MOSSES AND LIVERWORTS 115 
The mosses which approach most nearly to the He- 
paticee 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 Hepatice, 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 archesporium. 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 Bryinesw. While these show great diversity in their 
habits, their essential structure is remarkably uniform. 
