THE REPRODUCTIVE STRUCTURES iii 



long, spiral band. This band, lying in the protoplasm 

 between the nucleus and the periphery of the sperm, 

 develops thousands of long, slender cilia which pierce 

 through the protoplasm and extend some distance 

 beyond it (Fig. 59). The cilia are the motile organs 

 of the sperm, and they enable it to swim with consider- 

 able vigor. 



A constant and striking feature in the pollen-tube 

 structures of all of the cycads is the behavior of the 

 prothallial cell. From an early stage it continues to 

 press up into the stalk cell, until it finally looks as if it 

 were entirely surrounded. The function of these two 

 cells is not very well understood, but, like the rest of the 

 tube, they are abundantly supplied with starch, and 

 much of this is used up by the body cell and the sperms 

 derived from it. 



