achonologment of gynosperms
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Essay on the Evolutionary Trends of Gymnosperms
1. Essay on the Meaning of Gymnosperms:
Gymnosperms (Gr. gymnos=naked, sperma=seed) and angiosperms (Gr. angios=closed; sperma=seed) are the two sub-divisions of division Spermatophyta (Gr. sperma=seed; phyton=plant) of plant kingdom. Spermatophyta includes all those plants which bear seeds.
Gymnosperms literally mean naked seed, and, therefore, this sub-division is characterized by having the ovules borne unprotected on the surface of megasporophylls.
In angiosperms, on the other hand, the ovules are borne within a closed cavity, the ovary, formed by the megasporophyll, and after fertilization the ovary becomes a fruit which encloses one or more seeds. The word gymnosperm was first used by Theophrastus, one of Aristotle’s pupils.
The lower gymnosperms, such as members of Cycadales, show resemblances with the higher cryptogams (i.e. pteridophyta) whereas the higher gymnosperms, such as Gnetales and Coniferales, resemble members of angiosperms to some extent. Gymnosperms thus form a bridge between the pteridophytes and angiosperms, and have, therefore, been referred as “Phanerogams without ovary” by Goebel.
Gymnosperms form a small group of plant kingdom, and are represented by only about 70 genera and 725 species. In-spite of being represented by such a small number, the representatives of this group are widely distributed throughout the world, and on many mountains they form a dominant part of the vegetation.
Gymnosperms are most ancient and a history of the fossil records indicates that they once formed a predominant part of the earth’s vegetation. An idea of their history may be gained from Fig. 1.1, which shows the relative abundance of different orders during successive geological periods.
It becomes clear from this figure that both Pteridospermales and Cordaitales first appeared in the Upper Devonian or Lower Carboniferous, i.e. more than 300 million years ago. In addition to Pteridospermales and Cordaitales, two other groups of fossil gymnosperms, i.e. Bennettitales and Pentoxylales have been recognized.
Bennettitales were distributed during Mesozoic period from Middle Cretaceous to Upper Triassic while the Pentoxylales were distributed during Jurassic period of Mesozoic era. The living members of the gymnosperms are represented by Cycadales, Ginkgoales. Coniferales and Gnetales.
Precisely, gymnosperms are defined as those seed plants in which the ovules are not enclosed in ovary and the pollen typically germinate on the surface of the ovule. There is no double fertilization in gymnosperms, and the xylem in these plants lacks vessels (except in Gnetales). Theophrastus, a pupil of Aristotle, was the first to use the word “gymnosperms” in 300 B.C. in his book “Enquiry Into Plants”.