Gymnosperm Introduction

The word ‘gymnosperm’ was used in 300 B.C., by Theophrastus, a pupil of Aristotle, in his book “Enquiry into Plants”.
Theophrastus used this term to embrace all those plants whose 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 fertilization.
These groups are the gymnosperms and the angiosperms.
The gymnosperms have their ovules freely exposed before and after fertilization. They are not enclosed by any ovary wall.

GENERAL CHARACTER:

Vegetative Character:

Habit and habitat:

The gymnosperms are the most ancient seed plants that originated during the late Paleozoic (ca 265 million year ago) era, but flourished well during the Mesozoic.
The living gymnosperms include middle-sized or tall trees and shrubs and number approximately 70 genera and 725 species. .
Plant body sporophytic which is differentiated into root, stem, and leaf.

Root:

The roots are generally taproot, but mycorrhizae and coralloid roots are known in some genera. In root xylem is exarch and diarch to polyarch.

Stem:

The stem is aerial, erect, unbranched or branched.
In some genera branches may be of two types;
(i) the long shoot
(ii) the dwarf shoot.
Collateral, conjoint, endarch and open vascular bundle present in young stem.
In xylem vessels and in phloem companion cells are absent.

Leaf:

The leaves may be simple or compound and vary in size from a minute scale leaf to a large and more than six feet long.
They show great variation in shape also.
The venation may be reticulate, parallel or even dichotomous.
Leaves are of two types:
(i) Microphyllus leaf – These are small and scaly,
(ii) Megaphyllus – these are large and well developed.
The arrangement of leaves may be opposite and decussate [Gnetum, Ephedra], whorled[Cycas] or spirally arranged (Podocarpus, Taxus etc.).

Wood:

The secondary wood in the gymnosperms may be:

(i) Manoxylic:

This is porous, soft ,more parenchymatous in nature and found in cycadophyta It has wide medullary rays and is useless commercially.

(ii) Pycnoxylic:

This wood is compact and has narrow medullary rays and characteristic of coniferophyta.
It is of great commercially value.

Reproductive Character:

The gymnosperms are heterosporous i.e., Microspore and Megaspore.
But the two kind of spore produce two kind of gametophyte, i.e., the microspore or the pollen grains produce male gametophyte
whereas the single megaspore enclosed within the megasporangium develops into the female gametophyte bears two or more archegonia.
The two types of cone or strobili may be borne on the same tree [pinus] or on different trees [Ginkgo, Cycas].
The microspores and megaspores are haploid due to result of meiosis in the respective spore mother cell they are pioneers of male and female gametophyte.
The strobili or the cones are of varying sizes and shapes.
Their position on the plant also varies.
Male cones are ephemeral.
Microsporangia present at the abaxial surface of sporophyll.
Number of sporangia in a sporophyll may be fixed or not fixed.
Microspore is the first cell of the male gametophyte.
Megasporangium are naked on sporophyll.
Ovule is orthotropous and unitegmic generally.
In female gametophytes the number of archegonia varies according to species.
Archegonia consists of egg and one venter canal cell.
Pollination takes place by wind.
Pollen grains are collected at ovule in specific structure known as pollen chamber.
Zygote is the mother cell of the next sporophytic generation.
Embryo development is of meroblastic type i.e.embryo develops from the basal part of zygote.
Endosperm development starts before fertilization so it is a haploid structure.
Embryo development by free nuclear division at the initial stage.
Polyembryony is common in gymnosperm
The alternation of generations is heterologous in all the gymnosperms.