THE GYMNOSPERMAE : CYCADALES, ETC. 727 



Soon after pollination several peripheral cells at the micropylar end 

 of the prothallus begin to enlarge. These are the archegonium initials 

 and they vary from two to eight, with three as the commonest number. 



The initial cell divides transversely into a neck cell and a central cell. 

 The former divides again vertically into two cells, which constitute the neck. 

 The central cell enlarges to about 4 mm. long and forms the oosphere cell. 

 At first its protoplasm is scanty and the centre is occupied by a large vacuole. 

 When this growth phase is over the protoplasm increases and fills the whole 

 cell, the vacuole disappearing. The oosphere cell wall becomes thick and 

 amyloid and develops open pits connecting with the adjacent prothallial cells. 

 These cells are organized as a special jacket layer, and from them 

 food materials pass into the oosphere cell with which their protoplasm is 

 connected through the large pits. The turgidity of the oosphere cell is so 

 great that its cytoplasm protrudes into the jacket cells as " haustoria," and 

 eventually absorbs all the contents of these cells. The cytoplasm of the 

 oosphere cell becomes very dense and turbid, and contains starch, oil and 

 protein reserves. This nutritive phase lasts from two to three months. 



Finally the nucleus of the oosphere cell divides, forming a short-lived 

 ventral canal nucleus and an oosphere nucleus, which enlarges until it 

 may be half a millimetre across and visible to the naked eye. Its chromatin 

 becomes hidden by other contents, probably reserve protein. The archegonium 

 is now ready for fertilization. The prothallus tissue around the archegonium 

 grows upwards and forms a shallow basin, the archegonial chamber, in 

 which the archegonial necks lie (Fig. 732). The megaspore membrane is 

 ruptured above the chamber, and the bottom of the pollen chamber in the 

 nucellus also becomes disorganized, so that a free passage to the archegonia 

 is opened for the pollen. A drop of mucilaginous fluid fills the micropyle 

 and exudes as a pollination drop outside the ovule. In this the pollen grains 

 are caught and, as it dries up, are drawn down into the pollen chamber. 

 There the pollen grains are sealed in by the dried liquid. 



The Male Gametophyte 



The pollen germinates in about a week after pollination. The antheridial 

 cell divides into a stalk cell and a body cell which together represent an 

 antheridium. A pollen tube grows out from the thin- walled apex of the 

 microspore, but this is not an agent of fertilization as it is in Angiosperms. 

 It is simply a nutritive haustorium and it penetrates the nucellus laterally, 

 gradually digesting and breaking down tissues above the archegonial chamber. 



No further development takes place until the archegonia are mature, a 

 further period of about four months. The prothallial cell meanwhile 

 penetrates into the stalk cell, reducing it to the form of a ring. When 

 fertilization time approaches, the body cell enlarges enormously and two 

 granular blepharoplasts, surrounded by radiating fibrils, appear in it, which 

 take up positions transversely, one on each side of the nucleus. These 

 blepharoplasts are concerned in the production of cilia, and they have their 



