GNETALES 389 



of the endosperm. There can be no (juestion that the emljryo sac 

 of Gnetum is strongly suggestive of that of angiosperms, in which the 

 egg is also organized in the free nuclear stage of the gametophyte; 

 but there is nothing in Gnetum to suggest the remarkable fusion of 

 polar nuclei so characteristic]of the angiospermous sac. It is at least 

 a case of remarkable parallel development. 



In all of the Gnetales the megaspore membrane is very thin, and 

 according to Thomson (21) this confirms the evidence from other 

 structures that the group is relatively modern among gymnosperms. 



THE MALE GAMETOPHYTE 



The development of the male gametophyte of Gnetales was 

 unknown, except for occasional glimpses, until the investigation of 

 Ephedra trijurca by Land (18), and it still remains the only complete 

 account. 



A short time after the rounding off of the spore, its nucleus divides, 

 forming the first prothallial (vegetative) cell. This cell is pressed 

 closely against the wall of the spore by the enlargement of its sister 

 cell, from which it is separated by a wall, the only wall that appears 

 in the spore (figs. 436-441). A second division gives rise to the 

 second prothallial cell and the antheridium initial, which arc not 

 separated from one another by a wall. The second prothallial cell 

 and its nucleus become flattened also by the pressure of the growing 

 antheridium initial, whose nucleus enlarges very much, and upon 

 division produces the generative and tube nuclei. The generative 

 nucleus becomes surrounded by a zone of cytoplasm differentiated 

 from the general cytoplasm, the result being a free naked cell (fig. 

 439). The general cytoplasm still invests the second prothallial and 

 tube nuclei, with no evident boundary between them. The nucleus 

 of the generative cell divides, giving rise to the stalk and body nuclei, 

 which remain invested by the cytoplasm of the generative cell, the 

 body nucleus soon becoming much larger than the other. It is in 

 this condition that pollination occurs (during the first two weeks of 

 April), the spore containing five conspicuous nuclei (two prothallial, 

 stalk, body, and tube) (fig. 441). The short period of this species 

 has been referred to (p. 371), only two months elapsing between the 

 first appearance of the primordia of the sporangia and the five-nucleate 

 grain, which is shed about two weeks later. 



