238 ELEMENTS OF BOTANY 
THE STUDY OF PIGEON-WHEAT MOSS 
(POLYTRICHUM COMMUNE) 
306. Occurrence. — This moss is widely distributed over the sur- 
face of the earth, and some of its relatives are among the best 
known mosses of the northern United States. Here it grows com- 
monly in dry pastures or on hillsides, not usually in densely shaded 
situations. 
307. Form, Size, and General Characteristics. — Study several speci- 
mens which have been pulled up with root-hairs. Note the size, 
general form, color, and texture of all the parts of the plants exain- 
ined. Some of them probably bear on slender stalks spore-capsules like 
those shown in Fig. 167, while others are without them. Sketch one 
plant of each kind, about natural size. 
What difference is noticeable between the appearance of the 
leaves in those plants which have spore-capsules and those which 
have none? Why is this? 
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 enlarge- 
ment, out of the center of which a younger portion of the stem seems 
to proceed; and this younger portion 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 intervals of ten or fifteen minutes. 
Finally sketch a single leaf. 
308. Minute Structure of the Leaf and Stem. — The cellular struc- 
ture 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. 170. If any detailed 
