/w sulphuric acid, needled shaped crystals of calcium sulphate slowly separate out.
Fixed Oils and Fats
Fixed oils and fats are widely distributed in both vegetative and reproductive parts of the plant. They are more concentrated in the seeds as reserved lipids. Fixed oils occur as small refractive oil globules, usually present in association with aleurone grains. Fixed oil and fat show certain common characteristics and respond to the following tests:
1. They are generally soluble in ether and alcohol with few exceptions.
2. 1% solution of osmic acid colours them brown or black.
3. Dilute tincture of alkanna stains them red on standing for about 30 minutes.
4. A mixture of equal parts of strong solution of ammonia and saturated solution of potash slowly saponifies fixed oil and fat.
4.5 Cell Division
From the smaller plants like algae to the large trees like eucalyptus, all starts their growth from a single cell called as egg cell. It is brought about by the development of new cells. Two important processes are continued which ultimately helps in the vegetative growth and also in the preservation of hereditary characteristics. It includes the division of nucleus termed as mitosis and the division of cell cytoplasm, referred to as cytokinesis.
Mitosis
Mitosis is a somatic cell division which is responsible for the development of vegetative body of the plants. A German Botanist Stransburger (1875) first studied it in detail. The process of mitotic cell division consists of four important stages, viz. prophase, metaphase, anaphase and telophase (Figure 4.15).
Fig. 4.15 Phases of mitotic cell division
Prophase
This phase of chromosome fixation is the longest one in the mitotic cell division. Firstly, the indistinct chromosomes appear as the recognizable thread. Chromosomes are closely occurring double threads of which each longitudinal half becomes chromatid. Gradually chromosomes are thickened. Chromatid starts dividing longitudinally into two halves along with chromosomal substance matrix around it. Some gap start appearing in the chromosomes which is called as centromeres. At the end of prophase, nucleoli become smaller, matrix becomes clearer and the nucleus enters into metaphase.
Metaphase
During this phase nuclear membrane vanishes and the spindle formation takes place; Bipolar spindle is made up of delicate fibres. Later the nuclear membrane is removed; spindle appears into the nuclear region. Movement of chromosomes to the equatorial plane of spindle separates them from one another. Centromeres are along the equators while the arms of the chromosomes are directed towards the cytoplasm where they are most clearly revealed.
Protometaphase
Nucelear envelope fragments. Microtubes of spindle invade nuclear area and are able to interact with chromosomes. Chromosomes are more condensed. The two chromatids have kinetochore-protein structure. Microtubes attach to kinetochore and move the chromosomes back and forth. The kinetochore that do not attach interact with others from the opposite pole.
Anaphase
In anaphase, chromatid halves move away equatorially at two opposite poles with the tractile fibres. The chromatid separates completely from each other. The spindle undergoes maximum elongation to facilitate separation of diploid chromatids. It is a shortest phase of mitosis.
Telophase
In telophase, chromatids forms the close groups. The polar caps of the spindle disappear and the formation of nuclear membrane takes place around the groups of chromosomes. The matrix and spindle body disappears completely. Appearance of nucleoli and nuclear sap makes them recognizable as two distinct nuclei.
Once again nucleus formed grows in size and starts working as metabolic nuclei to enter again in the cycle of mitotic cell division. It mainly depends upon types of plants, plant part and temperature.
Cytokinesis
Cytokinesis is the partition of cytoplasmic material. It takes place either by formation of new cell walls or by cytoplasmic breakdown. New cells are formed by deposition of cellulosic material in the equatorial zones, which forms the membrane and divide cytoplasm into newly formed cells.
Meiosis
Meiosis is a process of nuclear division in which the numbers of chromosomes are reduced to half (n) from the basic nucleus of 2n chromosomes. A German botanist Stransburger (1888) was the first researcher of this complex genetic process. Chromosomes are called as the carriers of hereditary characters, so the meiosis is the process of transmission of these genetic characteristics. All sexually reproducing plants and animals are gametes with haploid number of chromosomes. Fusion of the male and female gametes results into zygote whereby doubling of chromosomes to 2n takes place to develop offspring.
Meiosis involves two successive divisions: the first process of division I is reduction division, while the second process of division II is similar to that of mitosis, (Figure 4.16).
Fig. 4.16 Phases of meiosis
Division I
In this process of meiosis mother nucleus undergoes complicated changes which can be subdivided into various phases as given below.
Prophase I: In this phase chromosomes are systematically arranged. This phase is again divided into five different stages:
• Leptotene: This is an early prophase in which diploid chromosomes are found as long, single threads of identical pairs. Coiling of these threads of chromosomes occurs.
• Zygotene: Identical chromosomes gets attracted towards each other and the pairs are developed throughout their length. This pairing is termed as synapsis. The Chromosomes thus paired are homologous in nature.
• Pachytene: The pairs of chromosomes go shorter and thicker due to coiling. Longitudinal splitting in it gives rise to four chromatids from each chromosome. This is a longer phase of prophase I.
• Diplotene: This is a stage where separation of chromatids takes place. Their point of attachment remains at a single point known as chiasmata. At this stage the exchange of the genetic material occurs due to crossing over, a prominent feature of meiosis. With further thickening and shortening of chromosomes, diplotene ends into Diakinesis.
• Diakinesis: In this last stage of prophase I, two halves of the chromosome starts moving equatorially. Chiasmata remain as a point of attachment. Nucleolus disappears and nuclear membrane gets dissolved to release the chromosomes in cytoplasm. Nuclear spindle formation begins at the end of diakinesis.
Metaphase I: In this phase both the chromatids starts moving to two opposite poles of the spindle. In mitotic metaphase chromosomes are lined up at the opposite poles while in meiosis chiasmata remains attached to spindle fibres at the opposite poles.
Anaphase I: The Chiasmata of the homologous chromatids repels each other to opposite poles. Chromosomes are carried away by the tractile fibres to the equators. This is an important stage at which reduction of chromosome number from diploid to haploid occurs.
Telophase I: At both the equatorial poles, pairs of chromatids start developing as the two haploid daughter nuclei. The nucleolus starts reappearing and the formation of nuclear membrane takes place. Two daughter nuclei thus formed enters in the second process of Division II.
Division II
All the phases of division II are similar to that of mitotic cell division. Telophase I passes into prophase II.
