Classification Of Plant

A. Bryophyta.

(1) Introduction : Bryophyta (Gk : Bryon = moss ; phyton = plants) includes the simplest and primitive land plants. De Jussieu (1789) placed mosses etc. under acotyledons along with algae and fungi. De Candolle (1813) placed liverworts and mosses in the class Aetheogamous of the division Cellulare. Robert Broun include algae, fungi, lichen and mosses under bryophyta. It occupies a position intermediate between algae and pteridophyta. Due to peculiar type of their habitats, they are regarded as 'the amphibians of the plant kingdom'.

(2) Habitat : Bryophytes usually grow in moist and shady places. The plants grow densely together and form green carpets or mats on damp soil, rock, walls, barks of trees and on decaying logs in forests, especially during the rainy season.

(3) Specialized habitats : Some bryophytes grow in diverse habitats such as – aquatic (e.g., Riccia fluitans, Ricciocarpus natans, Riella), epiphytes (e.g., Dendroceros, Radula protensa and many mosses), saprophytes (e.g.,Buxbaumia aphylla, Cryptothallus mirabilis), and in dry habitats such as dry heaths (e.g., Polytrichum juniperinum), deserts (e.g., Tortula desertorum) and dry rocks (e.g., Porella platyphylla).

(4) Gametophytic plant body

(i) The life cycle of bryophytes consists of two distinct phases – the gametophytic phase and the sporophytic phase. The haploid gametophyte is dominant, long lived, green and independent whereas the diploid sporophyte is short lived and dependent upon the gametophyte. The two phases come one after the other in alternating manner and both are morphologically distinct.

(ii) The plants are small, range from few millimetres (e.g., Zoopsis) to 30–40 centimetres. The tallest species may reach upto 70 cm in length (e.g., Dawsonia).

(iii) The gametophytes are either thalloid (i.e., not differentiated into true roots, true stem and true leaves) or leafy shoot having stem-like central axis and leaf-like appendages.

(iv) The roots are completely absent and they are replaced by unicellular or multicellular thread like rhizoids. In some higher forms the multicellular rhizoids form cords.

(v) The vascular tissue (i.e., xylem and phloem) are completely absent. In few mosses (Polytricum) the xylem like hydroids, which conduct water and phloem like leptoids, which conduct the assimilates, have been reported.

(5) Apical growth : The apical growth in bryophytes take place by a single apical cell or a group of meristematic cells arranged in a transverse row. In Riccia, Marchantia and many jungermanniales the apical growth takes place by a transverse row of apical cell. In mosses, it occurs by single pyramidate apical cell. In Anthoceros, on the other hand, there may be a single apical cell or a transverse row of such meristematic cells.

(6) Reproduction : The bryophytes reproduce vegetatively, asexually and sexually. Various methods involve in reproduction are discussed in the following account.

(i) Vegetative reproduction : The bryophytes reproduce vegetatively by following methods :

(a) Death and Decay : Most of these plants reproduce vegetatively by gradual death and decay of the older part of the plant body.

(b) Adventitious branches : Many plants like Riccia fluitans, Reboulia, Asterella, Pellia etc. reproduce by adventitious branches. They separate and produce new plants.

(c) Tubers : Several species of Riccia, Anthoceros, Sewardiella, Asterella etc. produce tubers which give rise to new plants on the arrival of favourable conditions.

(d) Gemmae : Several members, reproduce vegetatively by forming multicelled gemmae. In Marchantia, Lunularia, the gemmae are produced in gemma cups. In some liverworts, 1–3 celled gemmae are prodcued on the axis or on the 'leaves' or on thalli. Gemmae are also produced on the thallus of Anthoceros. Several mosses also produce gemmae on the 'leaves' (Bryum), or axis or rhizoids or on the protonema (Funaria).

(e) Leafy propagules : Some liverworts also reproduce vegetatively by forming leafy propagules.

(f) Primary protonema : The mosses generally reproduce vegetatively by breaking of the primary protonema. New gametophores now arise from the buds differentiated on it.

(g) Secondary protonema : In several mossess a secondary protonema may arise from the rhizoids or primary protonema or even from the injured sporophyte. It may produce buds which give rise to new gametophores.

(h) Rhizoids : Mosses may also reproduce vegetatively from the rhizoids e.g., Leucobryum.

