fungi take forms an interesting question for study. The common mushroom,
the _Agaricus_, the amanitas, and other central stemmed species grow
usually in an upright fashion; that is, the stem is erect. The cap then,
when it expands, stands so that it is parallel with the surface of the
earth. Where the cap does not fully expand, as in the campanulate forms,
the pileus is still oriented horizontally, that is, with the gills
downward. Even in such species, where the stems are ascending, the upper
end of the stem curves so that the cap occupies the usual position with
reference to the surface of the earth. This is beautifully shown in the
case of those plants which grow on the side of trunks or stumps, where
the stems could not well grow directly upward without hugging close to
the side of the trunk, and then there would not be room for the
expansion of the cap. This is well shown in a number of species of
_Mycena_.

In those species where the stem is sub-central, i. e., set toward one
side of the pileus, or where it is definitely lateral, the pileus is
also expanded in a horizontal direction. From these lateral stemmed
species there is an easy transition to the stemless forms which are
sessile, that is, the shelving forms where the pileus is itself attached
to the trunk, or other object of support on which it grows.

Where there is such uniformity in the position of a member or part of a
plant under a variety of conditions, it is an indication that there is
some underlying cause, and also, what is more important, that this
position serves some useful purpose in the life and well being of the
plant. We may cut the stem of a mushroom, say of the _Agaricus
campestris_, close to the cap, and place the latter, gills downward, on
a piece of white paper. It should now be covered securely with a small
bell jar, or other vessel, so that no currents of air can get
underneath. In the course of a few hours myriads of the brown spores
will have fallen from the surface of the gills, where they are borne.
They will pile up in long lines along on either side of all the gills
and so give us an impression, or spore print, of the arrangement of the
gills on the under side of the cap as shown in Fig. 12. A white spore
print from the smooth lepiota (_L. naucina_) is shown in Fig. 13. This
horizontal position of the cap then favors the falling of the spores, so
that currents of air can scatter them and aid in the distribution of the
fungus.