Mosses and Peat 
that its cushions are very close and crowded and soon 
tend to swell up into small dome-shaped or convex 
projections. Both leaf and stem are full of empty cells 
which act as water cisterns and possess fine spiral 
strengthening fibres to prevent collapse. The appear- 
ance of these cells reminds one of indiarubber tubing, 
strengthened by a spiral wire inside. 
Suppose now that a wet marsh is beginning to be 
dotted about by separate Sphagnum plants, Each is 
a circular cushion convex upwards and always widen- 
ing at the circumference. Every shower of rain saturates 
them; any surplus water simply collects under the 
cushions. In time the cushions bégin to touch one 
another, but there will be some inequality of growth 
so that the surface is uneven with holes full of water 
between the bulging convex surfaces. The moss, how- 
ever, still goes on growing upwards, and eventually 
forms a huge accumulation of peat, which may occupy 
the whole marsh. In the middle it will be on the 
whole higher, and it will slope as a very gently arched 
dome from the centre to every part of the circumfer- 
ence. This mass of peat is composed of the dead re- 
mains of the Sphagnum itself, and of all other plants 
which succeed in growing there; they do not decay, 
because the living crust of Sphagnum being saturated 
with water and very closely packed together, does not 
permit the free passage of oxygen to the dead vegetable 
matter below. Moreover, there are but few bacteria, for 
there are antiseptic substances in peat. Hence such 
peat mosses are extremely valuable from a historical 
point of view. One can not only distinguish the 
changes in vegetation that have occurred since the 
peat moss began to grow, but if there be bones of 
animals or prehistoric canoes and other implements, 
they are quite well preserved (see Chap. XX.). 
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