(ii) Sexual reproduction : The male sex organs is called as antheridium and the female as archegonium. The antheridial stalk is very distinct whereas the archegonial stalk is generally short. They may be of embedded type e.g., Riccia, Anthoceros or of projecting type e.g., Marchantia, mosses.

(a) Antheridia : They are generally borne on the dorsal surface of the thallus. While in Riccia the antheridial chambers are open, in Anthoceros they are closed. The antheridia lie embedded within the thallus in both the plants. In Jungermanniales the antheridia are borne in one or more rows. In Marchantiaceae they are present on a special branches, the antheridiophores or the male receptacles which may be stalked or sessile. While in most of the members the antheridia are superficial in origin, in Anthoceros they are endogenous. Each antheridium is distinguishable into a stalk and the body. The antheridial body consists of a mass of androgonial cells covered by a 1-cell thick sterile jacket. The terminal cell of the jacket, when distinct, is called as operculum. Each androgonial cell finally behaves as androcyte mother cell. The androcyte mother cell then forms two androcytes (antherozoid mother cell), each of which is metamorphosed into a biflagellate antherozoid.

(b) Archegonia : These are also borne on the dorsal surface of the thallus. In Riccia and Anthoceros they lie embedded in the thallus. In many members of Marchantiaceae they are borne on special branches called archegoniophores or the female receptacles, that may be stalked or sessile. The archegoniophore or carpocephalum has rows of archegonia protected by involucre or perichaetium. The archegonia are flask shaped structures distinguishable into a long neck and a globular, swollen venter. A multicelled stalk is also present in mosses but in others it is very short. The neck is one-cell thick. It is generally made up of six vertical rows of cells but in Jungermamnniales it is compose of 4 or 5 vertical rows only. The neck is capped by four cover cells and contain varying number of neck canal cells inside. While in Riccia there are only 4 neck canal cells, the mosses however, possess more than six of them. The venter is also 1-cell thick in most of the plant but in Jungermanniales it is 2–3 layered. In mosses it is double layered. The venter contains an egg and a ventral canal cell.

(7) Importance of water in bryophytes : The bryophytes are fundamentally terrestrial plants but require presence of water to complete their life cycle. The water is needed for dehiscence of antheridia, liberation of antherozoids, transfer of antherozoids from antheridia to archegonia, opening of archegonial neck, and the movement of antherozoids into the archegonial neck.

(8) Fertilization : Before fertilization the walls of androgonial cells disorganise to form a mucilagenous mass. The opercular cell is removed and the antherozoids are liberated. The neck canal cells and the ventral canal cell also disorganise. The cover cells split apart giving a free passage to incoming antherozoids. The antherozoids are attracted towards the egg by chemotactic stimulus, which in bryophytes, is provided in the form of sugars. Antherozoids enter in to archegonia and fertilized the egg.

(9) Sporophyte

(i) The diploid fertilized egg (zygote) is the first cell of sporophytic generation. It divides and develops into a sporophytic plant body, called sporogonium.

(ii) The wall of venter forms calyptra, which provides a protective covering to the developing sporogonium.

(iii) The sporogonium, in most of the cases, is differentiated into foot, seta and capsule.

(iv) The sporogonium is completely dependent on the gametophyte for water and mineral supply and, in most of the cases, partly or wholly for organic nutrition. The sporogonium remains attached to the gametophytic plant body throughout its life.

(v) The sporogonium is mainly concerned with the production of asexually formed haploid spores (or meiospores). The spores are produced inside the capsule of sporogonia as a result of meiosis in the spore mother cells.

(vi) The spores are the first cells of gametophytic generation. They germinate to produce the gametophytic plant body either directly or through a juvenile filamentous stage, called protonema.

(10) Alternation of generation : Bryophytes exhibit a distinct and heteromorophic alternation of generations in which two phase gametophytic and sporophytic follow each other in regular sequence. The sporophytic plant body (2 N) of bryophytes is dependent on the gametophyte (N).

(11) Classification : Eichler (1883), Engler (1892), Bower (1935) divided Bryophyta into two classes – Hepaticae and Musci. On the other hand, Campbell (1940), Smith (1955), Takhtajan (1953) divided into three classes namely Hepaticae, Anthocerotae and Musci. Proskauer (1957) changed the names of these classes in accordance with the recommendations of the code, into Hepaticopsida, Anthocerotopsida and Bryopsida.

