Gymnosperms are one of the most successful groups of seed plants. They have worldwide distribution. They constitute about 1/3 of the world, forests.
- The gymnosperms are heterosporous plants.
- The term Gymnospermae means ‘naked seeded’ (Gymno = naked and spermae = seed). The ovules in these plants are not enclosed. They are borne on the exposed surface of fertile leaves. These leaves are called megasporophyll.
- The show regular heteromorphic alternation of generation.
- Sporophyte: They have independent and dominant sporophyte. It produces two types of spores, microspores and megaspores. Microspores are developed on microsporophyll and megaspores are developed on megasporophyll.
- Gametophyte: The gametophyte is less conspicuous and dependent. The female gametophyte is permanently retained within the ovule. The megasporophylls cover ovule. These sporophylls are not folded or joined at the margins to form an ovary. That is why, seed are naked on megasporophyll.
- The important genera of Gymnospermae are:
- Cycas sago-palm
- Pinus pines
- Picea Hemlock
- Cedrus: deodar
Life Cycle of Pinus
Pinus is a conifer. It shows alternation of generations. The diploid sporophyte generation is dominant and the haploid gametophyte generation is inconspicuous. The main plant body is a sporophyte. Conifers are heterosporous. They develop two types of spores, microspores and megaspores.
These are spores are produced in sporangia. These sporangia are developed on reproductive structures, called cones. There are two types of cones, male and female cones. Both these cones develop on the same plant.
The male cones are small in size. They are produced in clusters on an axis (stem). Male cone contains many microsporangia. These microsporangia produce many microspores by meiosis. The microspore germinates within its own wall to form a small inconspicuous male gametophyte. Male gametophyte is also called microgametophyte. A microspore containing the male gametophyte (microgametophyte) including the gametes is called Pollen Grain (plural=pollen).
The transfer of male gametes (Sperm) to female gametes (egg) does not require water in gymnosperm. Pollens are produced in great numbers. The pollen grain of pinus has two lateral wings. Due to these wings the pollen can float ¡n air for longer time. So they are carried by wind to longer distances.
In this way, the gymnosperms have evolved a new mechanism for the transfer of male gamete to the female gamete through air. This is an important adaptation. It helps these plants to survive in the harsh dry terrestrial environment.
The female cone has large size and they are more conspicuous. The female cone contains many microsporangia. Each female cone is composed of a large number of scales called megasporophylls. These megasporophylls are spirally arranged and woody in texture.
An ovule is present at base of each scale. Ovule is a megasporangium, which is protected by two integuments. Each megasporangium has a single diploid megaspore mother cell. The megaspore mother cell divides by meiosis to produce four haploid megaspores. Three megaspores are degenerated.
The functional megaspore undergoes mitosis and forms female gametophyte or an embryo sac. The embryo sac contains one to many archegonia. The archegonia contain female gamete called egg.
Pollination and Fertilization of Class Gymnospermae
During pollination, the pollens fall directly on the ovule. Only a few pollens germinate to form pollen tube. The Pollen tube transfers the male gametes (sperm) to the embryo sac for fertilization.
More than one egg can be fertilized to form several zygotes. Only one zygote survives and forms a single embryo. After fertilization the ovule becomes seed. The seed contain an embryo and some stored food material. The seed germinates and develops new sporophyte.
The term angiosperm means “enclosed seeded” (angi = close and sperm = seed). In these plants fertile leaves bearing ovules are folded and joined to from ovaries. This ovary after fertilization is changed into a fruit. Fruit contains seed.
There are 235,000 species of angiosperms out of 360,000 known species of plants. Angiosperms are heterosporous plants. These are most successful plants of the earth. These plants produce flower, fruits and seeds.