178 COOK 



FUNCTIONS OF THE APOSPOROUS PROTHALLUS. 



The gymnosperms may be supposed to have the ovules and 

 seeds naked because they are still borne in an endosperm which 

 corresponds to the primitive, vegetatively functional prothallus, 

 though it now remains attached to the much more highly devel- 

 oped double-celled structure which corresponds, in turn, to the 

 capsule of the liverwort and the moss, the frond of the fern and 

 the leafy axis of the club-moss. The ovule and seed of the 

 angiosperm are borne, not on the original vegetative prothallus, 

 but on a structure which, if it arose by apospory, would never 

 have had independent vegetative functions. It need never have 

 been very large, and could have been reduced very readily in 

 becoming more specialized. There was no occasion to assume 

 protective or nutritive duties, since there was always an enclos- 

 ing capsule or carpel. 



The fern and the flowering plant are alike in that their ances- 

 tors can be traced back to the capsules of simple thallose plants 

 like Anthoceros^ but there appears to have been, at some very 

 remote point, a divergence of procedure, the group which gave 

 rise to the ferns and gymnosperms retaining for a much longer 

 period a functional prothallus which the adoption of apospory 

 enabled the ancestors of the angiosperms to completely eliminate. 



Lastly, a certain presumption of probability may be claimed 

 for a suggestion which enables the peculiarities of such plants 

 as JVephrolepis, Begonia and Bryofhylhim to be associated with 

 other phenomena of apospory and thus to find places in normal 

 evolutionary history, instead of remaining mere meaningless 

 "freaks of nature," because they may have appeared to lie far 

 outside the pale of former interpretations. 



