Classification of Plants- Botany
Classification of Plants
- 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).
- 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.
- 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.
- External features
- The plant body is sporophyte and differentiated into root, stem and leaves.
- The plant possess well developed tap root system. In some cases the roots are symbiotically associated with algae (e.g., Coralloid roots of Cycas) or with fungi (e.g., Mycorrhizal roots of Pinus).
- The stem is erect, aerial, solid, woody and branched (unbranched in Cycadales) but almost tuberous in Zamia.
- The leaves may be microphyllous or megaphyllous.
- Gymnospermous wood
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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).
- 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.
- 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
- 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.
- 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. 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.
Order 1. Cordaitales : All the members of this order are extinct. e.g. Cordiates, Dadoxylon.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.
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.
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