(12) Salient features of classes

(i) Hepaticopsida : The latin word Hepatica means liver. Thus the members of hepticopsida are popularly known as liverworts. The important characters of hepaticopsida are :

(a) The gametophytic plant body is small, dorsiventral, thallose or leaf axis (foliose).

(b) Chlorophyllous cells contain many chloroplasts and one to several oil bodies.

(d) Rhizoids are unicellular.

(e) Sex organs develop from single superficial cells.

(f) Sporogonium has little or no chlorophyllous tissue and stomata.

(g) The capsule is not linear. It lacks columella and intercalary meristem.

(h) Capsule dehisces by drying of capsule-wall, usually by more than two valves.

(ii) Anthocerotopsida : This class is characterised by the following characters –

(a) Gametophyte is thalloid. Thalli are lobed, dorsiventral, internally homogenous without any differentiation of tissues.

(b) Air chambers and air pores are absent but mucilage cavities may be present.

(d) Scales are absent.

(e) Each cell possesses single (some times more) large chloroplast with central pyrenoid.

(f) Oil bodies are absent.

(g) Antheridia are endogenous in origin, borne singly or in groups inside the closed cavities.

(h) Sporogonium is differentiated into foot, meristematic zone and capsule (the seta is absent).

(i) Capsule has central sterile columella.

(j) The capsule dehisces basipetally by two valves and shows hygroscopic twisting.

(iii) Bryopsida : The members of bryopsida are commonly known as mosses. The class is characterised by the following characters –

(a) Gametophyte is differentiated into two stages – prostrate protonema and erect radial leafy shoot.

(b) Leaf-like appandages are spirally arranged on stem – like axis.

(d) Sex organs develop from superficial cells.

(e) Sporogonium is differentiated into foot, seta and capsule.

(f) Wall of capsule is several layered with stomata on epidermis.

(g) The capsule has central columella.

(h) Elaters are absent.

B. Pteridophyta.

(1) Introduction : The pteridophytes (Gk. Pteron = feather and phyton = plants ; means plants with feather like fronds or ferns). They are flowerless, seedless, spore producing vascular plant which have successfully invaded the land. Pteridophytes represent an intermediate position between bryophytes and spermatophytes (Gymnosperm and Angiosperm). It is classified by Carolus Linnaeus (1754) under the class cryptogamia. They are also called vascular cryptogams. The group has a long fossil history. It is the most primitive group that flourished in Devonian and Carboniferous periods of Palaeozoic Era.

(2) Habitat : The plants of pteridophytes are mostly terrestrial. They prefer shady habitats. Some species of Selaginella and Adiantum are xerophytes. A fern, Acrostichum aureum is a halophyte. Some species e.g., Selaginella oregana, Psilotum flacidum, Lycopodium squarrosum and ferns like Asplenium nidus, Pleopeltis sp. are epiphytes. Marsilea occurs as a terrestrial, amphibious as well as an aquatic plant. There are true aquatics ferns likeSalvinia, Azolla and Ceratopteris.

(3) Sporophytic plant body

(i) The main independent plant body of pteridophytes is sporophyte. It is differentiated into true roots, true stem and true leaves.

(ii) The primary root is short lived. It is replaced by adventitious roots. The root has a permanent growing apex.

(iii) The stems are usually herbaceous (except in some woody ferns) and branched monopodially or dichotomously.

(iv) The leaves may be small microphyllous (e.g., Lycopodium, Equisetum) or large macrophyllous (e.g., Pteridium, Pteris and other ferns).

(v) All the vegetative parts possess vascular tissues (i.e., xylem and phloem) organized in definite groups or steles. Secondary growth does not occur in most of the living pteridophytes (except in Isoetes).

(4) Apical growth : The pteridophyte generally possess a single apical cell with three cutting faces in the shoot apex. The root tip also has a single apical cell but with four cutting faces.

(5) Spore producing organs : The plants of pteridophytes are sporophytes. They reproduce asexually by forming spores in sporangia. They are homosporous but a few plants are heterosporous also e.g., Isoetes, Selaginella, Marsilea, Regnellidium, Pilularia, Azolla and Salvinia. In Selaginella the sporangia are borne in relation to sporophylls which constitute a strobilus. In Equisetum they are borne on sporangiophores which constitute a cone. In ferns the sporangia are borne in sori on the sporophylls. The sori are of three types –

(i) Simple sorus : Here all the sporangia mature at the same time.

