174 MORPHOLOGY OF SPERMATOPHYTES 



a tendency that would seem very consistent with, the develop- 

 ment of megaspores, whose peculiar work holds so definite a re- 

 lation to abundant nutrition. For this very reason high num- 

 bers of microspores may be continued, and a diminishing number 

 of megaspores produced. This would reach its culmination in 

 the production of but a single megaspore by a sporangium, and 

 a proportionate increase in the size of the megaspore. With the 

 development of a single spore imbedded in a sterile tissue, shed- 

 ding becomes not only mechanically difficult, but meaningless, 

 since the necessity for scattering abroad of gametophytes, to 

 avoid competition, has disappeared. It is further true that the 

 development of such a spore involves nutritive supplies from 

 numerous neighboring cells, and a certain amount of retention 

 becomes necessary for this reason. Still further, the advantage 

 to a single megaspore in being retained, thus securing more abun- 

 dant outside nutrition during germination, would fix the habit 

 if any selective process were at work. For these various rea- 

 sons it would seem evident that when the sterilization of a mega- 

 sporangium had reached its extreme limit, by organizing a single 

 spore, retention is likely to follow sooner or later. If this line 

 of reasoning be true, the seed habit might have been developed 

 in any heterosporous line." 



Some unpublished results obtained by Miss F. M. Lyon in 

 a study of species of Selaginella are of interest in this connec- 

 tion. The usual condition was found in which the one or two 

 fully formed megaspores were retained in the sporangium until 

 they germinated. Not only this, but the eggs were fertilized 

 and complete embryos formed within the retained megaspores. 

 Fertilization was effected not by the entrance of sperms into the 

 sporangium, but by the entrance of microspores, which dis- 

 charged sperms through papillate protuberances. Moreover, 

 the embryos thus formed escaped from the megaspores and 

 developed into young plantlets, a strobilus often being cov- 

 ered with sprouting plantlets. It should be remarked that such 

 strobili had fallen from the parent plant, so that the megaspores 

 had been physiologically separated from the individual which 

 produced them, but the permanent retention of the megaspore 

 within the sporangium, without interfering with germination, 

 fertilization, or the development and escape of the embryo, are 

 facts which contain a morphological suggestion. 



