io6 THE LIVING CYCADS 



become widely separated, the pollen grains fall out from 

 the sporangia and are blown about by the wind, and 

 those happening to reach female cones sift in among the 

 cone scales, which are rather loose for a few days, while 

 the male cones are shedding their pollen. 



At just this period, while the pollen is sifting in, some 

 cells at the tip of the ovule are breaking down and secret- 

 ing a clear, mucilaginous substance which oozes out as 

 a sparkhng droplet. As the droplet dries, whatever 

 pollen has fallen upon it is drawn down into the chamber, 

 called the pollen chamber, formed by the disintegration 

 of the cells which produced the mucilaginous droplet. 

 Within the pollen chamber the development of the male 

 gametophyte continues for several months before it 

 finally results in the formation of sperms. The details 

 of spermatogenesis and sperms, which are more complex 

 in cycads than in any other group of plants or in animals, 

 will now be considered. 



As soon as the pollen grain arrives in the pollen 

 chamber the interrupted germination is resumed. The 

 exine at the apex of the pollen grain is ruptured, and the 

 intine protrudes, forming a long tube called the pollen 

 tube, which penetrates deeply into the tissues surround- 

 ing the pollen chamber, while the chamber itself con- 

 tinues to enlarge by disintegration of adjacent tissues 

 (Fig. 54). Almost immediately after the pollen grain 

 reaches the pollen chamber the generative cell divides, 

 giving rise to a ''stalk cell " in contact with the prothallial 

 cell, and a "body cell" which will finally produce two 

 sperms; but several months elapse between the forma- 

 tion of the body cell and the division by which it pro- 

 duces the sperms. 



