136 HEREDITY AND EVOLUTION IN PLANTS 



axis and foliar organs were both derived from an ancestral 

 thallus, branching dichotomously.^ 



The structural differences in the two generations are, 

 on the basis of this hypothesis, considered as due almost, 

 if not entirely, to differences in environment, the main 

 factor being the gradual transition from aquatic to dry- 

 land surroundings. Where the environment is uniform 

 and the same for both generations, as for Dictyota, the 

 gametophyte and sporophyte are identical in external 



Fig. 68. — Dictyota dicholoma. Left, sporogonial plant; right, sperma- 

 gonial (gametophytic) plant. (After W. D. Hoyt.) 



organs and general appearance (Fig. 68). In any event 



the hypothesis postulates a homology between the various 



organs of the two generations, however much these parts 



may differ in external appearance as a result of individual 



variation and environmental influence. 



110. A Third Hypothesis. — Viewing the matter from 



the standpoint of individual development (ontogeny) Lang 



has developed the ontogenetic hypothesis oj alternation. 



' In a forked manner, resulting from the occurrence of two growing 

 points at the tips of the axes. 



