426 THE AMERICAN NATURALIST. [Vol. XXXVII. 



ably a modified structure which demands some other form of 

 expression than the vegetative activities characteristic of the 

 species. Such a phase, intercalated after a sexual act, would 

 start its peculiar period of activity because the protoplasm had a 

 different chemical and physical composition from that of the par- 

 ent plant. Its fundamental characteristics are therefore per- 

 formed through the fusion of the gametes. 



This view of the origin of the sporophyte as primarily the 

 result of sexuality carries with it a comprehensive definition of 

 the phase. It becomes a generation, always intercalated after 

 the sexual plant, which is called a gametophyte by way of dis- 

 tinction. It is a generation always antithetic with the game- 

 tophyte because of potentialities within itself. When once 

 thoroughly established, the sporophytic generation would be 

 expected to have the power of developing the gametophyte only 

 after it had passed through its characteristic history. The 

 sporophyte could never be an homologous generation with the 

 gametophyte in the way that succeeding generations of Thallo- 

 phytes by asexual methods of reproduction are homologous with 

 one another. 



We should not expect the sporophyte to have arisen with the 

 potentialities of immediate and extensive growth developments 

 but rather as a small beginning such as may well be illustrated 

 in Ulothrix, Sphaeroplea and Oedogonium. Here the peculiari- 

 ties of the sexually formed spore seem scarcely more than an 

 increased vigor that expresses itself in the formation of a number 

 of reproductive cells. However, we know nothing of the nuclear 

 activities in these forms and the assumption that the protoplasm 

 of the sexually formed spore is structurally different from that 

 of the gametophyte is speculation but seemingly reasonable. As 

 has been stated, there are several thallophytes whose sexually 

 formed spores develop directly into the sexual plant and indeed 

 the egg of Oedogonium has been known to grow directly into a 

 filament (Fig. 4 d). These forms must also be studied in com- 

 parison with the types in which the sporophyte generation seems 



union of the gametes gives a zygospore (see Fig. 5 a) that finally 