Prophase II: Both the chromatid groups which have the loose ends go on coiling and become shorter and thicker. Nucleolus and nuclear membrane vanishes and spindle fibres show its appearance.
Metaphase II: In Metaphase II, chromatids once again starts separating equatorially at two opposite poles. Pairs of chromatids separate completely with its own centromere and ends in Anaphase II.
Anaphase II: At the stage of Anaphase II, two sister chromatids of each pair of chromosome move to opposite poles of the spindle as directed by the centromeres.
Telophase II: In Telophase II, both the polar groups of chromosomes are converted to the nuclei by formation of nuclear membrane.
Lastly via cytokinesis four daughter cells are formed each having the haploid or ‘n’ number of chromosomes.
4.6 Morphological Study
The abundance of plants and their size from bacteria to huge trees make it difficult to study their morphological characters. Classification of plants has solved the problem to a greater extent. Still it is impossible to define precisely the plant body as made up of certain parts only. Plants exhibit vividness in several respects.
The details of morphological characters of these plant organs are as under.
Morphology of Bark
The bark (in commerce) consists of external tissues lying outside the cambium, in stem or root of dicotyledonous plants. Following are the tissues present in bark:
Cork (phellum), phellogen and phelloderm (collectively known as periderm), cortex, pericycle, primary phloem and secondary phloem.
In Botany, the bark consists of periderm and tissues lying outside it, i.e. cork, phellogen and phelloderm.
Methods of Collection of Barks
Bark is generally collected in spring or early summer because the cambium is very active and thinwalled and gets detached easily. Following are the methods of collection of barks.
1. Felling method: The fully grown tree is cut down near the ground level by an axe. The bark is removed by making suitable longitudinal and transverse cuts on the stem and branches. The disadvantages of this method are (a) the plant is fully destroyed and (b) the root bark is not utilized.
2. Uprooting method: In this method, the stem of definite age and diameter are cut down, the root is dug up and bark is collected from roots, stems and branches. In Java, cinchona bark is collected by this method.
3. Coppicing method: The plant is allowed to grow up to certain age and diameter. The stems are cut at a certain distance from ground level. Bark is collected from stem and branches. The stumps remaining in the ground are allowed to grow up to certain level; again the shoots are cut to collect the bark in the same manner. Cascara bark and Ceylon cinnamon bark are collected by this method.
Morphology of Bark
The following features may be used to describe the morphology of bark.
1. Shape: The shape of the bark depends upon the mode of cuts made and the extent and shrinkage occurred during drying.
(a) Flat: When the large piece of the bark is collected from old trunk and dried under pressure, the bark is flat, e.g. Quillaia and Aarjuna barks.
(b) Curved: Here, both the sides of the bark are curved inside, e.g. Wild cherry, Cassia and Cascara barks.
(c) Recurved: Both sides of bark are curved outside, e.g. Kurchi bark.
(d) Channelled: When the sides of bark are curved towards innerside to form channel, e.g. Cascara, Cassia and Cinnamon barks.
(e) Quill: If one edge of bark covers the other edge, it is called quill, e.g. Ceylon, Cinnamon and Cascara barks.
(f) Double quill: Here, both the edges curve inward to form double quill, e.g. Cinnamon and Cassia barks.
(g) Compound quill: When the quills of smaller diameter are packed into bigger quills, it is called compound quills. Compound quills are formed to save the space in packing and transportation, e.g. Cinnamon bark.
Fig. 4.17 Morphological characters of bark
2. Outer surface:
(a) Smooth: When development of cork is even, e.g. Arjuna bark.
(b) Lenticels: They are transversely elongated holes formed on outer surface because of lateral pressure, e.g. Wild Cherry and Cascara barks.
(c) Cracks and fissures: They are formed due to increase in diameter, e.g. Cinchona bark
(d) Longitudinal wrinkles: They are formed because of shrinkage of soft tissues, e.g. Cascara bark.
(e) Furrows: If troughs between wrinkles are wide, it is called furrows, e.g. Cinchona calisaya bark.
(f) Exfoliation: Sometimes the cork of bark flakes off exposing cortex, e.g. in Wild cherry bark.
(g) Rhytidoma: It is composite dead tissue consisting of alternate layers of cork, cortex and/or phloem, e.g. Quillaia and Tomentosa barks. Sometimes it is removed during peeling.
(h) Corky warts: They are the small circular patches, found sometimes in old barks, e.g. in Cinchona succirubra and Ashoka barks.
(i) Epiphytes: Such as moss, lichen and liverwarts are sometimes seen in bark, e.g. Cascara bark.
3. Inner surface: The colour and condition of inner surface is of diagnostic value.
4. Fracture: The appearance of exposed surface of transversely broken bark is called fracture. Different types of fracture, their descriptions and examples are given in Table 4.3.
Table 4.3 Various types of fracture of bark
Histology of Barks
The bark shows following microscopical character: (i) Tabular, radially arranged cork cells, may be suberised or lignified (e.g. in Cassia bark), (ii) Thin-walled cellulosic parenchymatous phellogen and phelloderm, (iii) Collenchymatous and/or parenchymatous cortex, (iv) Parenchymatous or scleranchymatous pericycle; may contain band of stone cells and fibres, (v) Primary phloem which is generally crushed, e.g. in Cascara and Arjuna and (vi) Secondary phloem consisting of sieve tubes, companion cells, phloem parenchyma, phloem fibres and stone cells. Phloem fibres are thick walled, lignified, e.g. in Cinchona and Cascara; stone cells are thick, lignified with narrow lumen, e.g. Kurchi and Cinnamon barks; sometimes branched stone cells are seen in Wild cherry bark, (vii) Thin walled, living radially elongated medullary ray cells which are uni-, bi- or multiseriate and straight or wavy, (viii) Starch, calcium oxalate, oil cells, mucilage, etc., are often present in cortex.
Morphology of Roots
Root is a downward growth of the plant into the soil. It is positively geotropic and hydrotropic. Radicle from the germinating seed grows further into the soil to form the root. It produces similar organs. Root does not have nodes or internodes. Branching of the root arises from the pericyclic tissues. Roots are covered by root caps or root heads.
[A] Functions of Roots
1. Roots fix the plant to the soil and give mechanical support to the plant body.
2. Roots absorb water and the minerals dissolved in it from the soil and transport them to the aerial parts where they are needed.
3. At times, the root undergoes modification and performs special functions like storage, respiration, reproduction, etc.
