_sinuate_.

[Illustration: Figure 2.--Small portion of a section through the
spore-bearing layer of a mushroom which produces its spores on the ends
of cells called basidium. (a) Spores, (b) basidium, (c) sterile cells.]

In some genera the lower surface of the cap is full of pores instead of
gills; in other genera the lower surface is crowded with teeth; in still
others the surface is smooth, as in the Stereums. The gills, pores and
teeth afford a foundation for the hymenium or fruit-bearing surface. It
will be readily seen that the gills, pores and teeth simply expose in a
very economical way the greatest possible spore-bearing surface.

If a section of the gills be examined by a microscope, it will be
observed that upon both sides of the surface are extended hymenial
layers. The _hymenium_ consists of elongated cells or basidia
(singular, basidium) more or less club-shaped. Figure 2 will show how
these basidia appear on the hymenial layer when strongly magnified. It
will be seen that they are placed side by side and are perpendicular to
the surface of the gills. Upon each of these basidia are in some species
two, usually four, slender projections upon which the spores are
produced. In Figure 2 a number of sterile cells will be seen which
resemble the basidia except that the latter bear four sterigmata upon
which the spores rest. Among these basidia and sterile cells will
frequently be seen an overgrown bladder-like sterile basidium which
projects beyond the rest of the hymenium, and whose use is not as yet
fully known. They are called cystidia (singular, cystidium). They are
never numerous, but they are scattered over the entire surface, becoming
more numerous along the edge of the gills. When they are colored, they
change the appearance of the gills.

[Illustration: Figure 3.--Rootlike strands of mycelium of the
pear-shaped puff-ball growing in rotten wood. Young puff-balls in the
form of small white knots are forming on the strands. Natural
size.--_Longyear._]

The spores are the seeds of the mushroom. They are of various sizes and
shapes, with a variety of surface markings. They are very small, as fine
as dust, and invisible to the naked eye, except as they are seen in
masses on the grass, on the ground, or on logs, or in a spore print. It
is the object of every fungus to produce spores. Some fall on the parent
host or upon the ground. Others are wafted away by every rise of the
wind and carried for days and