GINKGOALES 205 



is entirely free from the megaspore membrane, the latter is pushed 

 outward by the development of tissue about the archegonial chamber, 

 forming a roof over it. The megaspore membrane becomes differ- 

 entiated into two layers, the inner one being very thin and serving 

 as a base for the transversely placed "rods" of the outer layer, as. 

 already noted in the cycads (p. 139). When the endosperm tissue has 

 become closed at the center of the sac, the megaspore membrane has 

 reached a thickness of over 6 /a. According to Thomson (41, 43), 

 the structure of the membrane is very similar to that of the Cycadales, 

 but the inner layer is not so thick. His general conclusion is that 

 Ginkgo is farther removed from the fern level in this regard than 

 are the Cycadales, and less so than are the Coniferales,. in which 

 the megaspore membrane is variable in thickness and simpler in 

 structure. 



The archegonium initials, usually two in number (figs. 242, 243), 

 but occasionally three, appear very early in the history of the gameto- 

 phyte, the two-celled neck and a considerably enlarged central cell 

 appearing before the tissue lining the embryo sac is equal in depth to 

 the breadth of the central cavity. The development of the arche- 

 gonium is exactly similar to that in the Cycadales, including the 

 organization of the conspicuous nutritive layer of jacket cells. At 

 the beginning of August (in Japan), when the tube nucleus is passing 

 back into the pollen grain end of the tube, as described by Hirase 

 (22), the endosperm begins to develop a beak between the two 

 archegonia. In about two weeks this beak resembles a small column, 

 with its summit against the settling nucellar beak, "like a tent 

 supported by its center pole." This endosperm beak much enlarges 

 and modifies the archegonial chamber described above. Just before 

 fertilization, early in September (in Japan), the small and ephemeral 

 ventral canal cell is cut off, and the egg is matured (fig. 244). In 

 this respect Ginkgo is more primitive than the cycads, in which no 

 wall is formed between the ventral canal nucleus and that of the egg. 

 Ikeno (30) reports that in one instance he observed a ventral canal 

 nucleus that had enlarged pari passu with the egg nucleus, whose 

 maturation resembles that described under Pinus Laricio (p. 266). 

 Hirase states that about twenty weeks elapse between pollination 

 and fertilization. 



