236 ELEMENTS OF BOTANY. 
\ rh 
ti <= 
“ae 
Fie. 202. — A Plant of 
Pigeon-Wheat Moss 
(Polytrichum commune). 
rh, root-like portion; s, 
bristle-like stalk of urn, 
or spore-case ; c, hoodlike 
cover of urn ; ap, knob at 
base of urn; d, cover of 
urn. (Natural size.) 
In some specimens the stem may be found, at 
a height of an inch or more above the roots, to 
bear a conical, basket-shaped enlargement, out 
of the centre of which a younger portion of the 
stem seems to proceed, and this younger por- 
tion may in turn end in a similar enlargement, 
from which a still younger part proceeds. 
Note the difference in general appearance 
between the leaves of those plants which have 
just been removed from the moist collecting- 
box and those which have been lying for half an 
hour on the table. Study the leaves in both 
cases with the magnifying glass in order to find 
out what has happened to them. Of what use 
to the plant is this change ? Put some of the 
partially dried leaves in water, in a cell on a 
microscope slide, cover, place under the lowest 
power of the microscope, and examine at inter- 
vals of ten or fifteen minutes. Finally sketch 
a single leaf. 
293. Minute Structure of the Leaf and Stem. 
—The cellular structure of the pigeon-wheat 
moss is not nearly as simple and convenient 
for microscopical study as is that of the smaller 
mosses, many of which have leaves composed, 
over a large part of their surfaces, of but a 
single layer of cells, as shown in Fig. 205. If 
any detailed study of the structure of a moss 
is to be made it will, therefore, be better for 
the student to provide himself with specimens 
of almost any of the smaller genera,! and work 
out what he can in regard to their minute 
anatomy. 
294. Spore-Capsules. — That part of the 
reproductive apparatus of a common moss 
which is most apparent at a glance is the urn 
or spore-capsule, Fig. 202. This is covered until 
it reaches maturity with a hood which is easily 
detached. Remove the hood from one of the 
1 As Mnium or Bryum. 
