THE NATURE OF ALTERNATION, ETC. 339 



polation of a stage in the life history, Bower has traced the 

 probable course of evolution of the sporophyte as illustrated 

 by such a series as QLdogoniiim, Coleochcete, Riccia. But, 

 going farther than Celakovsky, he has shown strong reasons 

 for considering the increase in the complexity of the Bryo- 

 phyte sporogonium to have been due to progressive ster- 

 ilisation, and has extended this comparison to the lower 

 Pteridophyta. The point of contact with ancestral forms, 

 the sporophyte of which was of similar construction to a 

 Bryophyte sporogonium, is sought among strobiloid types, 1 

 such as Lycopodinese and Equisetineae, and the working 

 hypothesis is put forward "that in the strobiloid Pterido- 

 phyta the apex of the sporogonium is the correlative of the 

 apex of the strobilus ". In the transition from the sporo- 

 gonial to the strobiloid form progressive sterilisation with 

 differentiation of the sterile tissue, the formation of ap- 

 pendicular organs, and the subdivisions of the archesporial 

 layer to form isolated patches instead of one continuous 

 tissue are assumed to have taken place. In addition 

 Bower has indicated the relation borne by these changes 

 to the alteration in mode of life, which is assumed to have 

 taken place on the spread of plants to the land, and has 

 thus brought the biological aspect of the subject into proper 

 prominence. 



The relation of the course of evolution to the probable 

 conditions is also dealt with by Atkinson, 2 who makes the 

 additional suggestion that the disturbance of the assimila- 

 tory function of the gametophyte, induced by its spread to 

 the land, would not only assist the sterilisation of some of 

 the sporogenous tissue, but would tend to force the function 

 of assimilation upon some of the sterilised regions of the 

 sporophyte. 



The views of Strasburger have been referred to above, 

 and is is sufficient to say that on the ground of the facts 

 known as to the periodic reduction of chromosomes he 

 arrives at conclusions which are in essential agreement 



1 Goebel, on the other hand, has compared Mosses directly with the 

 Ferns on the basis of resemblances in the sexual generation. Goebel (5). 



2 Atkinson (2), (3). 



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