(ii) Gradate sorus : Here the oldest sporangium lies in the centre and the sporangia on either side show successively younger stages.

(iii) Mixed sorus : It shows mixed arrangement of younger and older sporangia.

In Marsilea, Azolla, Salvinia etc. the sori are present in a box like structure called sporocarp. The sorus may be naked or covered by an inducium. The inducia may be true or false. A true inducium is a specially developed structure whereas a false inducium is formed by curving of the sporophyll margin.

(6) Sporangium : The sporangia are generally stalked structures. Each sporangium is distinguishable into a jacket enclosing a mass of sporogenous tissue. The sporangial jacket may be 2-4 layered. The innermost wall layer is the tapetum. No tapetum is formed in Psilotum and Tmesipteris. It is a nutritive layer which degenerates at maturity of the sporangium. The sporangial jacket in some ferns shows distinctions of annulus and stomium. On the basis of development the sporangia have been classified by Goebel, 1881 into two categories as under :

(i) Eusporangiate type : Such a sporangium develops from a group of superficial initials. They divide periclinally into outer and inner components. The outer cells form the wall whereas the inner cells give rise to sporogenous tissue.

(ii) Leptosporangiate type : Such a sporangium arises from a single superficial initial. It divides periclinally into outer and inner components. While the inner cell forms the stalk, the outer gives rise to sporagnium proper. In Marattia alata, the sporangia in a sorus may fuse to form a synangium.

(7) Spore : The plants may be homosporous, i.e., produce only one type of spores (e.g., Lycopodium, Pteridium) or heterosporous i.e., produce two different types of spores, smaller microspores and larger – megaspores (e.g., Selaginella, Marsilea etc.). The spore germination is homosporous pteridophytes may be bipolar (e.g., Lycopodium, Equisetum) or tripolar (e.g., Hymenophyllum) or amorphous (e.g., Angiopteris).

The spores germinate to produce haploid gametophyte, called prothallus. The homosporous pteridophytes produce bisexual (monoecious) gametophytes whereas heterosporous one produce unisexual (dioecious) gametophytes.

(8) Sex organs : The archegonia and antheridia are generally of embedded type. The archegonium consists of neck which usually projects from the surface of the prothallus. It contains 1-2 neck canal cells. There is no venter. The egg and the ventral canal cell remain surrounded by the cells of prothallus. The antheridia are generally sessile. They have a 1-cell thick jacket enclosing a mass of androgonial cells. They form the androcytes which are metamorphosed into biflagellate (Lycopodium, Selaginella) or multiflagellate (Equisetum,ferns) antherozoids.

C. Gymnosperm.

(1) Introduction : Gymnosperm (Gk. Gymnos = naked ; sperma = seed) are the plants with exposed or naked seeds or ovules. These plants represent the most ancient group of seed plants. They have been generally placed in the division spermatophyta (seed bearing plants) along with angiosperms. They were not grouped separately as gymnosperms. But Robert Brown (1827) separated them from angiosperms and placed under a distinct group due to presence of unprotected ovules in them. The gymnosperm originated much earlier then angiosperms. However, most of the members of this group have now become extinct and only few living forms are known today. The living gymnosperm are generally grouped under four orders (Cycadales, Ginkgoales, Coniferales and Gnetales).

(2) Distribution : Plants of Gymnosperms occur throughout the world. The group is presently represented by only 900 living species. Of these, about 500 species belong to 'Conifers' or cone bearing plants. Several species of conifer occur in north-west America and eastern and central China. In India several members are found in Himalayas, Podocarpus and Cupressus in the central and Larix, Tsuga, Cephalotaxus in the eastern. The Indian species of Ephedra are commonly found in Punjab, Rajasthan, Haryana and from Sikkim to Laddak. Gnetum sp. occur in Kerala, Assam, Naga hills, Orissa, Sikkim, Burma, Thailand and China. Welwitschia is endemic to south-west Africa. Ginkgo is native of South China.

