CHAP, i The Nature of Alternation of Generations 57 



differentiation of its whole mass into seta and capsule. 

 The whole group is, however, characterized by the fact 

 that the sporogenous tissue of each individual is normally 

 a continuous mass. 



Turning to the Pteridophyta or Vascular Cryptogams, 

 there is not any doubt that progressive sterilization 

 will account for many stages in their evolution. They 

 differ in degree from the Bryophyta, for the proportion 

 of sporogenous to vegetative tissue is but small and the 

 sporogenous cells are either separated into small masses 

 or are distributed singly. There has been a great develop- 

 ment of the external form, which, originating in a relatively 

 simple strobiloid structure, derived from a simpler sporo- 

 gonium-like fructification, has become more complex 

 through the putting out of appendicular organs. This was 

 no doubt at the outset a physiological rather than a morpho- 

 logical development, for the adoption of the foliar habit 

 in the group, correlated further with the acquirement of 

 a subterranean absorbing system, brought with it great 

 advantages, ensuring the physiological independence of the 

 sporophyte. 



Passing then to the question of the suggested intercalation 

 of the sporophyte, Bower examined the question from the 

 broader point of view of the relations of the two forms to 

 environment, pointing out that physiological relationships 

 were most probably determining factors, though he admitted 

 that the conclusions reached must be checked by considera- 

 tions of phylogeny . Taking the fern as his starting point, he 

 pointed out the evident adaptation of its gametophyte, the 

 prothallus, to a watery or at any rate moist environment, 

 showing as it does relationship to the Algae in its simple 

 structure and in the mode of fertilization of its ova. It is 

 indeed semi-aquatic. The sporophyte, the fern plant, is 

 on the other hand robust, with well differentiated tissues, 

 adapted to the more exacting conditions of life in air. 



