594 MOSSES AND FERNS chap. 



Liverworts, so the Anthocerotes suggest the course of develop- 

 ment which resulted in two other lines, the Mosses and the 

 Pteridophytes. Whether the former class constitutes a con- 

 tinuous series, beginning with Sphagnum, or whether the 

 Sphagnaceae and the higher Mosses represent two branches 

 from a common stock, it seems extremely likely that the thalloid 

 protonema of Sphagnum is the primitive condition derived 

 from some Liverwort-like form similar to Anthoceros, and 

 that the alga-like protonema of the higher Mosses is a sec- 

 ondary development from it. The extensively branched proto- 

 nema is probably an adaptation associated with the rapid propa- 

 gation of the gametophyte, as the number of leafy shoots pro- 

 duced from such a protonema, is far greater than is possible 

 from a thallose protonema like that of Sphagnum. 



In tracing the gradual evolution of the sporophyte among 

 the Muscineae we have seen how, starting with the simple spo- 

 rogonium 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 

 in Anthoceros. It is true that the sporogonium always remains 

 to some extent parasitic upon the gametophyte, but this para- 

 sitism is very slight in Anthoceros, where the formation of a 

 root would make the sporogonium quite self-supporting. This 

 increase in the vegetative tissues of the sporophyte is at the 

 expense of the sporogenous tissue, which becomes more and 

 more subordinated to the assimilative and conductive tissue of 

 the sporogonium, as is seen in the Bryales among the Mosses, 

 and in Anthoceros. 



In most of the Liverworts the sterile tissues of the sporo- 

 gonium are mainly concerned with the protection and dissemi- 

 nation of the spores. Only the foot, usually, can be properly 

 considered as an organ concerned in the nourishment of the 

 growing embryo. The seta, capsule wall, and elaters are 

 merely adaptations for facilitating the dispersal of the ripe 

 spores. In all of the Hepaticae, the whole of the central tissue 

 of the capsule constitutes the archesporium, all of whose cells 

 are devoted to the formation of spores or elaters. In the 

 Anthocerotes, however, the origin of the archesporium is quite 

 different, and it arises not from the central cells, but by a sec- 

 ondary division of the parietal ones. As yet there is no clear 



