420 



THE AM ERICA X NATURALIST. [Vol. XXXVII. 



phases as either homologous or antithetic They are liomologous 

 when they can be made to develop indefinitely the same form of 

 the plant successively, antithetic when the sexually formed spore 

 always gives rise to a distinctly new phase. This test of the 

 conditions in the life histories of the thallophytes is being gener- 

 ally applied throughout the group as opportunities are presented 

 and has led to some interesting results. 



Some forms, that at one time were thought to present tenden- 

 cies towards an alternation of generation, are now known to give 

 merely a succession of homologous phases. The zygospore of 

 the moulds (Mucorales) and the eggs of Vaucheria, the Sapro- 

 legniales and probably the Volvocaceai may on germination pro- 

 duce a series of asexual generations or they may pass at once 

 back to the sexual plant according to the environmental condi- 

 tions. So there can be no antithesis of sporophyte and game- 

 tophyte among these types. These facts have been thoroughly 

 established experimentally by Klebs and illustrate clearly the 

 method by which the problems are attacked. However, the 

 j letail * <>f the environmental conditions under which such plants 



Again, certain thallophytes whose sexuality is highly devel- 

 oped present life histories with but one phase, a sexual plant, 

 and without the least tendency towards the development of a 

 sporophyte generation. Notable examples are found in the 

 Characea? and Fucacea? where the eggs on germination produce 

 a sexual plant like the parent. The fact that the oospore of 

 Chara develops a small filamentous structure preliminary to the 

 establishment of the characteristic later growth does not affect 

 the general principle. Investigations on the nuclear history of 

 Fucus report a chromosome reduction after a peculiar manner 

 that cannot be reconciled with the conditions known for groups 

 above the bryophytes. The number of chromosomes is reduced 

 one half just before the differentiation of the oogonium so that 

 the gametes have half the number of chromosomes characteristic 

 of the parent plant. After fertilization the egg has again the 



ent. The life history may be outlined in this manner. 