[B] Various Parts of a Root
A typical underground root exhibits the following parts:
(a) Root cap: The tip of the root is very delicate and is covered by root cap. Root cap protects the growing cells and as and when it is worn out it is replaced by the underlying tissues immediately.
(b) Region of cell division: The next layer of tissue lying immediately after the root cap towards the stem is the meristematic tissue producing new cells, known as region of cell division or growing region.
(c) Region of elongation: The newly formed cells in the growing region grow further by elongation in this region resulting in the increase in the length of the root.
(d) Region of root hairs: Above the region of elongation is the region of root hairs wherein the root hairs, the unicelluar, tubular outgrowths formed by the epiblema are formed. They are responsible for strengthening the hold of root into the soil and also for the absorption of water.
(e) Region of maturation: It is located above the region of root hairs. It does not absorb anything, but is mainly responsible for the absorbed material by roots. The root branches or the lateral roots are produced in this region.
Fig. 4.18 Apex of root showing different regions
[C] Types of Roots
There are two types of root systems:
(a) Tap root system or primary roots and
(b) Adventitious roots.
(a) Tap root system: The radicle grows into the soil and forms main axis of the root known as tap root. It grows further to produce branches in the acropetal manner known as secondary roots which further branches to give tertiary roots. These are all true roots. This system is characteristic of dicotyledons.
(b) Adventitious root system: The roots that develop from any part of the plant other than radicle are termed as adventitious roots. They may develop from root base nodes or internodes. This type of root system is found in monocots and in pteridophytes.
Fig. 4.19 (a) Tap root system and (b) Adventitious root system
I. Modification for storage of food: This type of modification is shown by both the types of roots, i.e. tap roots and adventitious roots. They store carbohydrates and are used during early growth of successive season.
(i) Tap roots show the following three types of modifications:
(a) Conical: These are cone-like, broader at the base and tape-ring at the tip, e.g. carrot.
(b) Fusiform: These roots are more or less spindle shaped, i.e. tapering at both the ends, e.g. radish.
(c) Napiform: These are spherical shaped and very sharply tapering at lower part, e.g. beat and turnip.
Fig. 4.20 (a) Conical root, (b) Fusiform root and (c) Napiform root
(ii) Adventitious root show the following types of modifications. They store carbohydrates but do not assume any special shape.
(a) Tuberous roots: These get swollen and form single or isolated tuberous roots which are fusiform in shape, e.g. sweet potato, jalap, aconite.
(b) Fasciculated tuberous roots: When several tuberous roots occur in a group or cluster at the base of a stem they are termed as fasciculated tuberous roots as in dahlia, asparagus.
(c) Palmated tuberous roots: When they are exhibited like palm with fingers as in common ground orchid.
(d) Annulated roots: The swollen portion is in the form of a series of rings called annules as in ipecacuanha.
II. Modifications for support: Plant develops special aerial roots to offer additional support to the plant by way of adventitious roots.
(a) Clinging or Climbing roots: These types of roots are developed by plants like black pepper for support or for climbing purposes at nodes.
(b) Stilt roots: This type of root is observed in maize and screw-pine, which grow vertically or obliquely downwards and penetrate into soil and give additional support to the main plant.
(c) Columnar roots: In certain plants like banyan, the additional support is given by specially developed pillars or columnar roots. They even perform the function of regular roots.
Fig. 4.21 (a) Climbing root (b) Stilt root (c) Columnar root
III. Modifications for special functions:
(a) Respiratory roots or pneumatophores: The roots of the plant growing in marshy places on seashores due to continuous water logging are unable to respire properly. They develop some roots growing against the gravitational force (in the air) with minute openings called lenticels. With the help of lenticels they carry on the exchange of gases. They look like conical spikes around the stems. This type of root is observed in case of plants called mangroves found in creeks, i.e. avicinnia.
Fig. 4.22 Respiratory roots
(b) Sucking roots or Haustoria: The plants, which are total parasites on the host, develop special type of roots for the purpose of absorption of food material from, the host. These roots neither possess root caps nor root hairs, and are known as sucking roots, e.g. cuscuta, striga and viscum.
(c) Photosynthetic roots: Aerial roots in some cases, specially in leafless epiphytes become green in colour on exposure to sunlight and perform photosynthesis and are known as photosynthetic roots as in case of Tinospora cardifolia.
(d) Epiphytic or Assimilatory roots: The plants which grow on the branches or stems of the plants without taking any food from them are called epiphytic and the roots developed by them are the epiphytic roots. They consist of the following:
(i) Clinging roots with which they get fixed with the host and
(ii) Aerial roots which hang freely in the air, which are normally long greenish white in colour and absorb moisture from the atmosphere with the help of porous tissue. These roots are devoid of root caps and root hairs. They carry on photosynthesis. These are developed in the plants growing in humid atmosphere. Bulbophyllum, uanda are the examples of this type.
(e) Nodulated roots or root tubercles: The plants belonging to leguminosae family develop nodules or tubercles. These are formed by nitrogen fixing bacteria and getting carbohydrates from the plants. Roots and bacteria are symbiotic to each other. These swellings developed by roots are nodulated roots.
Uses of Roots
1. Source of food and vegetables: Most of vegetables constitute roots only, i.e. radish, turnip, beet, carrot, etc. They are rich sources of vitamins or their precursors. Some of them like sweet potato and tapioca are rich in starch, and hence are consumed as food.
2. Various types of medicinally important drugs are obtained from roots.
Morphology of Stems
The plumule develops to form the stem. Thus stem is an aerial part of the plant. It consists of axis and the leaves. Stem has got the following characteristics:
Depending upon the presence of mechanical tissues, the stems may be weak, herbaceous or woody.
[A] Weak stems: When the stems are thin and long, they are unable to stand erect, and hence may be one of the following types:
(a) Creepers or Prostate stem: When they grow flat on the ground with or without roots, e.g. grasses, gokharu, etc.
(b) Climbers: These are too weak to stand alone. They climb on the support with the help of tendrils, hooks, prickles or roots, e.g. Piper betel, Piper longum.
(c) Twinners: These coil the support and grow further. They are thin and wiry, i.e. ipomoea.
[B] Herbaceous and woody stems: These are the normal stems and may be soft or hard and woody, i.e. sunflower, sugarcane, mango, etc.
