Life Cycle in Gymnosperm plants
Gymnosperm reproduces by seed formation. Vegetative reproduction is absent in gymnosperm. Only cycas produce vegetatively by the appearance of bulbil like structures.
Cones or Strobili
Gymnosperms are heterosporous, They produce micro and megaspores. The development defines unsexual cones are strobila. There are two types of bones strobila.
These are male cones and female cones; male cones are usually smaller in size than female cones. They show a significant variation.
(a) Male cones: the male cone small in size. They got growth in clusters on an axis (stem). The male cone contains many microglia.
These microglia produced many microspores by meiosis. The microspore germinates with its wall to form a small in the inconspicuous male gametophyte.
(b) Female cone: the female cone has a large size, and they are more conspicuous. The female cone contains many microsporangia. The female cone is composed of a large number of scales called megasporophylls.
These megasporophylls are arranged and Woody in texture. An ovule is present at the base of each case. The ovule is the megasporangium.
- Microscope and Microsporangia
The microspores ( pollen grains) got growth in microsporangia (anthers). They are borne on microsporophyll (stamens). Microsporophylls are organized on the central axis of the male cone spirally.
Microsporangia have a microspore mother cell; each cell is divided by meiosis to form four haploid microspores. Microspore is depressed by the wind.
- Megaspores and Megasporangia
The megaspores (embryo sac) are produced in megasporangia (ovule). They are born on the megasporophylls (Carpels). The views are covered by a single integument.
It has a single megaspore mother cell. The cell divides meiotically to form four megaspores.
These megaspores degenerate; only one remains functional.
Microspore represents the beginning of the Gametophyte stage. It has only one prothalial cell, It Cuts of the generative cell. This abundant cell represents reduced antheridium.
Generative cell divides into a basal stroke cell and body cells; body cells give rise to two male gametes. Pollen tube carries the male gametes (pollen grain) to the oosphere. Male gametes are non-motile except for specific primitive form as cycads.
The gametophytic stage starts with the functionality of megaspore. Superficial cells give rives to two or more simple archegonia toward the micropylar end; each archegonium had a simple neck sale and venter. The venter had an egg and a ventral canal cell.
Pollination is the transfer of pollen grain from the male cone to the micropylar end of the ovule on the female cone.
It occurs by the wind; in some cases, it may occur by animals. A sticky fluid exudes from the micropile of the ovule. It is called pollination drop.
The microspore wall adheres to this drop of the flood. The liquid becomes dry. It pulls the male gametophyte down into the ovule. After that, the micropyle is closed.
Formation of pollen tube and fertilization
In some primitive gymnosperms, well-developed Chambers are present at the free apical and of the nucellus. The Chambers contain liquid flagellated Sperm to swim in it.
Such Chambers are absent in another gymnosperm like pinus. These plants only come in direct contact with the surface of the nucellar beak by pollination.
The male gametophyte produces a Pollen tube. It is an extension of the inner wall of pollen, which grows down into the nucellar tissue. It enters the archegonium and leaves the non-motile Sperm directly into it.
The bustle of the Pollen tube will burst At the time of fertilization. Elaborates on the two flagellated Sperm above the female gametophyte; one or both sperms may enter an archegonium. Only Sperm fertilizes the egg.