No. 460.] STUDIES ON PLANT CELL. V. 25 1 



is substituted for the union of gamete nuclei but the phylogenetic 

 and evolutionary aspects of sexuality are disregarded. Also, the 

 nuclei that fuse are sometimes very closely related, which is a 

 condition generally avoided in sexual processes except where 

 peculiarities of habit make close inbreeding necessary. It is 

 true that large groups, such as the Basidiomycetes, perhaps 

 certain regions of the Ascomycetes, some Phycomycetes, and 

 some forms of the higher plants and algae seem to have given up 

 normal sexual processes but there is much evidence that in many 

 cases this loss of sexuality is associated with a certain degree of 

 segregation and with peculiarities of life conditions apart from 

 the normal activities of all organisms or quite different from the 

 ancestral stock. The groups are likely to be distinguished by 

 highly specialized life habits of a sort that make it impossible 

 for inherited sexual organs to function, either through mechan- 

 ical difficulties or because one or both degenerate. It seems to 

 me much clearer to regard all illustrations of Blackman's 

 ''reduced forms of fertilization" under the general term of 

 apogamy even though it may be clear that they are physio- 

 logical substitutes for sexual acts and to reserve the term fertil- 

 ization for the union of gametes which can always be clearly 

 identified through morphology in ontogeny and phylogeny. The 

 success of a group even though ancestral sexual processes may 

 be suppressed does not enter into a problem which is at bottom 

 a morphological one. Success is relative and we really have no 

 means of estimating its degree save by actual experiment. It is 

 not likely that any biologist would claim that sexual degenera- 

 tion is advantageous to any species although the organic world 

 is full of forms which have dispensed with sexuality and still 

 hold their places. These are the reasons why I have grouped 

 cell unions and nuclear fusions as sexual and asexual on a mor- 

 phological basis founded on phylogenetic principles and why in 

 Section V, we shall devote some attention to the substitutes for 

 sexuality under the head of apogamy. 



The Ascomycetes present a phenomenon of nuclear fusion 

 within the ascus which may properly be considered at this time 

 since there is a certain resemblance to the nuclear fusions in 

 the teleutospore and basidium. Dangeard ('94-'95b) gave the 