1. Produce leaves and exposes them properly to sunlight for carrying out photosynthesis.
2. Conducts water and minerals from roots to leaves and buds.
3. Foods produced by leaves are transported to nongreen parts of the plant.
4. Produce flowers and fruits for pollination and seal dispersal.
5. Depending upon the environment it gets suitably modified to perform special functions like storage of foods, means of propagation, etc.
I Underground Modifications of Stems
Underground modifications of stems are of the following types:
1. Rhizome
2. Tuber
3. Bulb
4. Corm.
1. Rhizomes: Grow horizontal under the soil. They are thick and are characterized by the presence of nodes, internodes and scale leaves. They also possess bud in the axil of the scale leaves, e.g. ginger, turmeric, rhubarb, male fern, etc.
2. Tubers: Tubers are characterized by the presence of ‘eyes’ from the vegetative buds which grow further and develop into a new plant. Tubers are the swollen underground structure of the plant, e.g. potato, jalap, aconite, etc.
3. Bulb: In this case, the food material is stored in fleshy scales that overlap the stem. They are present in the axils of the scales, and few of them develop into new plant in the spring season at the expense of stored food material in the bulb. Adventitious roots are present at the base of the bulb. The reserve food material formed by the leaves is stored at their bases, and the new bulbs are produced next year, e.g. garlic, squill and onion.
Fig. 4.23 Bulbs (a) Onion (b) Garlic
4. Corm: Corms are generally stout, and grow in vertical direction. They bear bud in the axil of the scaly leaves, and these buds then develop further to form the new plant. Adventitious roots are present at the base of the corm, e.g. saffron, colchicum, dioscorea, etc.
II Sub-Aerial Modifications of Stems
These include (1) Runner, (2) Stolon, (3) Offset and (4) Sucker.
1. Runner: These creep on the ground and root at the nodes. Axillary buds are also present, e.g. strawberry, pennywort.
Fig. 4.24 Strawberry runner
2. Stolon: These are lateral branches arising from the base of the stems which grow horizontally. They are characterized by the presence of nodes and internodes. Few branches growing above the ground develop into a new plant, e.g. glycyrrhiza, arroroot, jasmine, etc.
3. Offsets: These originate from the axil of the leaf as short, thick horizontal branches and also characterized by the presence of rosette type leaves and a cluster of roots at their bottom, e.g. aloe, valerian.
4. Sucker: These are lateral branches developed from underground stems. Suckers grow obliquely upwards, give rise to a shoot which develop further into a new plant, e.g. mentha species, chrysanthemum, pineapple, banana, etc.
Fig. 4.25 Sucker of mentha
III Aerial Modification of Stems
As the name indicates they grow into the air above the soil to a certain height, as follows:
1. Phylloclades: At times, the stem becomes green and performs the function of leaves. Normally this is found in the plants growing in the desert (xerophytes). Phylloclades are characterized by the presence of small leaves or pointed spines, e.g. opuntia, ruscus, euphorbia, etc.
Cladode is a type of phylloclade with one internode, i.e. asparagus.
2. Thorns and prickles: This is another type of aerial modification meant for protection. Thorns are hard, pointed, straight structures, such as duranta, lemon, etc. Prickles and thorns are identical in function. Prickles get originated from outer tissues of the stem. Thus, they are superficial outgrowths. Prickles are sharp, pointed and curved structures. They are scattered all over the stem. Rose, smilax can be quoted as examples of the same.
Fig. 4.26 (a) Thorns of duranta (b) Rose prickles
3. Stem tendrils: In certain plants, the buds develop into tendrils for the purpose of support. Terminal buds in case of vitis, axillary bud in case of passiflora are suitable examples.
Uses of Stems
Depending upon the structural and chemical contents, stems are used for various purposes.
1. Underground stems in their various forms are either used as food spices or for culinary purposes like, potato, amorphophalus, colocasia, garlic, ginger and onion.
2. Jowar, rice and other stems are used as fodder.
3. Stems of jute, hemp and flax as sources of industrial fibres used for various purposes.
4. Sugarcane stems are used as source of sucrose while latex from stems of Hevea brasiliensis is used as rubber.
5. Woods from stems of several plants are used as drugs like quassia, guaicum, sandalwood, etc.
6. The stems of several plants are injured to produce gums for their multiple industrial uses like gum-acacia, gum-tragacanth, gum-sturculia, etc.
Morphology of Leaves
Leaves are flat, thin green, appendages to the stem, containing supporting and conducting strands in their structure. They develop in such a way that older leaves are placed at the base while the younger ones at the apex.
[A] A Typical Angiospermic Leaf Consists of the Following Parts
(a) Leaf base or hypopodium: By means of which it is attached to the stem.
(b) Petiole: It is the stalk of leaf with which leaf blade is attached to the stem. It is also known as mesopodium. It may be present in leaf or may be absent in leaf. Leaves with petiole are called petiolate, and those without petiole sessile. They may be short or long and cylindrical. Sometimes, it is flattened as in the case of lemon. Then it is described as winged petiole. In some plants the petiole undergoes modification to form the tendrillar petiole which helps the plant to climb, e.g. clematis. In few aquatic plants it enlarges to form the swollen petiole by enclosing air and thus keep the entire plant floating over the water. In few other cases, the petiole enlarges to such an extent to form the leaf like structure as in Australian acacia and is known asphyllode.
(c) Lamina or Leaf blade: The flat expanded part of the leaf is lamina or leaf blade (Epipodium). Lamina may be thick as in xerophytic leaves or thin as in hydrophytes or intermediate as in mesophytes.
(d) Stipules: These are the two small outgrowths found at the base of the leaf, to protect the axillary bud. Leaves may or may not have stipules. Leaves with stipules are described as stipulate, while those without stipules are described as ex-stipulate.
Some stipules perform special functions and hence are put into following types:
1. Tendrillar stipules: The stipules get modified into coiled, tendrils helping the plant to climb, i.e. Indian sarsaparilla (Smilax microphylla).
2. Foliaceous stipules: In case of plants with compound leaves some of the leaflets get converted into tendril and the stipules expand to form the flat surface and carry on photosynthesis, i.e. Lathyrus or pisum.
3. Bud stipules: Scaly stipules of the Ficus sp. are characteristic, which protect the terminal vegetative bud. With the development and unfolding of the leaf the bud stipule falls off.
4. Spiny stipules: In some plants, the stipules get converted into spines and help against browsing animals as in the case of Acacia and Zizyphus.