(3) Habit : Living gymnosperms are mostly perennials, xerophytic, evergreen, arboreal and woody plants. They grow as wood trees, bushy shrubs or rarely as climbers (e.g., Gnetales). None of them are herbs or annuals.

(4) External features

(i) The plant body is sporophyte and differentiated into root, stem and leaves.

(ii) The plant possess well developed tap root system. In some cases the roots are symbiotically associated with algae (e.g., Coralloid roots ofCycas) or with fungi (e.g., Mycorrhizal roots of Pinus).

(iii) The stem is erect, aerial, solid, woody and branched (unbranched in Cycadales) but almost tuberous in Zamia.

(iv) The leaves may be microphyllous or megaphyllous.

(5) Gymnospermous wood

(i) Manoxylic wood : Cambial activity is short lived, cortex and pith are broad, parenchymatous rays are broad, wood is soft and commercially useless. e.g., Cycas.

(ii) Pycnoxylic wood : Cambial activity is long lived, cortex and pith are reduced, parenchymatous rays are few, wood is hard and compact, wood is commercially most important and used as good quality timber. e.g., Pinus.

(6) Reproduction : Gymnosperms are heterosporous, i.e., produce two different kinds of spores – the male microspores and the female megaspores. The spores are borne inside the sporangia. The two types of sporangia are borne on special leaf-like structures, called sporophylls. The microsporangia (pollen sacs) are born on microsporophylls (= stamens) and the megasporangia (ovules) are borne on megasporophylls (= carpels).

The sporophylls are usually aggregated in the form of compact structures called cones or strobili. The cones are generally unisexual, i.e., the male cones are microsporangiate (pollen cones) and the female cones are megasporangiate (seed cones). The male cones are short lived whereas the female cones are long lived. The female cones remain attached on the plants for several years till the maturity or ripening of the seeds.

(7) Pollination : The microsporangium (Pollen sac) produces numerous light pollen grain. Pollination is anemophilous (wind pollination). The ovules are orthotropous and remain exposed on the megasporophyll. Each ovule surrounded by integuments. It incloses the nucellus and a female gametophyte formed from the haploid megaspore. The female gametophyte contains archegonia. The pollen grains are captured by the pollination drop secreted by the micropyle of the ovule. When it dries, the grains are sucked in the pollen chamber. The pollen grains now germinate. A pollen tube is formed due to elongation of the tube cell. In Cycas and Ginkgo the pollen tube is haustorial in nature. The lower end of the tube bursts and releases the male gametes which fuse with the egg to form the zygote.

(8) Fertilization : Fertilization occurs by siphonogamy, i.e. the male gametes are carried to the archegonia through pollen tube (except in Cycas where pollen tube functions as haustorium and fertilization occurs by zoodiogamy). Fertilization thus takes place in the absence of external water.

(9) Embryogeny : The zygote undergoes free-nuclear divisions in Cycas followed by wall formation. There are no free-nuclear divisions in Sequoia and Gnetum. The embryo is soon differentiated into an upper haustorial, middle suspensor and lower embryonal regions. In Pinus, on the other hand, the zygote gets differentiated into four tiers of four cells each, designated as open tier, rosette tier, suspensor tier and embryonal tier. Cleavage polyembryony is seen in Pinus. The embryonal part shows differentiation of radicle, hypocotyl, cotyledons and plumule.

(10) Seed : As a result of fertilization the ovule develops into a seed. The integument forms the seed coat. The outer fleshy layer of the integument forms the testa whereas the middle stony layer gives rise to tegmen. The nucellus persists as a cap like perisperm. In Taxus a fleshy aril is also present at the base as a cup like structure. The seeds of gymnosperms comprise tissue of three generations namely parent sporophytic (integument and nucellus), gametophytic (endosperm) and second sporophytic (embryo).

(11) Living fossils : When a group of plants is represented by a single genus or species while rest of the other representatives of the group have become extinct and fossilized the long surviving individual is called a living fossil e.g.,Ginkgo biloba. However,Cycas is also regarded as a living fossil because most of the cycad species are confined to tropical and subtropical region and the group is becoming endangered. Therefore, cycads have been referred as reptiles of plant kingdom or panda of vegetable kingdom.

