CHAP. XV SUMMARY AND CONCLUSIONS 509 



gametophyte. This suggests the possibiHty of a derivation of 

 all of them from some type in which this two-sided apical cell 

 was permanent. Aneiira and Metzgeria, among living genera, 

 have retained this condition, and in this respect are possibly 

 to be considered as representing the simplest type of the thallus. 

 The peculiar gemms of the former, which may properly be 

 compared to the zoospores of Coleochcete, strengthen this view. 



Starting from this primitive type, we have endeavoured 

 to show that development proceeded along three lines — the 

 Marchantiaceae, the Jungermanniaceae, and the Anthoceroteai. 

 In the first one the differentiation consists mainly in the 

 specialisation of the tissues, while the gametophyte retains its 

 strictly thallose character ; in the Jungermanniaceae it is rather 

 in the direction of the development of appendicular organs, 

 while the tissues remain nearly uniform. In both of these 

 groups the sporogonium is comparatively simple, in strong 

 contrast to the Anthoceroteae. Whether the peculiar chloro- 

 plasts of the latter are of secondary origin, or have been 

 inherited directly from ancestors like ColeochcEte, where the same 

 form occurs, it is not possible to determine. The great prepon- 

 derance of the foliose Liverworts indicates that they are com.- 

 paratively modern types, which have adapted themselves to 

 present conditions, and show no indications of being connected 

 directly with any higher forms. 



Just as the simplest Jungermanniaceae may have served as 

 a starting-point for the three main lines of development in the 

 Liverworts, so the Anthoceroteae show evidences of being the 

 ancestors of two other lines, the Mosses and the Pteridophytes. 

 Whether the former class constitutes a continuous series, begin- 

 ning with SpJiagnuin, or whether the Sphagnaceae and the 

 higher Mosses represent two branches from a common stock, 

 it seems extremely likely that the thalloid protonema is the 

 primitive condition derived from some Liverwort-like form allied 

 to Anthoceros, and that the alga-like protonema of the higher 

 Mosses is a secondary development from it. 



In tracing the gradual evolution of the sporophyte among 

 the Muscineae we have seen how, starting with the simple 

 sporogonium of Riccia, which, physiologically, is only a spore- 

 fruit and quite incapable of independent growth, it gradually 

 becomes more and more independent by the development of a 

 special system of assimilative tissues, which reaches its extreme 



