XV. NATURE OF THE ALTERNATION OF GENERATIONS 573 



the spore produces a gametophyte .... and a zygote produces 

 a sporophyte ....," can hardly be admitted. The spores of 

 all Archegoniates, if we admit the antithetic theory of alterna- 

 tion, are the direct descendants of those produced by the germi- 

 nating zygote of the ancestral form, where also the product of 

 germination is not directly a new gametophyte, but spores from 

 which the latter arises secondarily, as is the case/ in the Arche- 

 goniates. This is readily demonstrable, while on the other 

 hand, the development of any type of spore in the least resem- 

 bling those of the sporophyte is absolutely unknown in any 

 gametophytic structure. 



If it is admitted that the leafy sporophyte originally arose 

 as an apogamous bud, it would necessarily follow that the foli- 

 age leaves are more primitive than the sporophylls, and that 

 there is no genetic connection between Bryophytes and Pterido- 

 phytes; at present, however, it seems to the writer that the 

 weight of evidence is very much against such a supposition. 



That chlorophyll activity has been a very potent factor in 

 the evolution of the plant-body is of course beyond dispute, but 

 its bearing upon the origin of the higher land plants is not so 

 clear. All green plants, whether acjuatic or terrestrial, must 

 provide for photosynthesis, and we find the arrangements for 

 the most favorable exposure of the green tissue brought about 

 in various ways. Leaves are by no means confined to land 

 plants, many Algse, especially the large Laminariaceae and 

 Fucaceae having large and perfect foliar organs, which, al- 

 though of simple structure, are very efficient organs for photo- 

 synthesis. The independent development of the leaves in sev- 

 eral groups of Bryophytes show^s no evident connection with 

 adaptation to a terrestrial environment. 



If one were seeking among the Bryophytes a structure which 

 most nearly simulated the leafy Fern-sporophyte, it would be 

 found in such thallose Liverworts as Syiuphyogyna or Hymeno- 

 phyton, whose repeatedly forked thallus resembles superficially 

 to an extraordinary degree the fan-shaped leaf of a small Fern. 

 It is conceivable that W'hen the sporophyte first developed a 

 leaf, the latter might tend to assume the dichotomously 

 branched form so common in the gametophyte of the lower Liv- 

 erworts and of the P>rns also which presumably have arisen 

 from similar forms. 



Looking at the evidence from all sides, it seems to the writer 



