THE REPRODUCTIVE STRUCTURES 105 



the exine and the inner the inline. The exine is hard and 

 dry, but the intine consists largely of cellulose and is 

 capable of extreme growth. There is one nucleus, an 

 abundance of protoplasm, and some starch (Fig. 52). 



Germination begins within the spore before it is shed. 

 The first division results in the formation of two very 

 unequal cells, the one nearest the base of the spore being 

 much smaller. This small cell, called the "prothaUial 

 cell," is strictly comparable with the prothallium of a 

 fern, and also with the female gametophyte of the cycad, 



Figs. 52-53. — Dioon edule: Fig. 52, microspore, showing inner and 

 outer spore coats, several starch grains, and a large nucleus; Fig. 53, 

 later stage, showing prothallial cell at the bottom, immediately above it 

 the somewhat larger "generative cell," and at the top the large tube 

 nucleus. Very highly magnified. 



which consists of miUions of cells. ^ The prothaUial cell 

 increases in size but does not divide. The larger cell 

 divides unequally, forming a small cell called the genera- 

 tive cell, and a larger cell called the tube cell (Fig. 53). 

 At this stage the male gametophyte is ready to be shed 

 from the sporangium. At the shedding stage the male 

 gametophyte, not only in cycads but in all flowering 

 plants, is called the pollen grain. 



As soon^ as this three-celled stage is reached the 

 axis of the cone elongates, so that the microsporophylls ^ 