There are five types of stipules which are as under:
1. Free lateral: These are free and located on either side of the leaf as in China rose.
2. Adnate: When the stipules unite with the petioles forming wing like structure are known as adnate stipules, i.e. Groundnut, rose, etc.
3. Inter-petiolar: When stipules are located in between the two petioles of two leaves as in ixora.
4. Axillary: When two stipules unite becoming axillary to the leaves.
Before considering the further anatomical details of the leaves, it is very essential to know the basic difference botanically between the leaf and the leaflet which is as under:
| Sr. No. | Leaf | Leaflet |
| 1. | Bud or branch is present in the axil. | Bud is absent. |
| 2. | Leaves are solitary and are arranged spirally | These are arranged in pairs |
| 3. | These lie in different planes | Leaflets lie in the same plane |
| 4. | Symmetrical at the bases, i.e. Belladonna, vasaka, eucalyptus, etc. | Asymmetrical at bases, i.e. Rose, senna, acacia, etc. |
[B] Shape of the Lamina of Leaves
Various shapes of the leaves are due to various types or shapes of lamina. It may be one of the following:
1. Acicular: Needlelike, i.e. pinus.
2. Subulate: With acute apex and recurved point, i.e. Ephedra sinica.
3. Linear: When it is long, narrow and flat, i.e. Grasses.
4. Oblong: Broad leaves with two parallel margins and abruptly tapering apex, i.e. Banana.
5. Lanceolate: Which look like lance or spear shaped, e.g. nerium, senna.
6. Ovate: Egg shaped or broad base and narrow apex, e.g. China rose, Buchu.
7. Obovate: Broad apex and narrow base, e.g. Jangalibadam.
8. Obcordate: Inversely heart shaped, i.e. base is narrow but apex is broad, e.g. Oxalis.
9. Spathulate: Like spatula or spoon shaped as in calendula and drosera.
10. Cuneate: Wedge shaped as in pista.
11. Cordate: Heart shaped, i.e. betel.
12. Sagittate: Arrow shaped such as in arum.
13. Hastate: When the two lobes of sagittate leaf are directed outwards as in ipomoea.
14. Reniform: Kidney shaped, i.e. Indian pennywort.
15. Auriculate: When the leaf has got ear like projections at the base.
16. Lyrate: When it is lyre shaped or the blade is divided into lobes with large marginal lobe, i.e. radish mustard.
17. Runcinate: With the lobes convex before and straight behind, pointing backward like the teeth of the double saw, i.e. dendelion leaf.
18. Rotund (Orbicular): When the blade is circular or round, e.g. lotus.
19. Elliptical or oval: When the leaves are narrow at the base and apex but broad in the middle such as guava, vinca, etc.
20. Peltate: When the lamina is shield shaped and fixed to the stalk by the centre.
Fig. 4.29 Shape of the lamina of leaves
[C] Leaf Margins
Leaf margin may be of the following types:
1. Entire: When it is even and smooths, i.e. senna, eucalyptus.
2. Sinuate or wavy: With slight undulations like Ashok.
3. Crenate: When the teeth are round as in digitalis.
4. Dentate: Toothed margin, teeth directing outwards such as margosa, melon.
5. Serrate: When it is like the teeth of the saw such as rose, China rose, etc.
6. Ciliated: It is fringed with hairs.
7. Biserrate: Lobed serrate margin.
[D] Leaf Apices
The apex of the leaf may be one of the following kinds:
1. Obtuse: Rounded tip, i.e. banyan.
2. Acute: When it is pointed to form acute angle, but not stiff, i.e. hibiscus.
3. Acuminate: Pointed tip with much elongation, peepal.
4. Recurved: When the apex is curved backward.
5. Cuspidate: With spiny tip like date palm.
6. Mucronate: Rounded apex ending abruptly in a short point i.e vinca, ixora.
7. Retuse: Broad tip with slight notch, i.e. pistia.
8. Emarginate: When tip is deeply notched as in bambinia.
9. Tendrillar: Tip forming a tendril such as Gloriosa—superba.
Fig. 4.31 Leaf apices
[F] Leaf Surface
It may be of the following types:
(a) Glabrous: When surface is smooth and free of hair or any outgrowth, i.e. vasaka, datura.
(b) Rough: When harsh to touch, digitalis.
(c) Hairy: When covered with hairs.
(d) Glutinous: When covered with sticky substance, tobacco.
(e) Glaucous: When covered with waxy coating, castor.
(f) Pubescent: Covered with straight, short hair, i.e. senna.
[G] Types of Leaves
Taking into consideration the nature of the lamina of the leaves, they are classified into two main groups:
1. Simple leaves and
2. Compound leaves.
1. Simple leaves: A leaf which has only one leaf blade or lamina is called a simple leaf. It may be stipulate or exstipulate, petiolate or sessile, but always possess axillary bud in its axil. It may have an undivided lamina or may be lobed, e.g. vasaka, digitalis, eucalyptus, datura, carica, castor and argemone.
2. Compound leaves: A compound leaf consists of more than one leaf blade or the lamina, the compound leaf is divided into several segments called leaflets or pinnae, e.g. senna, tamarind, acacia.
Compound leaves have been further classified as (a) pinnate compound leaves and (b) palmate compound leaves.
(a) Pinnate compound leaves: These are sub-classified as under depending upon the number of rachis (an axis bearing the leaflets in pinnate compound leaf is known as rachis):
1. Unipinnate compound leaves: Wherein only one rachis bearing the leaflets is present. When an even number of leaflet is present, it is known as paripinnate, e.g. tamarind, gul mohor; if the number of leaflet is odd, it is described as imparipinnate, e.g. rose, margosa, etc.
2. Bipinnate compound leaves: It consists of primary rachis and secondary rachis. The secondary rachis only bears the leaflets, e.g. acacia.
3. Tripinnate compound leaves: These contain primary, secondary and even tertiary rachis. Tertiary rachii only bear the leaflets as in moringa, oroxylon.
4. Decompound leaf: Wherein compound leaf is much divided irregularly as in coriander, carrot, anise, etc.
Fig. 4.33 Pinnate compound leaves
[H] Venation
The arrangement of veins in the lamina or leaf blade is known as venation. Veins are nothing but vascular bundles. Water and minerals absorbed by roots is conveyed to various parts of leaf by veins and the food synthesized by leaf by way of photosynthesis is translocated to other parts of plant through veins only. Veins also offer strength, support and shape to the lamina of the leaf. The prominent vein in the centre of the leaf is known as midrib. In the flowering plants two types of venations exist: (1) Reticulate and (2) Parallel.
