300 STRUCTURAL BOTANY 



i 



Possibly the aerial, asexual spores of the Bryophyta 

 may be homologous with the aquatic, asexual spores of 

 the Green Algffi, but, if so, the former have been so 

 completely modified that the homology can no longer be 

 traced with any certainty. The origin of the Arche- 

 goniatse must have taken place in enormously remote 

 geological ages, when plants were first adapting them- 

 selves to terrestrial life, and we cannot be surprised that 

 no transitional forms connecting them with the Algae are 

 known to us. It is commonly assumed that the Bryo- 

 phyta represent an earlier stage of evolution than the 

 Pteridophyta, but palreontological evidence lends no 

 support to this assumption. 



So far as the vegetative structure of the thallus is 

 concerned, Pellia is a very simple Liverwort ; others are 

 more complex, but in all alike the archegonium and 

 antheridium are totally different from the sexual organs 

 of any Thallophytes. 



In the true Mosses both the sexual and asexual genera- 

 tions are more highly developed than in the Hepaticse. 

 Not only is the thallus replaced by a leafy stem (a change 

 which is already accomplished in many Liverworts), but 

 the anatomical structure is much more perfect, and a 

 definite system of conducting tissue is differentiated. The 

 sporophyte never develops into anything more than a 

 fruit, yet it is anatomically the more elaborate of the two 

 generations, as shown not only by the arrangements for 

 dispersing the spores, but also by the vegetative tissues of 

 the sporogonium, which have some resemblance to those of 

 vascular plants, especially in the possession of true stomata. 

 The mosses are highly organised plants in their own way, 

 but appear to have no direct affinity with superior groups. 



If we found a wide gap to cross in passing from Algoe 

 to Bryophyta, this is equally the case when we pass on to 



