734 



THE AMEBIC AN NATURALIST [Vol. XLII 



ism, with its potentially unlimited power of regeneration and 

 growth, is extremely plastic and responds promptly to any ex- 

 ternal stimuli. Nevertheless, the facts of comparative morphol- 

 ogy are too evident to be ignored, and although the modern 

 student must carefully check his work by the data furnished 

 from experimental morphology, physiology, and paleontology, it 

 still is evident that the surest clues to relationship must be 

 sought in a comparison of corresponding structures and the facts 

 of ontogeny. 



The phenomena of alternation of generations as exhibited by 

 the higher plants are familiar to the botanist. The plant shows 

 two marked life phases, first the sexual phase, or gametophyte, 

 and second the neutral generation, arising from fertilization, 

 the sporophyte. In all but the highest plants, the seed plants, 

 the gametophyte betrays more or less clearly its aquatic origin. 

 It is usually poorly adapted to resist dryness. Water is neces- 

 sary for fertilization, as the male gametes are ciliated and the 

 mature sexual organs require water for their proper dehiscence. 

 In the lower types, like the simple liverworts, the gametophyte is 

 relatively larger and plays a much more important role in the 

 plant life than does the insignificant sporophyte, which is short- 

 lived and dies as soon as it has shed its spores, whose production 



never an aquatic structure, but receives its water indirectly from 

 the gametophyte with which it is permanently in intimate as- 

 sociation. 



With the increasing specialization of the Archegoniates, several 

 lines of development were inaugurated showing several different 

 types of specialization both in the gametophyte and in the sporo- 

 phyte. The gametophyte reaches its culmination in the higher 

 mosses which seem to show the limit of the possibility of adapting 

 the essentially aquatic gametophyte to life on land. The further 

 development of the land plants is therefore bound up with the 

 elaboration of the terrestrial phase of the plant's life history, 

 i. <:.. the sporophyte. With the increasing importance of the 

 latter there is a progressive reduction of the gametophyte which 



gametophytes of the heterosporous forms like Marsilia or Selagi- 



It is not likely that any existing liverworts represent very 

 nearly the direct ancestors of the Pteridophytes or "Vascular 