(12) Classification : Robert Brown (1827) recognised the gymnosperms as a group distinct from Angiosperms. However, Bentham and Hooker (1862-83) in their 'Genera Plantarum' placed them between Dicotyledonae and Monocotyledonae, Chamberlain (1934) divided gymnosperms into following two sub-classes.

(i) Cycadophyta

(ii) Coniferophyta

(i) Sub-class I. Cycadophyta : These are characterised by the presence of unbranched stem and large foliage leaves. Internally, the stem has large pith and cortex but the wood is relatively small. It includes following 3 orders.

Order 1. Cycadofilicales : It is a group of fossil plants. These plants resembled with ferns, hence they were given the name Pteridospermae (i.e., seed bearing ferns). e.g., Lyginopteris, Medullosa.

Order 2. Bennettitales or Cycadeoidales : It is also a group of fossil forms. These plants resembled with modern cycads. e.g., Cycadeoidea, Williamsonia.

Order 3. Cycadales : It includes both living and fossil forms. e.g., Cycas, Nilssonia, Zamia.

(ii) Sub-class II. Coniferophyta : These are characterised by long profusely branched stem and simple small leaves. In stem the amount of wood is much more than cortex and pith. It includes following four orders.

Order 1. Cordaitales : All the members of this order are extinct. e.g. Cordiates, Dadoxylon.

Order 2. Ginkgoales : All the members of this order, except for Ginkgo biloba are extinct. Ginkgo biloba is a medium sized tree with branched stem and bilobed leaves. Because of the resemblance of the leaves of this plant with those of Adiantum (maiden hair fern), the name Maiden hair tree has been given.

Order 3. Coniferals : The order includes both fossils and present day forms. e.g., Pinus, Cedrus, Sequoia.

Order 4. Gnetales : Gnetales are modern group consisting of living forms. The order differs from other gymnosperms in the presence of vessels in the xylem. e.g., Ephedra, Gnetum, Welwitschia

D. Angiosperms.

(1) Introduction : The angiosperms, or flowering plants, constitute the most dominant and ubiquitous vascular plants of present day flora which changed the green and yellow melancholy of the earth's vegetation by the colourful brightness and fragrance of their flower. The term angiosperm means 'enclosed seed' because the ovules or potential seeds are enclosed within a hollow ovary. In this respect they are considered most highly evolved and advanced as compared with the naked seeded gymnosperms.

(2) Characteristic features

(i) Angiospermous plants grow in almost every kind of habitats. In the deserts, these plants grow, flower, shed seeds and complete their life cycle in a few weeks of rainy season. Some flowering plants like Zostera, occur in shallow seas. A small orchid even lives underground. It survives as a saprophyte on decaying organic matter because of the mycorrhizal association which helps to obtain nourishment. In rain forests, some plants grow on the branches of other plants but do not obtain water or food from them. They are called epiphytes (e.g., Vanda).

(ii) The angiospermous leaves show reticulate or parallel venation forming areoles. The libriform fibres are present in the xylem and the companion cells are present in the phloem. The true vessels are present in the xylem of angiosperms.

(iii) The angiosperms produce flowers which normally consist of 4 whorls of appandages – the two outer accessory and reproductive structure such as sepals and petals and the two inner essential parts – stamens and carpels.

(iv) The stamens (microsporophylls) are bilaterally symmetrical. Each stamen consists of a filament and an anther.

(v) The anthers produce tectate pollen grains with exine differentiated into rod-like columellae covered by a tectum.

(vi) In angiosperms, the insects and animals also act as pollinating agents. For this purpose the flowers possess bright and showy petals, edible pollen and nectar.

(vii) The carpels (= megasporophylls) are rolled and partly sterile so that they enclose the ovules within a hollow ovary that is connected with the stigma and style.

(viii) The female gametophyte is highly reduced and consists of single egg cell, two synergids, three antipodals and two polar nuclei. The archegonia are absent.

(ix) The most characteristic feature of angiosperms is double fertilization.

(a) The male gamete fuses with the egg producing diploid zygote that develops into embryo or new sporophyte.

(b) Another male gamete fuses with the polar nuclei (triple fusion) resulting in the formation of triploid endosperm.

(x) After fertilization, the ovules ripens into seeds and ovary ripens into fruits.

(3) Size

(i) The smallest angiosperm is Wolffia. The plant body of Wolffia consists of tiny flat oval green stem (phylloclade) having a few small roots. The plants are about 1 mm in diameter and found free floating in aquatic habitats like ponds, etc.

