Gymnosperm- Naked seed bearing plant

 Overview

• Gymnosperms evolved from Pteridophyte.
• As they are seed bearing plants but their seeds are naked because the ovule are not enclosed by an ovary wall and remain exposed before and after the fertilization.

• They have vascular tissue for transportation of food and water, their plant body is divided into roots, stems and leaves but they do not  bear flowers. 

• The term gymnosperm signifies the naked seed because their ovule is not covered by an ovary wall.

• Gymnosperms are grouped under the  embryophyta. They  are like shrubs , medium height plants and some are long  trees.

• Sequoia is one of the tallest trees of gymnosperm, called as giant redwood gymnosperm.

• Gymnosperms can tolerate moist or dry conditions.

• Gymnosperms evolved  in the Triassic period. Approx 1200 species have been recognized under the gymnosperm. 

👌👌Remember -Gymnosperms are classified into four types -Cycadophyta, Ginkgophyta, Gnetophyta and coniferophyta. 

General characters of Gymnosperms

Habitat -

• They are found in colder regions and  They develop needle-like leaves to reduce surface area.

• In addition to this, the leaves of gymnosperm are well adapted to withstand adverse conditions like temperature , humidity etc.

• They also have thick cuticles and sunken stomata to reduce the loss of water.

Symbiotic association 

• Some gymnosperms have fungal symbiotic association in their roots.

• In Pinus, fungus make association in the root of pinus. This association is called mycorrhiza.

• In cycas, nitrogen fixing cyanobacteria like Nostoc make association in the root of cycas. Such roots are special in structure  termed Coralloid roots.

Morphological feature

• The roots in gymnosperm are tap roots because the primary branch of the root has various branches.

• The leaves may be simple or compound.
• The stem is branched like in Pinus or unbranched like in cycas.

Microsporophyll and Megasporophyll 

• The gymnosperm has a group of two kinds of  leaves called sporophyll.

• These sporophyll bear sporangia that also produced two kinds of spores - microspore and megaspore.

• In gymnosperm, small leaves are arranged in the form of microsporophyll which bear microsporangia and produce microspore.

• The bunch of small leaves in gymnosperm is called female strobili or female cone. 

• Similarly, Large leaves  are arranged in the form of megasporophyll which bear megasporangia and produce megaspore.

• The bunch of large  leaves in gymnosperm is called male strobili or male cone.

• In Pinus, Male and female cones are found in a single plant whereas in Cycles male and female cones are present at different plants.

Reproductive structure

• At microsporophyll, inside the microsporangia, microspores divide and produce male gametophyte or pollen grains.

• On the other hand, At megasporophyll, inside the megasporangium, an ovule is present and it is surrounded by the protective envelope called integuments. The cells of integuments are called nucellus.

• A single cell of nucellus is differentiated and forms a megaspore mother cell.

• These megaspore mother cells divide by meiosis and form four megaspore.

• Out of four megaspore , one megaspore develops into a female gametophyte. This female gametophyte contains one or two archegonia bearing eggs.

Reproduction 

• In gymnosperm, male gametophyte and female gametophyte are retained within microsporangia and megasporangia respectively.

• Pollen grains are carried by the wind and reach the ovule.

• At the ovule, pollen grain germinates the pollen tube and discharges male gamete at the mouth of Archegonia.

• As a result, fertilization occurs by the fusion of male gamete of pollen grains and egg of archegonia.

• After fertilization, zygote is changed into embryo and ovule into seed that is without covering. Example-Cycas,Pinus,Ginkgo, Thuja, Cedrus,Picea etc.