1. Reticulate venation: This type of venation is characterized by the fact that many veins and veinlets in the lamina of the leaf are arranged in the form of network or reticulars. This type of venation is characteristic to dicotyledonous leaves. It is further sub-classified as:
(a) Unicostate-reticulate venation: Where the leaf contains only one midrib and several veins are given out on both the sides to form the network such as henna, eucalyptus, peepal, etc.
(b) Multicostate-reticulate venation: In this type many veins of equal strength arise from the end of the petiole. Each vein further branches to give rise to veinlets that form the network. The veins may be convergent (meeting at the apex) or divergent (diverge towards the margin) as in castor, carica and cucurbita.
Fig. 4.35 Reticulate venation
2. Parallel venation: In this type the vein and veinlets in leaf blade are arranged parallel to one another. It is characteristic to monocotyledonous plants with few exceptions like dioscorea and sarsaparilla.
Like reticulate venation, it may also be unicostate parallel venation or multicostate parallel venation as under:
(a) Unicostate parallel venation: Wherein the leaf consists of only one midrib running from apex to the petiole of the leaf. The veinlets and veins arise parallel to one another on each side as in banana and canna.
(b) Multicostate parallel venation: In case of multicostate parallel venation many number of main veins of equal strength arise from the tip or the petiole and run parallel to each other. It may be convergent as in case of several grasses and bamboo or divergent as in case of fan palm.
Fig. 4.36 Parallel venation
[I] Phyllotaxy
It is the mode of arrangement of leaves on the stem. Since the leaves are the chief organs of photosynthesis they must be exposed to sunlight favourably. This is done by arranging the leaves in systematic manner. Following are the various types of phyllotaxy:
1. Alternate or spiral: This phyllotaxy is characterized by the presence of one leaf at each node and all leaves together make a spiral path on the axis, i.e. tobacco, mustard and sunflower.
2. Opposite: When two leaves are placed at the same node and are opposite to one another. This is farther divided into two:
3. Whorled: When more than two leaves are present in a single node and are arranged in a circle as in nerium, alstonia.
4. Leaf mosaic: In this type, the leaves are so arranged that there will not be any overshading and all the leaves are exposed properly. The older leaves have longer petiole while younger leaves have short petiole and are placed in the space left by the older leaves. It recalls the arrangement of glass bits in a mosaic and hence the name, e.g. Oxalis and acalypha.
Fig. 4.37 Types of phyllotaxy
[J] Modifications of Leaves
Under the functions of leaves, it is stated that leaves have to perform two types of functions, i.e. primary functions and secondary functions. Under the primary function, leaves are known to perform three main functions like photosynthesis, gaseous exchange and transpiration. The secondary functions which the leaf has to perform are support, protection, storage of food material etc.
To perform these secondary functions the leaf undergoes structural and physiological changes called modifications. There are at least five types of leaf modifications known.
1. Leaf tendrils: Leaves get modified into slender, coiled and wiry structures as seen in Lathyrus peas and gloriosa for support to the plant.
2. Leaf spines: For the sake of protection certain leaves get converted into spines as seen in Aloe, argemone, acacia, etc.
3. Phyllode: Petiole gets modified to flat leaf-like phyllode to reduce the transpiration, e.g. Australian acacia.
4. Scale Leaves: In ginger and potato they protect the terminal buds, while in onion and garlic they store food material.
5. Pitcher and bladder: These are specially developed modifications of leaves to capture and digest insects in case of carnivorous plants, e.g. Utricularis Bladder wort and Nepenthes.
Fig. 4.38 Leaf modifications
Morphology of Flowers
The flower is actually a modified shoot meant for production of seeds. It consists of four different circles (whorls) arranged in a definite manner. A flower is built up on stem or pedicel with the enlarged end known as thalamus or receptacle. The four whorls of the flowers can be described as under:
1. Calyx: It is the outermost whorl of flower and is generally green in colour, the individual member of which is called sepal.
2. Corolla: It is the second whorl of flower and is either white or bright coloured, each member of which is known as petal.
3. Androecium: It is the third circle of flower and constitutes the male part. The individual component is called stamen and each stamen consists of filament, anther and connective.
4. Gynoecium: This is the fourth circle of the flower and constitutes the female part. Each component is known as carpel or pistil and is made of stigma, style and ovary.
Fig. 4.39 Typical parts of flower
When all the four whorls, are present in a single flower, it is described to be a complete flower, absence of any one of them describes it as incomplete flower. A flower is described to be hermaphrodite or bisexual when it contains stamens and carpels. Absence of any one of them describes it as unisexual flower. When calyx and corolla in a flower are similar in colour and shape, then both of them (calyx and corolla) together are called Perianth, i.e. garlic, onion, asperagus.
When a flower is divided into two equal parts by any vertical section passing through the centre, then it is described as regular or symmetrical or actinomorphic flower as in ipomoea, rose, datura and shoe flower. But when it cannot be divided equally into two parts by one vertical section, then it is described as irregular or asymmetrical or zygomorphic flower.
When the stamens arise from petals instead of thalamus, the petals are called epipetalous. When the stamens get united with gynaecium the structure is known as gynastemium. The union of stamens among themselves is known as cohesion. When the filaments of stamen get united to form a single bundle, it is known as monoadelphous. When it forms two bundles, it is known as diadelphous. When anthers get united to form a column (but filaments are free), the stamens are known as syngenesious. When ovary consists of only one carpel, it is said to be monocarpellary and when it contains more than one carpel, it is said to be polycarpellary. When the carpels in ovary are free, the ovary is described as apocarpous and when they are united it is known as syncarpous.
Fig. 4.40 (a) Actinomorphic flower (b) Zygomorphic flower
Arrangement of Floral Parts on Thalamus
Depending upon the arrangement of floral parts on thalamus, the flowers may be of three types.
(i) Hypogynous flower (Superior ovary): Herein the thalamus is conical, flat, convex and stamens, sepals and petals are arranged at base and ovary at the apex, e.g.: brinjal, china rose, mustard, etc.
