Introduction to botany  © punam kumar

 

CHAPTER 18:

Gymnosperms

Introduction

Gymnosperms are seed plants that do not produce flowers. The word gymnosperms originates from the words gymnos meaning naked and sperm meaning seed, the name is so because of the fact that seeds of gymnosperms are not surrounded by a protective wall which exists in the angiosperms. However, usually when the seeds of gymnosperms are immature they are enclosed within and protected by modified leaves or a cone. In flowering plants or angiosperms, the ovary wall or fruit encloses the seeds, whereas in gymnosperms there is no equivalent structure; hence, the interpretation of the seeds as "naked" or not enclosed.

Diversity and OriginThere are between 700 and 900 species of Gymnosperm. The gymnosperms are the most ancient naked seed plants that originated during the late Paleozoic era but flourished well during the Mesozoic. The Cycadales and Ginkgoales include living and non living members and have a long fossil history and known as 'Living fossils' of gymnosperms. During long evolutionary history three group of gymnosperms :-

became extinct.Gnetophytes are considered from morphological and molecular evidence to share a common ancestry with the flowering plants Conifers are by far the most abundant gymnosperms with around 600 species. Cycads are the next most abundant group with about 130 species. Approximately 75 - 80 species of Gnetales exist and only one species of Ginkgo remains today.

Classification of gymnosperms :- Gymnosperms have been variously classified by different workers from time to time.Theophrastus, a pupil of Aristotle in his book 'Enquiry into Plants' used the word gymnosperm in 300 B.C. By the beginning of the present century, Engler (1897), Coulter and Chamberlain (1910), Engler and Prantl (1926), Rendle (1930) and others recognized gymnosperms as a primary division of Spermatophyta . They divided the group into classes or orders of coordinate rank, viz. (1) Pteridospermae (Cycadofilicales) (2) Cycadales (3) Bennettitales (4) Cordaitales (5) Ginkgoales (6) Coniferales and (7) Gnetales.Between 1950-1981, gymnosperms are included within Division Tracheophyta:- the vascular plants The class essentially included the conifers and their allies (by which term is meant "related species of plants"), including several groups of extinct plants known only from fossils. In these earlier classification schemes, the "naked seed" plants were clearly set off from the other classes of higher plants (that is, the ferns and flowering plants), essentially as they are today. However, fossil evidence suggests that the angiosperms evolved from a gymnosperm ancestor, which would make the gymnosperm taxon paraphyletic. Modern cladistics attempts to define taxa that are monophyletic, traceable to a common ancestor and inclusive therefore of all descendants of that common ancestor. So, while the term gymnosperm is still widely used to distinguish the four taxa of non-flowering, seed-bearing plants from the angiosperms, plant species once treated as gymnosperms are distributed among four groups given equal rank as divisions within the Kingdom Plantae.The four division into which gymnosperms are classified represent four evolutionary lines. These groups are:


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Habit and habitat :- The gymnosperm is represented by a perennial, evergreen, woody plant. Most of them are trees and some are shrubs.A few gymnosperms may be lianas or climbers. There are no herbs.Gymnosperms include one of the world's tallese trees - Sequoia sempervirens (the Giant Red Wood Tree) measuring about 125 meters in height and 30 meters in girth.

Living gymnosperms are distributed worldwide, with a majority, particularly the conifers, in temperate and subarctic regions. Cycads and gnetophytes are mainly tropical to subtropical.

General features of Gymnosperms :-Morphology (external structure of plants)

Anatomy(internal structures of plant):-There is a well-developed vascular system of xylem and phloem and have true roots, stems, and leaves.All gymnosperms are woody plants.

Reproduction :-Vegetative reproduction are rare in gymnosperms but vegetatively propagating bulbils are known in Cycas.

Pollination and Fertilization

Embryogeny:- In all gymnosperms except Welwitschia, Gnetum, and Sequoia sempervirens, seed development begins with a series of free nuclear divisions at the start of embryogeny. Later embryo becomes cellular after wall formation and gradually differentiates into a suspensor, shoot apex, cotyledons, hypocotyl and radicle. Polarity is endoscopic with the shoot end directed away from the micropyle. An immature sporophyte is developed within the ovule and is surrounded by the megagametophyte. The immature sporophyte is an embryo. While this is happening the outermost layer of the ovule, the integument, develops into the seed coat. Mature embryo is differentiated into root, stem and leaves.

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Polyembryony:-In most gymnosperms a common feature in embryogeny is polyembryony - formation of several embryos in a single gametophye inyoung seed. Usually from fertilization of more than one archegonia or from cleavage polyembryony where cells of developing embryo become separated and each give rise to a new (usually 4) embryo.The mature seed normally has only one embryo.

The young embryo draws its nutrition from the endosperm which develops before fertilization and is haploid (n). Endosperm develops from female gametophyte that has absorbed the food from nucellus.Histological maturation of various layers of integument continues and the stony layer becomes an extremely hard, resistant shell which effectively encloses and mechanically protects the female gametophyte and the embryo.

Seeds:- The fertilized egg (zygote) develops into an embryo which is contained within the seed. A seed is a matured ovule. The matured ovule consists of a seed coat, megagametophyte, and embryo. The seed coat is diploid tissue from the original parent sporophyte. The megagametophyte is a haploid plant that developed from a megaspore produced by the original sporophyte. The embryo is a new diploid sporophyte The detached seeds of all gymnosperms (except for cycads and Ginkgo) remain dormant for sometime undergoing a resting period. Gymnosperms being mostly temperate plants (except for cycads, Gnetum and Ephedra) growing in tropics and subtropics show very little activity in the development of reproductive structures during winter. The activity is renewed in the spring. Under favourable conditions the embryo resumes growth and after rupturing the seed coat develops into a new sporophyte plant, thus completing the life cycle. . Gymnosperms are plants with naked seeds (no fruit). Angiosperms are plants in which the seeds are enclosed within a fruit.

 

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