(ii) The tallest angiosperm is Eucalyptus. Their trees may attain a height upto 100 meters or more.

(iii) Banyan (Ficus bengalensis) tree covers a large area. It's slanting aerial branches spread in all directions. The tree spreads with the help of prop or pillar roots.

(4) Longevity : Based on the duration of life, the plants are divided into following 4 categories :

(i) Ephemerals : This category includes the plants which live only for a few weeks because of a very short growing season. Such plants are found near deserts or in very cold countries. For example, Arabidopsis species have a life span of 20–28 days.

(ii) Annuals : The plants of this category live and complete their life-cycle in a single favourable season. During this period, they grow in size, produce flowers, shed their seeds, undergo senescence and die. They pass the unfavourable period in the form of seeds. Many crop plants (e.g., wheat, rice, maize, etc.) are annuals. The smallest angiosperm – Wolffia is an aquatic annual.

(iii) Biennials : The plants of this category complete their life-cycle in two favourable seasons (i.e., in two years). They grow vegetatively in the first season and produce flowers and set seeds in the next. Often they produce some storage organs, as in the sugar beet, where food is stored in their swollen roots.

(iv) Perennials : Plants of this category live for more than two years. Generally they live for many years and bear the flowers and fruits during specific seasons. Some perennials continue their vegetative growth for several years and produce fruits and seeds only once in their life time, e.g., Agave, Bamboos, etc. They are called monocarpic. Others produce flowers and fruits every year after attaining a definite stage of maturity, e.g., Mango, Lemon, Apple, etc. Such plants are called polycarpic.

(5) Habit : Depending upon the habit of plants, the angiosperms belong to following categories –

(i) Herb : These are small, soft, non-woody plants without persistent parts aboveground. The height of plants usually reaches upto 1 m. The plants may be annual (Brassica), biennial (Sugar beet) or perennial (Canna). The perennial herbs usually possess underground rhizomes which form the new aerial shoots every year. The plants of banana are perennial herbs.

(ii) Shrubs : These are woody plants of relatively low height (1-4 m). They typically branch at or near the base and do not have a main trunk, e.g., Rose. They are mostly perennial.

(iii) Trees : These are perennial woody plants with one main trunk. The trunk may or may not be branched. These are of the following types :

(a) Caudex : The stem is unbranched and usually bears a crown of leaves at the apex. e.g., Date-palm.

(b) Excurrent : The lower part of stem is thicker which gradually tapers above. Branches arise from the main stem in acropetal succession and plant appears conical e.g., Pinus.

(c) Deliquescent : The apical bud of the main stem dies after some time and branches and sub-branches spread in different directions. e.g., Tamarindus, Ficus.

(iv) Culms : In these plants, nodes and internodes are extremely clear. Internodes of such plants are usually hollow. These plants are grasses but cannot be considered as herb or shrub or tree. e.g., Bambusa (Bans).

(6) Habitat : Warming (1895) divided the plants, on the basis of their adaptation to water, into four major groups – hydrophytes, mesophytes, xerophytes and halophytes. A fifth group epiphytes can also be included.

(i) Hydrophytes : The plants which grow in aquatic habitats are called hydrophytes. They are further grouped as –

(a) Submerged (e.g., Hydrilla)

(b) Attached floating (e.g., Nymphaea)

(d) Amphibious or partly emerged hydrophytes (e.g., Sagittaria).

(ii) Mesophytes : These are the plants which grow under moderate moisture and temperature conditions. They have no special adaptations to grow either in very dry or in very wet conditions (e.g., Sun flower, Brassica). These plants do not possess special adaptations to reduce transpiration.

(iii) Xerophytes : The plants which grow in dry or xeric habitats (i.e., under deficient supply to available water) are called xerophytes. These plants face acute shortage of water and therefore, develop morphological, structural and physiological adaptations in order to survive under such habitats. The adaptations in plants are mainly to check the transpiration and survive under acute shortage of water. e.g., Cynodon (Doob grass), Casuarina, Euphorbia tirucalli, Asparagus, etc.

