_, hymenium layer.
The long, dark cells are brown cystidia, termed spicules by some to
distinguish them from the colorless cystidia. The long cells bearing the
oval spores are the basidia.]

[Illustration: FIGURE 250.--Inocybe repanda (Bull.) Bres. (= Entoloma
repandum Bull.). _t_, trama of pileus; _sh_, sub-hymenium; _h_, the
hymenial layer; the long cells with a drop of moisture at the ends are
cystidia (sing. cystidium).]

=The hymenium.=--The term _hymenium_ is applied to the spore-bearing
tissue of many fungi. In the _Agaricaceae_ the hymenium covers the entire
surface of the gills and usually the portion of the pileus between the
gills. It originates in the following manner: the threads forming the
trama of the gills grow out from the lower side of the pileus and
perpendicular to its under surface. As growth advances many branches of
the threads turn outward toward either surface of the gill and finally
terminate in club-shaped cells. These cells, therefore, lie side by
side, perpendicular to the surface, forming a pavement, as it were,
over the entire surface of the gills. Some of them put out four little
prongs, on each of which a spore is borne, while others simply remain as
sterile cells (Figs. 249, 250). The spore-bearing cells are _basidia_;
the others are called _paraphyses_. They resemble each other very much,
except that the basidia bear four _sterigmata_ and a spore on each. In a
few species the number of sterigmata is reduced to two and in some low
forms the number is variable. The layer just beneath the basidia is
usually more or less modified, being often composed of small cells
different from the rest of the trama. This is called the _sub-hymenial_
layer or _sub-hymenium_ (Fig. 250).

Other cells called _cystidia_ occur in the hymenia of various species
distributed through nearly all the genera of the agarics. Cystidia are
large, usually inflated, cells which project above the rest of the
hymenium (Fig. 250). They originate either like the basidia, from the
sub-hymenial cells (Fig. 250), or from special hyphae deeper down in the
trama of the gill (Fig. 249). They are scattered over the entire surface
of the hymenium, but become more numerous on the edge of the lamellae.
Their number is much smaller than that of the basidia, but in some
species where they are colored they may greatly change the appearance of
the gills. Cystidia often secrete moisture which collects in drops at
their tips, a phenome