3io 



BOTANICAL GAZETTE [may 



(7, 8, 9),Vaizey (71), Strasburger (65, 67), Beard (3), Campbell 

 (11, 12), Scott (61), Lang (36, 37), Klebs (34, 35), Hartog (27), 

 Coulter (18), Davis (19,20), Williams (73), Blackman (4, 5), 

 Wolfe (77), Lotsy (41,42), Gregoire (25), Chamberlain (16), 

 Christman (17), Oltmanns (49), Harper (26), Yamanouchi (80), 

 and many others. 



Among thallophytes no generalization for the whole group is pos- 

 sible at present, partly because of extreme diversity, and partly on 

 account of the meagerness of our knowledge regarding the life-cycle 

 of the majority of the forms. Different opinions are held con- 

 cerning the nature of the phenomena in various forms, and some 

 even question the existence of an alternation of generations. How- 

 ever, it is now being gradually established by actual investigation, and 

 quite recently cytological proof has been obtained from several forms, 

 as Dictyota, Phragmidium, Nemalion, Polysiphonia, and some others. 



Pteridophytes and bryophytes have been regarded as the best 

 illustrations. Discussion in connection with the pteridophytes has 

 not been in reference to the existence of alternation, but has centered 

 about the question whether it is to be interpreted as of antithetic or 

 homologous origin. 



These two views represent different conceptions as to the origin 

 of the sporoplryte. Those who advocate the theory of antithetic 

 origin regard the sporophyte of pteridophytes as a gradual elaboration 

 from the zygote of some aquatic algal ancestor, a new phase having 

 thus been intercalated in the life-history. This view was first clearly 

 stated by Celakowsky (13,14). Bower (7,8,9) supported it 

 and endeavored to explain it as an adaptation to external conditions. 

 Strasburger (65), restating the position in terms about identical 

 with Bower's, based the theory upon nuclear details. Those who 

 maintain the theory of homologous origin consider that the sporophyte 

 arose as a modification of the gametophyte, and not as a new structure. 

 Pringsheim (55, 56), and more recently Scott (61), Lang (36, 37), 

 Coulter (18), and others advocate the homologous theory. This 

 theory is largely based upon the phenomena of apogamy and apospory 

 and also to a certain extent upon experiments in regeneration. 



When these two theories were proposed, cytological investigations 

 had not yet developed, and even Lang's admirable work did not 