(iv) Halophytes : Halophytes are those plants which grow in saline habitats, i.e., in salt marshes, alkaline soils, river estuaries, saline ponds near seashore or sandy and heavy soils having excess of salts. In such habitats, the water is present in sufficient amount but due to high osmotic concentrations it is physiologically not available to normal plants. Such conditions are said to be physiologically dry. e.g., Spartina, Atriplex, Portulaca etc.

(v) Epiphytes : These are the plants which grow on other plants for space only. The plants are autotrophic and occur both in aquatic and terrestrial habitats. e.g., Vanda (an orchid).

(7) Modes of nutrition : On the basis of modes of nutrition plants are classified as follows –

(i) Autotrophs : These plants manufacture their organic matter from inorganic matter.

(a) Photoutotrophs : These are green coloured due to the presence of chlorophyll. In the presence of light they are capable of synthesizing their food from CO₂ and H₂O. e.g., Mango, Mustard etc.

(b) Chemoautotrophs : Those plants which synthesize their food from CO₂ and H₂O by using energy product in the chemical reactions. e.g., Many bacteria.

(ii) Heterotrophs : They are either unable to photosynthesize their food or are unable to take their water and minerals directly from the soil or unable to synthesize protein. They are classified as follows :

(a) Parasites

(b) Saprophytes

(d) Insectivorous plants

A detailed discussion of these group is given in chapter "Plant nutrition".

(8) Reproduction : (See in embryology).

(9) Classification : The plants of Angiosperms divided into two major groups as – Dicotyledons and Monocotyledons.

(i) Dicotyledons : They are show following distinguished characteristics.

(a) Tap roots found in the members of this group.

(b) The leaves in members of these class exihibit reticulate (net like) venation.

(d) The vascular bundles arranged in a ring, numbering 2–6, open and with cambium.

(e) The seeds of dicotyledons are with two cotyledons as the name indicate.

(ii) Monocotyledons : They are show following distinguished characteristics :

(a) Adventitious roots found in the members of this group.

(b) The leaves are simple with parallel venation.

(d) The vascular bundles scattered in the ground tissue, many in number, closed and without cambium.

(e) The seeds of monocotyledons are with one cotyledons as the name indicate. e.g., Cereals, bamboos, sugarcane, palms, banana, lilies and orchids.

E. Outline Of Bentham And Hooker's System Of Classification

❒The phanerogams, based on their morphological characters, such as leaf arrangement and venation pattern number of members in floral whorls, like calyx, corolla, androecium and gynoecium number of cotyledons in the seed and seeds with or without cover, were divided into 3 classes–Dicotyledons (165 families) Gymnosperms (3 families ) and Monocotyledons (34 families).

Class I : Dicotyledons (Gk. Di = two, Kotyledon = seed leaf)

❒Radicle of seed forms persistent primary root.

❒Perennial stems, exhibiting secondary growth, possess concentric arrangement of tissues.

❒The vascular bundles of stem are arranged in the form of a ring (eustele) and open i.e., cambium is present between the xylem and phloem.

❒The leaves exhibit reticulate (net-like) venation and show varied arrangement like alternate, spiral or whorled.

❒The flowers are tetramerous or pentamerous having four or five members in the various floral whorls, respectively.

❒The embryo of seeds of dicotyledons are with two cotyledons as the name indicates.

❒Bentham and Hooker further divided dicotyledons into three sub-classes, namely Polypetalae, Gamopetalae and Monochalamydae on the basis of the number and nature of floral leaves. The three sub-classes have been further divided into series considering mainly the position of ovary with respect to the other floral parts. Series have been further categorized into ‘cohorts’ (equivalent to orders in other systems) and ‘orders’

Class II : Gymnospermae

❒This class has been placed in between the dicotyledons and monocotyledons.

❒Perianth absent, ovules naked, cotyledons two or more, etc.

❒It includes 3 families–Cycadaceae, Coniferae and Gnetaceae.

Class III : Monocotyledons (Gk. Monos = single, Kotyledon = seed leaf)

❒Radicle of seed forms short – lived primary root, which is replaced by adventitious root.

❒Root is with pith and several vascular bundles (6–20).

❒Stem contains scattered vascular bundles present in uniform ground tissue. Vascular bundles are closed.

❒Secondary growth is absent.

❒The leaves are simple with parallel venation.

❒The flowers are trimerous having 3 members in each floral whorl.

❒The seeds possess only one cotyledon.