134 



GYMNOSPERMS 



The 2 sperms begin to move while still within the sperm mother- 

 cells, but soon the peripheral part of the wall between them breaks 

 down and leaves them free within the wall of what was called the 

 "body cell" before its division. This cell increases in size, and the 

 sperms, which stick together, move around in the cavity. The cilia, 

 at first, move slowly and then more vigorously, so that the pair of 

 sperms roll about in the rather small space. The movement of the 



Fig. 144 



Fig. 14s 



Figs. 144 and 145. — Microcycas calocoma: fig. 144, pollen tube with four body cells, 

 and stalk cell in telophase of division to form another; fig. 145, pollen tube with three 

 body cells and stalk cell in prophase of division to form another; p, prothallial cell; s, 

 stalk cell; b, body cells. No centrosomes have been found in the stalk cell or in the body 

 cell immediately after its formation; X1280. — After Dr. Dorothy G. Downie.'"-> 



cilia is accompanied by pulsating and amoeboid movements, and 

 when the apex of a sperm strikes the wall, there is a sudden, convul- 

 sive movement which makes one think of VorliccUa. They swim for 

 an hour or more in the cavity of the body cell before they separate, 

 and then for another half-hour before they escape into the general 

 cavity of the pollen tube. When free from each other, the general 

 movement is straight ahead, with a rotation upon the longer axis. 

 They swim up into the tube as far as the diminishing diameter will 

 permit, and then come back. When a nucellus is inverted, with a 



