The Spermatophyta includes the most highly organised plants characterised by the production of seeds. The seed plants are generally divided into two groups, gymnosperms and angiosperms.
The word 'gymnosperms' was used by Theophrastus in his book 'Enquiry into plants'. The term embrace all those plants whose seeds are unprotected(Gymnos-naked,sperma-seeds). The gymnosperms have their ovule freely exposed before and after fertilisation. They are not enclosed by ovarywall and hence sometimes called as phanerograms without ovary.On the contrary, in angiosperms,(angios-vessel,sperma-seed) the ovule is completely enclosed within an ovary. Due to the protection afforded to the ovule and the seed, the angiosperms are considered to be advanced than the gymnosperms. Gymnosperms are regarded as intermediate between-pteridophytes and angiosperms.
The gymnosperms are the most ancient seed plants that originated during the late paleozoic era(350 million years ago),but flourished well during the mesozoic era. The gymnosperms include both fossil forms and living members.
Distribution
The living gymnosperms include 70 genera and 725 species. The gymnosperm component of Indian flora comprises of 14 genera and 52 species of living gymnosperms (Raizada and Sahani 1960).They belong to 8 families families of the orders cycadales,coniferales, taxales, Ephidrales and Gnetales. The living gymnosperms include 22 genera that are monotypic. Gymnosperms are distributed in tropical and temperate region of the world. Some species are found even in Arctic region. In India they are mostly distributed in hilly areas very few grow in plains of India.
Habit:
The gymnosperms comprises of Woody plants which may be large or small trees or shrubs. A few maybe Woody climbers(lianas).Herbs and annuals are completely absent. Most of the gymnosperms are evergreen, some are deciduous like Larix and Taxodium. The plants are mostly xerophytic.
Gymnosperms have some of the tallest tree known in the plant kingdom. Sequoia sempervirens(California coast Redwood) is probably the tallest living tree reaching a height of nearly 112m and attaining a girth of 15 metres. It is as tall as a 36 storey building. Smallest gymnosperm is a cycad, Zamia pygmea whose fronds are only for 4 or 5 cm long.
External Characters:-
The plant body is a sporophyte and is differentiated into root,stem and leaves.
Plants have a well developed tap root system formed from radicle. Lateral roots may have symbiotic association with blue green algae as in coralloid roots of cycas or with fungus as in mycorrhizal roots of pinus and other conifers. The stele in root is diarch or polyarch. Secondary growth occurs in the roots. Xylem is exarch.
The stems are aerial erect underground (Welwitschia), Unbranched(Cycas,Zamia,Welwitschia) or branched (Pinus ,Cidrus).
In some genera (Pinus,Cedrus,Gingko)the branches are of two types first the long shoots and second the dwarf shoots that bear at their apices bundles or clusters of green Leaves and are collectively called as spur.
The vascular bundles in the stem are arranged in a ring. The bundles are conjoint,collateral, endarch. Xylem is composed of xylem parenchyma and tracheids with bordered pits. Xylem vessels are absent except in Ephedra, Welwitschia and Gnetum. The xylem is usually mesarch or endarch.
The phloem is composed of sieve tubes and phloem parenchyma. The companion cells are absent.
The stem show secondary growth due to which to which to sec
to which secondary phloem is produced on the the outer side and secondary xylem on ineerside. the secondary wood forms distinct annual rings.
The secondary wood in the gymnosperms is of two types first manoxylic and pycnoxylic. The manoxylic wood is soft, porous with wide with wide medullary wide medullary rays,more parenchymatous, large pith and and cortex.because of its small amount amount the Manoxylic wood is not important commercially. Example- Cyadophyta.
The pycnoxylic wood is hard, dense with small and narrow medullary rays and small amount of cortex and pith. The wood forms main bulk of trunk. It is a great commercial use and provide the timber of Commerce. example- Coniferophyta.
The manoxylic wood is related to megaphyllous leaves and radial symmetry of seeds. On the other hand the pycnoxylic wood is associated with microphyllous leaves and bilateral symmetry of seeds (spome 1974).
There is a great diversity in both forms and arrangement of leaves in addition to foliage leaves scale leaves may also be present. The leaves may be microphyllous or megaphyllous. The microphyllous leaves are small and scaly. the megaphyllous leaves are large and well developed and their vascular supply always leaves a leaf gap in the Stele.
