THE REPRODUCTIVE STRUCTURES 95 



The plant developed from the spore of an ordinary 

 fern is almost always called the prothallium, but the 

 plant developed from the germinating megaspore of 

 Selaginella is just as regularly called the female gameto- 

 phyte, although it is strictly comparable with the pro- 

 thallium of the fern. In the cycads the plant developed 

 from the megaspore is usually called the endosperm. It 

 is unfortunate that there should be so many names for 

 the same thing; one name would be sufficient for all 

 three cases. Since the plant developed from the spore 

 bears the gametes — as the eggs and sperms are called — 

 we prefer the term gametophyte, and we shall call the 

 plant developed from the megaspore the female gameto- 

 phyte, and that developed from the microspore the male 

 gametophyte. 



The development of the female gametophyte is more 

 easily illustrated than described. At the very beginning 

 a point which many fail to realize is that the megaspore 

 is really a spore like that of the fern or Selaginella, and 

 that instead of falling out from the sporangium (or ovule) 

 it remains within, germinates there, and never escapes. 

 It is this retention of the megaspore and the female 

 gametophyte developed from it that distinguishes the 

 seed plants from the ferns and their allies. However, 

 Selaginella shares this seed-plant character to such an 

 extent that no definition has yet been devised which will 

 include all seed plants and exclude some species of 

 Selaginella. 



When the megaspore begins to germinate it does not 

 form a cellular tissue, but its nucleus divides without 

 the formation of a wall, the two resulting nuclei divide, 

 the four thus formed divide, and the process continues 



