September 23, 1898.] 



SCIENCE. 



383 



intended to relate the embryo properly to 

 its food supply, and not of phylogenetic 

 significance. 



The testimony of history and morphology 

 seem to combine in pointing to a very 

 generalized Palseozoic type as the origin of 

 gymnosperms. This type is characterized 

 by its advancement towards seed produc- 

 tion rather than by its habit, which must 

 have been extremely varied to have given 

 rise to such types as cycads and conifers. 

 The usually recognized Cordaites show but 

 • one tendency of a much more extensive 

 group, for which the name Cordaites may 

 be extended for convenience. Cordaites in 

 this larger sense occur in such association 

 with groups of homosporous eusporangiate 

 Filieales, and approach them so much 

 nearer in the important morphological 

 structures mentioned than they do living 

 heterosporous Filieales, that an independ- 

 ent heterosporous line is suggested. If 

 such be the case, in the passage from the 

 Marattia forms to the Cordaites forms both 

 heterospory and the retention of the mega- 

 spore were attained, and probably sipho- 

 nogamy begun. 



THE SEED HABIT. 



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 functions as the production of 

 antheridia and 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 con- 

 ditions 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 an expres- 

 sion first in a dicBcious tendency, and finally 

 in a dioecious habit. With the habit once 



fixed the morphological differentiation of 

 spoi'es becomes inevitable, since the nutri- 

 tive requirements of the two prothallia 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. From 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 reten- 

 tion would not seem practicable until reduc- 

 tion had proceeded far enough to make the 

 gametophyte endosporic. Even greater re- 

 duction, 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 reduction does not 

 compel retention. It has seemed to me 

 that this phenomenon is to be explained by 

 Bower's law of sterilization, developed in 

 reference to the strobilus. This law cer- 

 tainly finds expression in the megasporangia 

 of heterosporous pteridophytes, in which 

 the sterilization of mother cells is conspicu- 

 ous. This method of increasing the nutri- 

 tion of the fertile cells is too common a 

 phenomenon to need illustration ; but it is 

 a tendency that would seem very consistent 

 with the development of megaspores, whose 

 peculiar work holds so definite a relation to 

 abundant nutrition. For this very reason 

 high numbers of microspores may be con- 

 tinued, and a diminishing number of mega- 

 spores produced. This would reach its 

 culmination in the production of but a sin- 

 gle megaspore by a sporangium, and a 

 proportionate increase in the size of the 

 megaspore. With the development of a 

 single spore imbedded in sterile tissue, 

 shedding becomes not ooly mechanically 