The leaves may be simple or compound and vary in size form minute scale leaves( example- Juniperus) to a large and more than 6 feet long Megaphylls of small cycads.
Besides size, there is variation in the leaf shapes too. The leaves may be pinnate or pinnate or fern like (cycads)or a circular or needle like (conifers) fan-shaped (Ginkgo) .The leaves of Gnetum superficially resemble those of dicotyledons.
The arrangement of leaves may be opposite and decussate(Gnetum, Ephedra,Welwitschia), whorled (Cedrus),spirally arranged (podocarpus, taxus).
The venation may be reticulate (Gnetum), parallel (Welwitschia)for dichotomous (Gingko) or single midrib (pinnae of cycas) ,open dichotomous (other genera of cycadales).
(the leaves exhibit xerophytic characters in their form and texture include there cuticle sunken stomata stomata cuticle sunken stomata sclerenchymatous hypodermis)
The stomata maybe haplocheilic or Syndetocheilic type . In haplocheilic type the guard cells and subsidiary cells originate from different mother cell or initials, example- pteridospermales, cycadales, Gingkgoales and coniferales. In syndetocheilic type the guard cell and subsidiary cells originate from same mother cell or initial cell, example- Bennettitales .Florin 1911 observed that haplocheilic type of stomata are primitive and syndetocheilic type are advanced. The stomata may be distributed on the both leaf surfaces (Gingko)or on the lower epidermis alone (cycas,Tatus).
The mesophyll may ( cycas, Taxus, Gnetum) or may not (pinus) be differentiated into palisade and spongy parenchyma.
Resin canals occur in the leaves and all almost all conifers (Pinus,Cidrus)except Taxus. Cycas and Gingko shows mucilage ducts. In Gnetum latex tubers are present.
The leaves of conifers and cycads posses a a transfussion tissue.
Reproduction: -
Vegetative reproduction is rare in gymnosperms. Cycads do propagate through bulbils.
All gymnosperms are heterosporous and produce two types of spores- microspores and mega spores .The two kinds of spores are produced inside sporangia that are borne on sporophylls which are arranged spirally along the axis to form strobilus or cones. The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili. The strobili bearing megasporophyll with ovules or megasporangia are called microsporangiate or female strobili.
Two types of cones or strobili may be borne on the same tree (pinus) on different trees (Gingko, Cycas).
The microsporangia are borne on the abaxial or lower surface of the microsporophyll. The microsporangium develops from a meristem that differentiates into epidermis and archesporium. The archesporial tissue divides to form periclinally to form an outer primary parietal layer and an inner primary sporogenous layer.
The primary parietal layer undergoes periclinal division to form a multi layered microsporangia wall .The innermost of which functions as the tapetum.
The primary sporogenous cells, on repeated divisions form a mass of sporogenous cells that mature into microspore mother cell. The microscope mother cell divide meiotically giving rise to haploid tetrads of microscopes.
The ovules of megasporangium are naked and are borne on the adaxial or upper surface of the megasporophyll scale. The ovule is generally orthotropous and sessile. Ovules of cycas are the largest in the plant Kingdom.
The ovule consists of a central parenchymatous mass of cells called the nucellus or megasporangium. The nucellus is normally surrounded by a single envelope or integument. The single integument consists of three layers 1.Sarcoterta layer , 2. scleroterta or middle stony layer and 3. The inner Sarcoterta. the integument envelops the nucellus and leaves a small passage above it called the micropyle. This is the micropylar end of the ovule.
Female gametophyte: -
One to several hypodermal cells if the nucellus acts as a archesporial cells. The archesporial cells undergoes periclinal division and give rise to the primary parietal cell and the primary sporogenous cell.
The primary parietal cell further undergoes periclinal division and forms the parietal tissue and in this process pushes the sporogenous cell deep into the nucellus.
The sporogenous cell functions as the megaspore mother cell. The megaspore mother cell after meiosis forms a tetrad of megaspores of which the lower are functional and forms the female gametophyte .Thus,the development of the female gametophyte is monosporic (formed from a single functional megaspore),except Welwitschia and Gnetum where it is tetrasporic.
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