Gametophytic generation Gnetales

Gametophytic generation Gnetales:-

Male gametophyte:-

> Microspore (pollen grain) is the first cell of the male gametophyte. The outer exine is thick-walled and the inner intine is thin-walled.

> In Gnetum, a microspore consists of a prothallial cell, a generative cell, and a tube nucleus. The generative cell divides within the pollen tube into two non-motile naked male gametes.

Female gametophyte:- 

> The functional megaspore is the first cell of the female gametophyte.

> In Gnetum, the megaspore mother cell takes part in the development of the gametophyte. It’s containing four megaspore nuclei and forms a female gametophyte (embryo sac). The four megaspores divide continuously and form thousands of female nuclei. The embryo sac is tetrasporic, in which 256-1500 nuclei are found.

> At this time cell wall is formed from the chalazal end. A few free nuclei in this part become larger and differentiate as eggs. Archegonia are not formed in Gnetum female gametophyte.

Fertilization:- In Gnetum, the pollen tube comes in contact with the female gametophyte and the male gamete enters the embryo sac. One gamete attaches to the female nucleus and forms a zygote. While the second gamete attaches to the other nucleus and as a result double fertilization occurs. At this time, the endosperm is formed due to wall formation at the micropylar end.

Embryo development:- In Gnetum, the zygote divides and forms two cells. Both the cells enlarge and form tubes. Each tube divides and forms suspensor and embryonal cells.

Seed germination:- Seeds are endospermic. Process of germination of the seed is hypogeal in Gnetum.