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Fig. 295.—Polytrichum commune. f,f, fertile plants, one on the left in fruit, m, antheridial plant.
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Fig. 296.—Section of Leaf of Polytrichum commune.
Mosses (Bryophyta)
If we have followed carefully the development of marchantia,
the study of one of the mosses will be comparatively easy. The
mosses are more familiar plants than the liverworts.
They grow on trees, stones, and on the
soil both in wet and dry places. One of the
common larger mosses, known as Polytrichum commune, may serve as an
example, Fig. 295. This plant
grows on rather dry knolls,
mostly in the borders of open
woods, where it forms large
beds. In dry weather these
beds have a reddish brown
appearance, but when moist
they form beautiful green
cushions. This color is due,
in the first instance, to the
color of the old stems and
leaves, and, in the second instance,
to the peculiar action
of the green living leaves
under the influence of changing
moisture-conditions. The
inner or upper surface of the
leaf is covered with thin, longitudinal
ridges of delicate
cells which contain chlorophyll.
These cells are shown
in cross-section in Fig. 296, as dots or granules. All the other
tissue of the leaf consists of thick-walled, corky cells which do
not allow moisture to penetrate. When the air is moist the green
leaves spread out, exposing the chlorophyll cells to the air, but in