298 STRUCTURAL BOTANY 



Possibly the aerial, asexual spores of the Bryophyta 

 may be homologous with the aquatic, asexual spores of 

 the Green Algse, 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 

 Archegoniatse must have taken place in enormously 

 remote geological ages, when plants were first adapting 

 themselves to terrestrial life, and we cannot be surprised 

 that no transitional forms connecting them with the 

 Algse are known to us. 



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 

 generations 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. 

 Algse to Bryophyta, still more is this the case when 

 we advance from Bryophyta to the Vascular Cryptogams. 



