MOSSES AND LIVERWORTS 119 



knowledge goes at present, the gulf between Algse and 

 Archegoniates is a deep one. 



The dependence of all Archegoniates upon water for 

 fertilization, and especially the presence of ciliated 

 spermatozoids, are strong arguments for the derivation 

 of the group from aquatic ancestors, but at present this 

 is about all that can positively be asserted. 



Among the Archegoniates themselves, the relation- 

 ships are much more obvious. Undoubtedly the lowest 

 forms are the Hepaticse, shown both by comparison with 

 the algae and with the other Archegoniates, and probably 

 these are to be considered as the primitive forms from 

 which the others have sprung. 



Among the Hepaticse, the lower Jungermanniacese, 

 such as Metzgeria (Fig. 27, C), seem, on the whole, to be 

 the simplest, although the sporophyte even in the low- 

 est ones is more perfect than in Riccia, which has the 

 lowest type of sporophyte found among the Archegoni- 

 ates. Assuming that the lower thallose Jungermanni- 

 aceee are the most primitive of Hepatics, we have seen 

 that, from this type, several others have been developed. 

 In one line (Marchantiacese) differentiation has resulted 

 in the specialization of tissues, the plant retaining its 

 primitive thallose form (Fig. 27, A, B). In the leafy 

 liverworts, the tissues have remained very simple and 

 the differentiation has been purely external, resulting 

 in a definite axis or stem bearing three rows of leaves 

 (Fig. 27, D, E). A third line of development has given 

 rise to the complex leafy gametophyte of the true 

 mosses. 



In the simpler Hepaticse the sporophyte is small 

 and exclusively devoted to spore-production, e.g. Riccia. 



