DEVELOPMENT OF PLANTS 305 



There is also to be noted the longer postponement in the for- 

 mation of the spores. In certain of the algae the gametospores 

 produce the spores directly on germinating while in Ricciocarpus 

 a small mass of cells is first formed and certain of these cells 

 soon become spore mother cells. In Marchantia, a larger number 

 of cells are formed by the gametospore and the spore mother cells 

 originate after a few weeks in a definite region of the sporophyte. 

 These features are more noticeable in the Jungermaniales and 

 especially in the long-lived sporophyte of Anthoceros. Recall 

 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 Pteridophyta, 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 may 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 

 21 



