by the threads of mycelium growing in
great numbers out from the side of the cords. These enlarge and elongate
and make their way toward the surface of the ground. They are at first
very minute and grow from the size of a pinhead to that of a pea, and
larger. Now they begin to elongate somewhat and the end enlarges as
shown in the larger button in the figure. Here the two main parts of the
mushroom are outlined, the stem and the cap. At this stage also the
other parts of the mushroom begin to be outlined. The gills appear on
the under side of this enlargement at the end of the button, next the
stem. They form by the growth of fungus threads downward in radiating
lines which correspond in position to the position of the gills. At the
same time a veil is formed over the gills by threads which grow from the
stem upward to the side of the button, and from the side of the button
down toward the stem to meet them. This covers the gills up at an early
period.

[Illustration: FIGURE 6.--Agaricus campestris. Under view of two plants
just after rupture of the veil, fragments of the latter clinging both to
margin of the pileus and to stem. (Natural size.)]

=From the Button Stage to the Mushroom.=--If we split several of the
buttons of different sizes down through the middle, we shall be able to
see the position of the gills covered by the veil during their
formation. These stages are illustrated in Fig. 4.

As the cap grows in size the gills elongate, and the veil becomes
broader. But when the plant is nearly grown the veil ceases to grow, and
then the expanding cap pulls so strongly on it that it is torn. Figure 5
shows the veil in a stretched condition just before it is ruptured, and
in Fig. 6 the veil has just been torn apart. The veil of the common
mushroom is very delicate and fragile, as the illustration shows, and
when it is ruptured it often breaks irregularly, sometimes portions of
it clinging to the margin of the cap and portions clinging to the stem,
or all of it may cling to the cap at times; but usually most of it
remains clinging for a short while on the stem. Here it forms the
annulus or ring.

[Illustration: FIGURE 7.--Agaricus campestris. Plant in natural position
just after rupture of veil, showing tendency to double annulus on the
stem. Portions of the veil also dripping from margin of pileus. (Natural
size.)]

=The Color of the Gills.=--The color of the gills of the common mushroom
varies in different s