SPERMATOPHYTES 



125 



FIG. 99. Two views of the sperm of a Cycad, 

 showing its spiral form and many cilia. 



is called pollination, and in the Gymnosperms the agent of 

 this transfer is the wind. Such plants, therefore, are said 

 to be wind-pollinated. 



Before the pollen grains (microspores) leave the pollen 

 sacs, the male gametophyte has begun to develop within 

 them (Fig. 97, D), and 

 after the pollen has 

 reached the nucellus, 

 the gametophyte con- 

 tinues to develop, form- 

 ing an antheridium, 

 which in almost every 

 Gymnosperm produces 

 two sperms. In the 

 Cycads these sperms 

 have cilia and swim (Fig. 99), just as do those of the 

 Pteridophytes ; but in the Conifers the sperms have no 

 cilia, and of course do not swim. 



74. Fertilization. After pollination has been accom- 

 plished, and the pollen grain (microspore) of the pine, with 

 its contained male gametophyte, is resting on the tip of the 

 nucellus, the sperms and the eggs are separated from one 

 another by the mass of tissue that forms the top of the nucel- 

 lus. This tissue must be penetrated, and the pollen grain 

 (really the male gametophyte within it) puts out a tube 

 (pollen tube) which grows into it, crowding its way among the 

 cells, absorbing nourishment from them like an internal 

 parasite, and finally reaches the egg (Fig. 98, B). In the tip 

 of the advancing tube the two sperms are tying, and when the 

 vicinity of the egg is reached, they are discharged from the 

 tube, and one of them penetrates the egg and the two nuclei 

 fuse. The result, of course, is a fertilized egg lying deep in 

 the ovule. 



Pollination and fertilization should not be confused, as 

 they often are. When pollen is carried to an ovulate cone 



