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A TEXTBOOK OF THEORETICAL BOTANY 



B 



C 



D 



Fig. 438. — Diagram illustrating the possible evolution of the antheridium and the 

 archegonium from a gametangium. A, Gametangium. B, Walled antheridium. 

 C and D, Stages in evolution of an archegonium. (After Davis.) 



derived from the fertilization of the ventral canal cell proceeds for more than 

 a few cell divisions. 



It must be clearly understood that in postulating this origin of the 

 archegonium we are considering the matter from a purely theoretical aspect, 

 we have no examples living at the present time showing stages in such an 

 evolutionary series. In fact, gametangia of the multicellular type are only 

 found in the Phaeophyceae, and it is not suggested that the Bryophyta 

 originated from this group ; the question of pigmentation alone makes it 

 very improbable. 



In fact the evolutionary picture thus outlined is so vague and unfinished 

 that one is driven to question the accepted idea of the derivation of the 

 Bryophyta from the Algae. There is considerable evidence of reduction 

 having taken place in some phases of the Bryophyte evolution, and it is not 

 impossible that reduction, that is to say simplification, has been the keynote 

 of evolution throughout the group. This idea, if accepted, would lead to the 

 conclusion that the simplest members, which are those most aquatic in their 

 habits, are really derived from more complex Bryophyta and have taken to 

 an aquatic life secondarily. 



Following out this scheme we would regard the highest Bryophyta, such 

 as the larger Mosses, as being derived from a common stock with the 

 Lycopodiales, and as having diverged from them by the development of the 

 gametophyte rather than the sporophyte as the chief phase in the life history. 

 This course did not prove as successful as the sporophyte line of evolution 

 in the Pteridophyta, and the Bryophyta were gradually driven back more 

 and more, upon moist, and finally upon aquatic habitats. 



