GYMNOSPERM

Gymnosperm:

Introduction:

Gymnosperms are small group of plants, which constitutes a sub-division of spermatophyte or phanerogams. Gymnosperm acts as a connecting link between pteridophytes and angiosperm.

Occurrence:

Living members are founding in colder region of earth where snow is the source of water. Only the members of cycadales and gnetales thrive in warm dry climate. There are about 70 genera and 900 species of gymnosperm distributed in tropical and temperate areas. Most of them are conifers generally evergreen with needle like leaves. They are found in the form of coniferous forest. Generally conifers are species of pine, fir, cedar, spruce, sequia giagntica.

Salient features of gymnosperm:

Ø  Gymnosperm is derived from greek word gymnos-naked, sperma-seed. They are naked seeded plants. Here the ovules are not enclosed and thus the seeds remain naked. Rather the ovules are borne directly on open carpellary leaves termed as megasporophylls.

Ø  Most of the gymnosperm are evergreen, woody perennial with shrubby or tree like habitat. They store xerophytic characterstics.

Ø  The main plant body is well differentiated sporophyte consisting of root, stem and leaves. Gametophyte stage is reduced and is dependent on sporophyte.

Ø  Plants possess well developed root system.

Ø  Vascular bundles are conjoint, collateral, open and endarch.

Ø  Reproductive parts are generally in the form of compact and hard cones or strobili. Cones are generally unisexual.

Ø  In male cones, many microsporophylls are arranged on a central axis. Microsporophyll bears microsporangia containing microspores or pollen grains.

Ø  Pollination is anemophilous type.

Ø  Endosperm is formed before fertilization and is always haploid.

Ø  Fertlization occurs by pollen tube (sphinogamy). Double fertilization is absent.

Ø  Polyembryony is the usual feature.

Distinguishing feature of gymnosperm:

Ø  Life cycle of gymnosperms shows heteromorphic alteration of generation.

Ø  They are more advanced than pteridophytes but are primitive than angiosperms.

Ø  Plant body is the sporophyte (diploid) with well developed roots, stem and leaves.

Ø  Sporophyte bears two types of fertile leaves, microsporophyll that produces microspores and megasporophyll that produce megaspores. Mostly the spores are grouped into compact cones or strobili.

Ø  Spores on germination develop into gametophyte which is very much reduced, microscopic and dependent on sporophyte.

Ø  Fertilization involves only one fusion female gametophyte provides nutrition to the developing embryo. The endosperm (female gametophyte) is pre- fertilization tissue and is haploid sac and the embryo (of the next sporophyte generation). All the nutrients for life are supplied.

Ø  Vessels are absent in xylem (except gnetales).

 

Classification of gymnosperms:

Gymnosperm is classified into following classes:

1.       Cycadophyta: Cycads are the seed bearing plants which are also called primitive gymnosperm. These plants usually have large compound leaves with circinate venation, thick trunks and small leaflets which are attached to the single central stem. They go upto a height of about 25cm. All are dioecious. Young leaves show circinate venation. Eg: Cycas, microcycas, Zamia pygmaea (smallest gymnosperm), chigua, stangeria.

2.       Ginkophyta: It is a strong leaf like structure looking like plant. It has a number of medicinal values. It possess fan like leaves that improve and sharpen the memory of human-beings. Ginkophyta includes only one living member i.e. Ginkgo biloba (Maiden hair tree) and is the oldest living seed plant persisted with little change till now. It has a sturdy leaf like structure. These plants were served Chinese people as they use it for various medicines and it is believed that its fan type leaves and seeds help in improving the memory and sharpening the thinking level. Wood is pycnoxylic. Ovules arise in groups of 2-10. Endosperm has beak like protuberance called tent pole.

3.       Gnetophyta: A Small but highly evolved group of gymnosperm represented by three genera i.e. Ephedra, Gnetum, Walwitchia. This is another type of species that consists of tropical plants, trees and shrubs. They are characterized by flowery leaves that have soft coating. This coating gives them an ancestral connection with the angiosperm. Ovules are othrotropous with long tubular micropyle.

4.       Coniferophyta: These are the most commonly known species among the gymnosperm.  They are dominant forest makers in the colder regions of earth due to xerophytic nature, evergreen nature, and presence of mycorhizza, scale, leaves, and resins to plug injury, enzyme activity even at -35°C. These are mainly characterized by male and female cones which are in the form of needle like structure. Conifers are usually found in places where the climate is temperate.  Wood is pycnoxylic. Sporophyll form cones.

Eg: Pinus sequoia, red wood tree

       Sequia Sempervines is also known as Red Douglas of California, tallest tree with height                            of  366 ft.

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