May 1900.] BOTANICAL SOCIETY OF EDINBURGH 301 



plants may have developed from a simple strobilus which 

 was evolved by a process of variation from the simple 

 sporogonial head of the Bryophyta. 



On this theory, sporogenous organs must be held as 

 phylogenetically older than vegetative, and according to 

 Van Baer's law ought to appear first in the developmental 

 history of the individual. Instead of this, the ontogeny 

 reverses the phylogeny, and the vegetative organs are 

 the first to appear, the sporogenous subsequently develop- 

 ing, but not uncommonly after long delay which may 

 amount to years. 



In dealing with this difficulty of incongruence between 

 individual and racial history, Bower (" Phil. Trans." loo. clt.) 

 states that " the development of the individual sporophyte 

 cannot be taken en bloc as illustrating the history of the 

 sporophyte at large," and that the extensive vegetative 

 phase preceding spore production in the individual was 

 of recent origin, appearing as an intercalation in the life 

 history, and that spore production at first, as evidenced by 

 the lower forms, was the sole function of the sporophyte. 

 He gives premier place to the phylogenetic conclusions 

 based on a comparative study of the lower forms, and 

 would have them used as guides and checks in the 

 interpretation of the individual development. 



Following this advice, and returning to the lower forms, 

 let us see what illustrations are afforded in them of 

 Mehnert's principle, before we apply it to this case of 

 inversion of the order of sequence in the development 

 of the higher sporophytes. Looking for a purely sporo- 

 genous type of sporophyte, we find it exemplified in such 

 green algte as CEdoyonium and Coleochcctc. In the former 

 it is very rudimentary — the protoplast of the zygote 

 segmenting into a few naked cells, which ultimately 

 escape as zoospores ; in Coleochccte, the cells formed by 

 segmentation of the zygote are enclosed in membranes, 

 but each cell ultimately emits its protoplast as a free- 

 swimming spore. Passing to the Heimticce, we find an 

 advance on this simple type of sporophyte : a distinction 

 can now be drawn between sporogenous and vegetative 

 tissues — the differentiation being rendered necessary by 

 the change in environment from aquatic to subaeri-u 