(ii) Perigynous flowers (Half-superior Ovary): Thalamus is flat, sepals and stamens grow around the ovary. The flowers are said to be perigynous as in Rose, Strawberry peach.
(iii) Epigynous flower (Inferior ovary): The thalamus is fused with ovary wall, calyx, corolla, stamen appear at the top and the gynaecium at the bottom as in Sunflower, cucumber, apple, etc.
Fig. 4.41 (a) Superior ovary (b) Half-superior ovary (c) Inferior ovary
Placentation
The type of distribution of placentae in the ovary is called placentation. They are of the following types.
1. Marginal: It is characteristic to monocarpellary ovary and placentae arise on ventral suture, e.g. bean and pea.
2. Axil: It is characteristic to polycarpellary syncarpous, bilocular or multilocular ovary. Ventral sutures of each carpel meet at the centre and each of them have marginal placentation, e.g. onion, china rose, ipomoea.
3. Parietal: It is characteristic to polycarpellary syncarpous ovary and the placentae develop on the ventral suture but the ovary is unilocular as in papaya and cucurbita.
4. Free Central: It is characteristic to polycarpellary syncarpous ovary, which is unilocular. Ovules arise on the central axis, but it is not connected with the peripheral wall, e.g. Dianthus, saponaria, portulacca.
5. Basal: It is characteristic to polycarpellary and unilocular ovary. Only one ovule is present and it arises from its base as in sunflower.
Fig. 4.42 Types of placentation
Pollination
Pollination is the process of transference of pollen grains from the anther of a flower to the stigma of the same flower or another flower of the same or allied species. Pollen grains are produced by bursting the anther and are carried by various agencies to the stigma.
The agencies may be insects of various types, wind or even water. There are two types of pollination, i.e. (a) Self-pollination or autogamy and (b) cross pollination or allogamy.
Pollination by insects is known as entomophily. To attract various insects, plants adapt different means such as colour, nectar and scent. Entomophilous pollination is very common in plants.
Morphology of Inflorescence
Plant bear flowers either solitary or in groups. The flowers which are large and showy are normally borne solitary, but which are not so prominent and are small, occur in group or bunches.
The form of natural bunch of flowers in which they occur is called inflorescence. Depending upon the type of branching various forms of inflorescences are known. The axis on which the flowers are arranged is known as peduncle while the stalks of flowers are known as pedicels.
Types of Inflorescences
Following are the types of inflorescences:
(A) Racemose or indefinite inflorescence:
1. Raceme: In this type of inflorescence the peduncle is long. Flowers are stalked and born in acropetal succession and peduncle has indefinite growth and goes on producing flowers as in mustard, radish, dwarf gold mohor, etc. When the main axis is branched and the lateral branches bear the flowers, it is said to be Compound raceme or panicle or branched raceme as in gul mohor, peltophorum, yuchr, etc.
2. Spike: This is similar to raceme, with sessile flowers as in Rangoon creeper, vasaka. A branched spike of polyanthes and terminalia species is known.
3. Spadix: In this inflorescence the peduncle is short with numerous small unisexual flowers, which are sessile and covered with boat shaped bract known as Spathe, i.e. banana, arum, palms and coconut are the example of compound spadix.
4. Catkin: A spike with unisexual sessile flowers on long peduncle as in mulberry and oak.
5. Umbel: Axis is shortened and bears flowers at its top which are having equal stalk and arranged in centripetal succession. A whorl of bracts is present at the base of inflorescence as in coriander, caraway, cumin, fennel, etc.
Fig. 4.43 Types of inflorescence
6. Spikelet: It is present in family Graminae characterized by small and branched spikes. Spikelets are provided with two bracts at the base known as glumes, and bracteole called palea.
7. Corymb: Peduncle is short, flowers bracteate, bisexual oldest flower is lower most and youngest at apex. Lowermost has longest stack and youngest has shortest, lying at same level.
8. Capitulum or Head: In this type flattened and expanded peduncle is present, called as receptacle. Base of receptacle is covered with bracts. The flowers are small and sessile (florets). Flowers towards the periphery are older, while at the centre, they are younger and open later. Two types of flowers are present, i.e. ray florets (strap shaped) and disc florets (tubular shaped), e.g. zinnia, cosmos, sunflower.
9. Capitate: Inflorescence similar to umbel type, except the flowers are sessile, i.e. acacia.
(B) Cymose inflorescence: In this type the growth of the main axis or peduncle is stopped by producing the flower. The order of opening is centrifugal. Its types are as under:
1. Solitary cyme: In this type the inflorescence ends in a single flower as in datura, capsicum, China rose, etc.
2. Uniparous or monochasial cyme: In this type, axis ends in a flower only; one branch arises just behind and ends in a flower. Uniparous depending upon the type of branching is again subdivided into (a) Hellicoid uniparous and (b) Scorpioid uniparous.
Hellicoid uniparous is characterized by branching on one side only, while scorpoid uniparous cyme by branching on alternate side.
Fig. 4.44 (a) Uniparous helicoids cyme, (b) Uniparous scorpoid cyme (c) Biparous cyme
3. Biparous or Dichasial cyme: This type of inflorescence is characterized by the end of main axis in a flower, which is followed by two lateral branches ending in flowers again. Actually this is a true cyme as in case of ixora, teak, jasmine, etc.
4. Multiparous or Polychasial: The main axis ends in a flower and numbers of flowers are produced laterally in the same manner, i.e. in nerium, calotropis, etc.
5. Special type: In this type hypanthodium (like peepal and fig), verticillasters (sacred basil, mentha, coleus blumi) cymose-umbel (onion) are included. Each of them has its special characters not covered in any type described above.
Morphology of Seeds
The seed is a fertilized ovule and is a characteristic of Phanerogams. Parenchymatous body of the ovule known as nucellus contains embryo-sac in which fertilization of pollen cells takes place giving rise to embryo. The seeds are characterized by the presence of three parts known as embryo, endosperm and seed coat.
Seed Coat
It is the outermost layer of the seeds providing necessary protection to the embryo lying inside the seed. In case of dicotyledonous seeds normally, it is hard and may contain two layers; the outermost thick layer is known as testa while the inner one which is thin is known as tegumen. In monocotyledonous seeds, it is thin or even may be fused with the wall of the fruit.
Embryo
It is the main part of the seed. It consists of an axis having apical meristem for plumule, radicle the origin or root and adhered to it are one or two cotyledons, differentiating the plants as monocot or dicot.
