THE PHYLOGENY OP GYMNOSPERMS 173 



The origin of both Gymnosperms and Angiosperms is bound 

 up with the evolution of heterospory and the seed habit, a state- 

 ment concerning which is presented in the following quota- 

 tion : * " The evolution of heterospory seems simple enough. 

 The physiological differentiation of the spores was complete 

 when prothallia became persistently dioecious. This division 

 of labor is to be expected in the case of two such distinct func- 

 tions as the production of antheridia and of archegonia. A 

 prothallium producing both sex organs equally well may be 

 regarded as in a state of equilibrium, an equilibrium which is 

 ■disturbed by any conditions which favor the production of one 

 sex organ rather than the other, in this case probably nutritive 

 conditions. This disturbance of the equilibrium of a bisexual 

 prothallium would certainly find expression first in a dioecious 

 tendency, and finally in a dioecious habit. With the habit 

 once fixed the morphological differentiation of spores becomes 

 inevitable, since the nutritive requirements of the two pro- 

 thallia are so different. The evolution of heterospory seems 

 to be one of the simplest of selective processes, with inequalities 

 of nutrition to furnish the variations. Prom this point of view 

 it would seem natural to expect that it may have been derived 

 frequently from homospory. 



" The retention of the megaspore, however, does not seem to 

 be so simple a problem. In a certain sense it is correlated with 

 the reduction of the gametophyte, since retention would not 

 seem practicable until reduction had proceeded far enough to 

 make the gametophyte endosporic. Even greater reduction, 

 however, is attained by the male gametophyte, but the spore is 

 shed. It should be noted that even in the case of the microspore 

 the male gametophyte is usually completely organized before 

 pollination; but the fact remains that the reduction does not 

 compel retention. 



" It has seemed to me that this phenomenon is to be ex- 

 plained by the law of progressive sterilization. This law cer- 

 tainly finds expression in the megasporangia of heterosporous 

 Pteridophytes, in which the sterilization of mother cells is con- 

 spicuous. This method of increasing the nutrition of the fertile 

 cells is too common a phenomenon to need illustration; but it is 



* Coulter, John M. The Origin of Gymnosperms and the Seed Habit. 

 Bot. Gaz. 26 : 153-168. 1898. 



