Gametophytes in Ginkgo
Gametophytes in Ginkgo:-
Male gametophyte:-
> Male gametophyte starts developing in situ, i.e. within the sporangium. Microspore is the first cell of the male gametophyte.
> Each microspore is a rounded structure having thin intine and thick exine layers.
> A centrally located nucleus contains one or two nucleoli and remains surrounded by dense cytoplasm. > An un-thickened portion, called pore, is also present in each microspore. It is a region where exine is not covering the intine.
> Two unequal cells are formed by the first mitotic division in the microspore.
> These are called 1st prothallial cell and inner cell. The 1st prothallial cell is smaller than that of inner cell.
> The inner cell again divides forming a 2nd prothallial cell and an antheridial initial. By this time the 1st prothallial cell starts to degenerate.
> The antheridial initial then divides forming a generative cell near the 2nd prothallial cell and a tube cell.
> The tube cell does not divide any further. This is the 4-celled stage of the male gametophyte and the microspores are dispersed by wind at this stage.
> Pollination in Ginkgo biloba takes place sometime in April. At the time of pollination the ovule secretes a mucilaginous substance.
> Further development of the male gametophyte takes place in the pollen chamber. In pollen chamber the generative cell of the 4-celled young male gametophyte divides into a stalk cell, which is close to the 2nd prothallial cell, and a body cell.
> Now the intine protrudes out near the pore and functions as a pollen tube. The pollen rube swells up and advances towards the archegonia by protruding into the nucellus. Just before fertilization the body cells divides into two. Two cells which function as sperms or male gametes.
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Female gametophyte:-
> Out of the four megaspores formed from the megaspore mother cell, only the lowermost remains functional and the remaining three degenerate.
> The nucleus of the functional megaspore migrates towards the micropylar end and divides into two followed by a number of free- nuclear divisions forming hundreds of free nuclei.
> First the anticlinal walls are formed followed by the vertical walls.
> Each cell generally contains one nucleus but in some cells 2-3 nuclei are also seen. Due to more rapid cell divisions on the micropylar end a pole-like structure develops.
> The female gametophyte possesses abundant chlorophyll.
> The development of archegonium starts from the cells towards the micropylar end of the female gametophyte.
> Each archegonium possesses a short neck made up of only four cells and a small venter having a central cell. The central cell later on forms a ventral canal cell and an egg cell.
> Two cells are present in the archegonial neck.
> The pollen tube reaches up to the neck of the archegonium just after its (archegonium) differentiation.
> The tube ruptures releasing the sperms and the other contents in the archegonial chamber.
> A sperm passes through the neck of the archegonium, comes in contact and fuses with the egg nucleus.
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