Fig. 4.45 (a) Castor seed (b) L.S. of Castor seed
Endosperm
It is the nutritive tissue nourishing the embryo. It may be present or may not be present in the seed. Depending upon the presence or absence the seeds are classified as under:
1. Endospermic or albuminous seeds.
2. Nonendospermic or exalbuminous seeds.
3. Perispermic seeds.
1. Endospermic or albuminous seeds: In this seed, the part of the endosperm remains even up to the germination of seed and is partly absorbed by embryo. Therefore, seeds are known as endospermic seeds as in colchicum, isabgol, linseed, nux vomica, strophamthus, wheat and rice.
2. Nonendospermic or exalbuminous seeds: During the development of these seeds, the endosperm is fully absorbed by embryo and endosperm, and is not represented in the seeds; hence, they are known as nonendospermic, e.g. sunflower, tamarind, cotton and soyabean.
3. Perispermic seeds: Herein the nucleus develops to such an extent that it forms a big storage tissue and seeds are found to contain embryo, endosperm, perisperm and seed coat; e.g. pepper, cardamom, nutmeg, guinea grains.
Seeds are characterized by the following descriptive terms:
(a) Hilum: This is the point of attachment of seed to its stalk.
(b) Micropyle: It is the minute opening of the tubular structure, wherefrom water is provided for the germination of seeds.
(c) Raphe: Raphe is described as longitudinal marking of adherant stalk of anatropous ovule.
(d) Funicle: It is the stalk of the ovule attaching it to the placenta.
(e) Chalza: This is the basal portion of ovule where stalk is attached.
Special Features of Seeds
Sometimes, apart from the regular growth of seeds, additional growth is visible in the form of appendages which attribute to their special features. They are described as:
(i) Aril: Succulent growth from hilum covering the entire seeds as in nutmeg (mace) and yew seeds.
(ii) Arillode: Outgrowth originating from micropyle and covering the seeds as in cardamom.
(iii) Arista (awn): Stiff bristle-like appendage with many flowering glumes of grasses and found in strophanthus.
(iv) Caruncle: A warty outgrowth from micropyle, i.e. castor, croton, viola moringa.
(v) Hairs: Gossypium and calotropis are examples of this type of outgrowth.
Fig. 4.46 Special features of seeds
Morphology of Fruits
Phanerogams are said to be matured when they reach the flowering stage. The ovules of the flowers after fertilization get converted into seeds, whereas the ovary wall develops further to form the protective covering over the seed, which is known as fruit. In botany, this particular coating is also called pericarp.
Pericarp consists of three different layers, one after the other as:
It is not necessary that the fruits should have seeds. If the ovules do not fertilize, the seedless fruits are formed. Depending upon the number of carpels present in the flowers, and other structures, the fruits fall into (1) simple fruits, (2) aggregate fruits and (3) compound fruits.
Simple Fruits
Formed from the single carpel or from syncarpous gynoecium. Once again depending upon the mesocarp, whether it is dry or fleshy, they are classified as dry fruits and fleshy fruits. Dry fruits are further sub-classified into dehiscent and indehiscent fruits.
Compound Fruits
In this particular case many more flowers come together and form the fruits, e.g. figs, pineapple.
False Fruits
Sometimes it so happens that apart from the ovary and the other floral parts like thalamus, receptacle or calyx grow and form the part of the fruit, known as false fruit or pseudocarp. Following are the few examples of pseudocarp in which other parts of the flower forming important part of the fruits are shown in the bracket. Strawberry (thalamus), cashew nut (peduncle and thalamus), apple (thalamus), marking nut (peduncle) and rose (thalamus)
I. Dehiscent capsular fruits:
1. Legume or pod: It is a dry monocarpellary fruit developing from superior ovary, dehiscing by both the margins, i.e. senna, tamarind, pea.
2. Capsule: It is a dry one to many-chambered fruit, developing from superior or poly carpellary ovary dehiscing in various forms, i.e. cardamon, cotton, datura, lobelia, colchicum, digitalis, poppy.
3. Follicle: Similar to legumes and dehisces at one margin only, i.e. rauwolfia, anise, calotropis.
4. Siliqua: A dry, two-chambered fruit, developing from bicarpellary ovary, multiseeded. It dehisces from base upwards as in radish mustard, etc.
II. Indehiscest fruits:
1. Achene: A dry, one-chambered, one-seeded fruit developed from superior monocarpellary ovary. Pericarp is free of seed coat, i.e. clemantis, rose.
2. Caryopsis or grain: Small, dry, one-seeded fruits, developing from simple pistil, pericarp fused with seed coat as in maize, rice, bamboo.
3. Nut: Dry, one-seeded fruits developing from superior ovary, pericarp hard and woody, i.e. areca nut, marking nut, cashew nut.
4. Samara: Dry, one- or two-seeded, winged fruit from superior bi- or tricarpellary ovary, i.e. dioscorea, shorea, etc.
5. Schizocarp: These are further divided into two subclasses.
(i) Lomentum: In this type of pod of legume is partitioned into one-seeded compartments as observed in acacia, ground nut, cassia fistula.
(ii) Cremocarp: Dry, two-chambered fruit, developing from an inferior bicarpellary ovary. Splitting into two, indehiscent one-seeded pieces are called mericarps, i.e. coriander, cumin, fennel, dill, etc.
Fleshy fruits:
1. Drupe (Stone fruit): A fleshy one or more seeded fruit, with pericarp well differentiated into epicarp, fleshy mesocarp and hard endocarp as in mango, olive, coconut, etc.
2. Berry: A fleshy, many-seeded fruit developed from superior, single carpel, i.e. tomato, guava, grapes, banana.
3. Pepo: Pulpy, many-seeded fruit developing from one- or three celled inferior ovary, i.e. cucumber gourd, colocynth, water melon.
4. Pome: Fleshy, one- or more-celled syncarpous fruit. Fleshy, part is thalamus, while actual fruit lies inside, e.g. apple, pear.
Uses of fruits
1. Apart from the main source of food grains, i.e. wheat, jowar, fruits are also used for their high sugar value, minerals and vitamins.
2. Fleshy fruits like, papaya, mango, apple are used commercially as source of pectin.
3. Bayberry wax and olive oil are obtained industrially from fruits only.
4. Several fruits like chilies, black pepper caraway and cumin are used on large scale for the preparation of spices.