Plant Kingdom Test - 3

Gymnosperms are those seed plants in which the seeds remain exposed over the surface of the megasporophylls because the latter are not folded to form pistils and thus lack ovary. Flowers are absent and thus fruits are not formed.

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 (microsporangia and megasporangium) are borne on special leaf-like structure, called microsporophylls (=stamens) and megasporophylls (=carpels) respectively. The sporophylls are usually aggregated in the form of compact structures called cones or strobiles. The cones are generally unisexual, i.e., the male cones are microsporangia (pollen cones) and the female cones are megasporangium (seed cones).

Heterospory is the condition of producing two types of spores, i.e., megaspores and microspores. Heterospory occurs in all seed bearing plants i.e. gymnosperms and angiosperms.

Cycas is an archegoniate plant, female gametophyte contains archegonia. The archegonia are formed from the gametophytic cel|s lining the archegonial chamber. The number of archegonia formed in a gametophyte is variable among species.

In Gymnosperms, the male and female gametophytes do not have an independent free living existence. They remain within the sporangia retained on the sporophytes i.e., female gametophyte within megasporangium and male gametophyte within miaosporanoium. In Pteridophytes the gametophyte is although small and inconspicuous, but is usually independent.

Conifers have a number of xerophytic characters such as needle -like (e.g., Pinus), scale-like (e.g., Thuja) or small and leathery leaves (e.g., Araucarias), thick cuticle, sclerenchymatous hypodermis and sunken stomata to reduce transpiration. They are, thus, well adapted to tide over the winter period when the soil becomes frozen and availability of water is very little.

In gymnosperms 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). The diploid zygote develops into an embryo while still enclosed within the ovule and the ovule becomes a seed. The wall of ovule becomes the seed coat. During seed development, the food laden tissue of the female gametophyte (or endosperm) supplies nutrient for the development of embryo and for future growth of embryo into seedling. Conifers are usually monoecious, i.e., male and female cones are borne on the same plant example, Pinus.

Pinus and Cedrus have branched stem. The main stem shows monopodial branching. Stem branches are of two types, long and dwarf. The dwarf branches possess 1-5 needle-like follage leaves Which are surrounded at the base by a sheath of scale leaves.

The coralloid roots of Cycasare symbiotically associated with nitrogen fixing blue-green algae, Anabaena cycadae and Nostoc punctiforme. In the middle cortex of coralled  roots, there occurs a broad blue green zone, which is known as algal zone. Coralloid roots are irregular, negatively geotropic, dichotomously branched coral likes roots which do not possess root hair and root caps.

A mycorrhiza is a symbiotic association of a fungus with a root system. The mycorrhizal roots of Pinus occur near the soil surface. They are devoid of root hair and root cap. The fungus commonly associated with mycorhizic roots of Pinus is Boletus elegans.

Stem portion of Cycas revoluta and C. circinalis is a good source of Sago - a kind of starch used in making breat by poor people. Seeds of Pinus gerardiana (Chilgoza) are edible. Oil obtained from Juniperus species is medicinally, important. An important alkaloid ephedrine is obtained from Ephedra (a gymnosperm) and is used in treatment of cough, cold and other respiratory disorders.

Canada balsam is obtained from Abies balsams The resin does not crystallise or granulate on drying. It has a high refractive index as that of glass and is used as a mounting medium for microscopic object and as a cement for lenses in optical work.

Thuja plicata possesses the wood which is known for possessing certain antibiotics. It is resistant to weathers and is widely used for commercial purposes. It also provides as essential oil which is used in perfumery. Agar is obtained from cell wall of Gelidium and Gracilaria.

Living gymnosperms are predom inantly middle sized trees (Cycas) to tall trees (Pinus) and shrubs (Ephedri Rarely they are woody climbers (Gnetum montanum). Their are no herbs in gymnosperms.

The gymnosperms are comparatively more ancient than the angiosperms in evolutionary terms. Most of the gymnosperms have now become extinct and the group is presently represented by only 900 living species. The living gymnosperms are widely distributed in the cold climates where snow is the source of water. Cycas and Ginkgo are referred to as living fossils.