82 Mutations and Evolution. 



on the facts of apogamy and apospory in Ferns, and experimentally 

 induced apospory in Bryophytes (Lang, 1901). But even when of 

 natural occurrence, these are generally admitted to be exceptional 

 conditions of recent development. The careful cytological studies 

 of apogamy and apospory, however, led Farmer and Digby (1907) to 

 the conclusion that these phenomena leave the question of alternation 

 essentially where it was. It need only be pointed out here that the 

 theory of homologous alternationof generations in plants corresponds 

 to such a view as would restrict the significance of recapitulation to a 

 great degree ; while an important phase of the antithetic theory is 

 that it implies a lengthening of the sporophyte generation in 

 connection with the adaptation of plants to a terrestrial habitat. 



In his original studies of apogamy and apospory, Lang (1898) 

 carefully refrained from giving his views a bias in either direction. 

 More recently (Lang, 1909) he has propounded an interesting con- 

 ception of homologous alternation from an ontogenetic point of 

 view. 1 Briefly the view begins with the concept of the species cell, 

 based on the fact that any cell of the species is potentially able to 

 reproduce the whole plant. This being the case, "the haploid and 

 diploid germ-cells have potentially the same morphological 

 properties." But they are believed to develop different generations 

 because the germ cell is exposed to different conditions in initiating 

 the two generations. This interesting suggestion, however, 

 encounters many serious difficulties, some of which were pointed 

 out in a criticism by V. H. Blackman (1909). 



It cannot be said that the homologous view has received wide 

 support as regards Archegoniates and their descendants, but on the 

 other hand important evidence has developed for the occurrence of 

 homologous alternation in Algae. It is significant that in this group 

 the gametophyte and sporophyte generations develop under the 

 same relatively uniform conditions, exposed to sea water, and they 

 are morphologically alike. Without discussing the subject in any 

 detail, 1 it may be pointed out that Yamanouchi (1906) found in the 

 red alga Polysiphonia an alternation of generations, which he 

 regarded as antithetic, between tetrasporic plants having 40 chromo- 

 somes and sexual plants with 20, the cystocarp being a part of the 

 sporophytic phase. Lewis (1909), in a study of another of the 

 Rhodophyceae, Griffithsia, has taken up a more advanced position 



1 See also the discussion on alternation of generations at the Linnean 

 Society, published in NEW PHYTOL. 8 : 104-116, 1909. 



1 For recent discussions of the life-cycles in Algae see Fritch (1916) and 

 Davis (1916). 



