MOSSES AND LIVERWORTS 115 



The mosses which approach most nearly to the He- 

 paticse are undoubtedly the species of Sphagnum, the 

 common peat-mosses. In these the protonema arising 

 from the germinating spore is a flat thallus, very much 

 like a simple liverwort in appearance (Fig. 29, A). 

 From the margin of this, secondary protonemal branches 

 arise which are filamentous and closely resembling those 

 of the higher Musci. 



If, as seems probable, the Musci have arisen from 

 the Hepaticse, Sphagnum probably represents an inter- 

 mediate form, and the flat, liverwort-like protonema 

 must be considered to be more primitive than the fila- 

 mentous type which has been derived secondarily from 

 it. The suppression of the flat thalloid stage is prob- 

 ably correlated with the development of the leafy 

 gametophoric branches, which become more and more 

 important. 



The sporophyte in Sphagnum is, in its early stages, 

 remarkably like that of Anthoceros, especially in the 

 origin of the archesporiurn. Like Anthoceros the 

 sporophyte possesses a well-developed assimilative sys- 

 tem of green tissue with numerous stomata, which are 

 not always, however, functional. 



The gametophyte of Sphagnum, in spite of its large 

 size, shows a simpler structure than that of the typical 

 mosses, the central strand of tissue being absent from 

 the stem, and the leaves being destitute of a midrib. 

 There are a few forms intermediate, to some extent, be- 

 tween Sphagnum and the typical mosses ; but a very 

 large majority of the Musci belong to a single order, 

 the Bryinese. While these show great diversity in their 

 habits, their essential structure is remarkably uniform. 



