l82 THE MORPHOLOGY OF PTERIDOPHYTES 



those removed in later stages of development grew into 

 normal sporophytes. Whether the environment is entirely 

 responsible, however, for the normal regular alternation of 

 generations has been questioned. BelP'^ suggests that there 

 must be some internal factor at work and looks upon 

 gametophyte and sporophyte as two levels of complexity, 

 reflecting different states of the cytoplasm, which can be 

 accounted for in terms of cell chemistry. This interesting 

 hypothesis should stimulate further research into the causes 

 of alternation of generations in hving plants. 



The present position, then, seems to be that there is no 

 fundamental distinction between gametophytes and sporo- 

 phytes, since they can be induced to change from one to the 

 other in either direction. They are 'homologous', as far as 

 can be judged from living plants, and one is led to speculate, 

 therefore, that they were probably ahke in form and structure 

 in the earliest ancestors of land plants. Merker's suggestion,^^ 

 already mentioned in Chapter 2, that the horizontal axes of 

 the Rhyniaceae were gametophytes, instead of sporophytic 

 rhizomes, is of enhanced interest, therefore, because if con- 

 firmed it will provide the only kind of evidence which can 

 really settle the controversy. As with most problems of 

 macro-evolution, it is the palaeobotanist who has the key 

 within his reach. 



