Essay on Gymnosperms!
The word ‘gymnosperm’ was used in 300 B.C., by Theophrastus in his book ‘Enquiry into Plants’. He used this term to include all those plants where seeds are unprotected, (Gymnos = naked; sperma = seeds). The seed plants (spermatophyta) are grouped into two major groups on the basis of protection afforded to the ovule before and after fertilisation.
These groups are the Gymnosperms and the angiosperms. The gymnosperms have their ovules freely exposed before and after fertilisation. They are not enclosed by any ovary wall, whereas in the angiosperms (angios = vessel; sperma = seed) the ovule or ovules are completely enclosed within a structure called the ovary which forms the lower swollen part of the carpel.
Due to the protection afforded to the ovule and the seed, the angiosperms are considered to be advanced than the gymnosperms, which are therefore, regarded as intermediate between the pteridophytes and the angiosperms.
The gymnosperms have long been considered as a group of seed-bearing plants. Together with the angiosperms they are known as phanerogams or spermatophytes. They are distinguished from the angiosperms by having naked ovules and seeds. The ovules are not enclosed in an ovary and the seeds are not enclosed in a pericarp.
The gymnospermae comprises of trees and shrubs which are characterized by xeromorphic acicular or fern-like leaves and unisexual inflorescence.
The ovules lie exposed on the megasporophylls which are open and therefore, devoid of a style and stigma. The nakedness of the ovules was clearly recorded by Robert Brown in 1928.
The pollen grains are spherical or oval, often with a bladder-like extension of the exine. The pollination is direct in the sense that the pollen grains or at least the pollentubes reach the nucleus of the ovule without the intervention of the stigma and styles through which the pollen tubes have to penetrate in angiosperms.
The pollen grains possess a prothallus of two or more cells, one of which produces two non- motile or rarely motile male gametes or spermatozoids (e.g., Cycas. Ginkgo).
The pollen grains are disseminated by wind. They directly settle upon the micropyle.
The embryo sac (female gametophyte) contains a tissue before fertilization which is known as rudimentary prothallium, as two or more archegonia are formed in it, each consisting of a large egg cell and a short neck.
Only one embryo usually develops to maturity. The embryo consists of an axis bearing two to several cotyledons and ending in a radicle. The whole embryo remains enclosed in a mass of endosperm which represents a continuation of the gametophytic tissue. The sporophylls are usually arranged in a cone-like structure.
The following key may be used in the identification of the gymnosperms:
Trees or shrubs usually resinous; ovules naked, not enclosed in an ovary; flowers unisexual, rarely bisexual; leaves needle-shaped scale-like, linear, pinnate, rarely fan-shaped or oblong elliptic, mostly evergreen.
From the point of view of their botany the interest in the group is several folds. They are all wood plants. They range from the tallest known trees, e.g., Sequoia sp. to dwarf shrubs, e.g., Ephedra. The diversity of their vegetative parts is very great.
They possess greater diversities in their reproductive organs. They are, in many points of structure and details of behavior, intermediate between vascular cryptogams and angiosperms. Several major groups of gymnosperms are extinct and are known only from a study of their fossils in Palaeozoic or Mesozoic strata.