MOSSES. 7 
tion and return to the lateral walls of the cells. 
These plasmic movements are exceedingly curious 
and interesting. 
The lowest forms of scale-mosses consist simply 
of a patch of green membrane spreading over 
the ground composed of a single or double 
layer of cells containing chlorophyll. Higher 
types have a more definite outward appear- 
ance, a greater complexity of internal structure, 
and possess a skin investing both surfaces—the 
upper portion of the frond containing stomata or 
breathing pores like the leaves of flowering-plants. 
From the slight groove which runs along the middle 
line on the upper surface of some species, and 
which looks like a mid-nerve, arise minute leaf- 
like bodies called amphigastria, which resemble 
the stipules of Phanerogamous plants ; while from 
the projecting rib on the lower surface correspond- 
ing to the groove on the upper, arise numerous 
radicles or rootlets, although in some species they 
are scattered indiscriminately over the whole under- 
side. Their substance is very loosely cellular, 
easily reviving, after being dried, by the application 
of moisture. The species that have stomata or 
breathing pores, however, when once dried, revive 
very slowly and imperfectly ; being in this respect 
analogous to flowering plants. Their colour varies 
from a pale white to the darkest green and the 
