320 DECLINE OF THE GAMETOPHYTE 



also the considerable growth of the sporophyte of the Bryales 

 and the final development of the spore mother cells In a definite 

 and restricted region In the capsule. In the Pterldophyta, the 

 sporophyte not only becomes larger and more highly differenti- 

 ated, but the formation of the spores is often deferred for years 

 and the spore mother cells are localized in special organs. This 

 postponement of the spore formation ma}^ appear at first sight 

 as a disadvantage since the sporophyte is exposed to many dan- 

 gers during the long period of preparation for Its work. However, 

 the delay Is more than compensated for by the large number of 

 spores that are finally formed and also by the fact that the sporo- 

 phyte does not entirely perish as In previous cases, but lives on, 

 sending up annually new spore-bearing leaves. It would appear 

 that a point had at last been reached In the evolution of the 

 sporophyte where it Is so well organized as to ensure Its exist- 

 ence. It can consequently with safety defer the formation of 

 spores until well-developed roots, stems and leaves have been 

 formed. The independent existence of the sporophyte brings 

 out very clearly the two phases In the life history of the plant 

 which we call the alternation of generations. The formation of 

 the spores marks the beginning of the gametophyte or sexual 

 generation, which ends with the formation of the gametes. The 

 formation of the gametospore through the fusion of the gametes 

 starts the sporophyte or asexual generation which ends with the 

 division of the spore mother cells. 



Why has not the gametophyte varied as well as the sporophyte? 

 Perhaps the simplicity of its structure has become fixed by in- 

 heritance from a long line of algal ancestors. The occurrence 

 in the Bryophyta and Pterldophyta of motile male gametes and 

 the consequent necessity of water for fertilization, points to the 

 inheritance of this peculiarity from aquatic ancestors. It is evi- 

 dent that there can be no considerable modification of the gameto- 

 phyte, as long as this feature Is retained, without greatly decreas- 

 ing the possibilities of fertilization. Consequently the gameto- 

 phyte must remain of necessity a primitive structure. The 

 sporophyte, on the other hand, was accustomed from the first to 

 terrestrial conditions, or to -such conditions as were unfavorable 



