THE STANDPOINT OF BOTANY 67 



number of chromosomes, which in some groups 

 of plants may vary from species to species, seems 

 to be practically a fixed number in the whole 

 assemblage. There seems to be among them lit- 

 tle or no visible response in nature to changing 

 conditions of the most extreme kinds. It would 

 iseem that selection among these relatively inva- 

 riable forms can hardly be more than the accident 

 of crowding. Certainly one can lay hold of no 

 kind of variation in nature that even suggests 

 the coming characters of another species, much 

 less of another genus or family. And yet the 

 group as a whole shows that certain distinct evo- 

 lutionary tendencies have been worked out in a 

 progressive way. Students of the group may 

 differ as to the details of the phylogenetic his- 

 tory, but there is no difference of opinion as to 

 its general features. Some of these general fea- 

 tures may be instructive in this connection. 



The plant which produces the female sex or- 

 gans, known as the female gametophyte, is not 

 only in the midst of an ovule invested by a thick 

 integument, but is also directly inclosed by the 

 heavy wall of the megaspore that produced it. 

 If any structure is shut away from the influences 

 of a changing environment, it would seem to be 

 this one. And yet, through the whole series of 

 Gymnosperms, this gametophyte shows a pro- 

 gressive transformation. In the most primitive 

 forms it matures as a relatively large mass of 

 tissue, and late in its history the female sex organs 



