THE REPRODUCTIVE STRUCTURES 97 



but is itself soon absorbed by the encroaching gameto- 

 phyte, which then draws upon the inner fleshy layer 

 until all its nutritive substances have been appropriated, 

 and only a dry membrane composed of the vascular 

 strands and some dead cell walls remains. 



At an early stage in the development of the female 

 gametophyte in Dioon edule, when it is about an eighth 

 of an inch in diameter, a few of the outer cells in the 

 upper portion become noticeably larger than the rest. 

 These cells are called "archegonium initials," because 

 they develop into the archegonia which produce the 

 eggs (Fig. 41). 



Soon after the archegonium initial appears it divides, 

 forming two very unequal cells, the upper being much 

 smaller. This upper cell then divides, and the two 

 resulting cells are called neck cells (Figs. 42 and 43). 

 The lower cell, which is called the "central cell," does 

 not divide but increases greatly in size, sometimes reach- 

 ing a length of an eighth of an inch. During this growth, 

 which extends over a period of several months, the proto- 

 plasmic content of the central cell increases, and in the 

 later stages of its growth various food materials accumu- 

 late. These come from the neighboring cells of the 

 female gametophyte and are passed into the central cell 

 by a layer of modified cells called the "jacket," which 

 surrounds the central cell just as a jacket, previously 

 referred to, surrounded the entire gametophyte. After 

 the central cell has reached its full size and has become 

 filled with nutritive materials, its nucleus divides, but 

 no wall is formed between the two resulting nuclei. The 

 upper nucleus, called the ventral-canal nucleus, imme- 

 diately disorganizes, but the lower increases immensely 





^m I 



