ally, but not always, in
the veins of the leaf; the intertwining twigs form an uninterrupted
cavity, in which is often enclosed the shrivelling tissue of the leaf.
The whole body swells to a greater thickness than that of the leaf,
and protrudes on the surface like a thickened spot. Its form varies
from circular to fusiform; its size is also very unequal, ranging
between a few lines and about half a millimetre in its largest
diameter. At first it is colourless, but afterwards its outer layers
of cells become round, of a brown or black colour, and it is
surrounded by a black rind, consisting of round cells, which separate
it from the neighbouring tissue. The tissue within the rind remains
colourless; it is an entangled uninterrupted tissue of fungus
filaments, which gradually obtain very solid, hard, cartilaginous
coats. The sclerotium, which ripens as the rind becomes black, loosens
itself easily from the place of its formation, and remains preserved
after the latter is decayed.

[Illustration: FIG. 29.--_Peziza Fuckeliana._ _a._ Natural size. _b._
Section enlarged. _c._ Ascus and sporidia.]

The sclerotia are, here as in many other fungi, biennial organs,
designed to begin a new vegetation after a state of apparent quietude,
and to send forth special fruit-bearers. They may in this respect be
compared to the bulbs and perennial roots of under shrubs. The usual
time for the development of the sclerotia is late in the autumn,
after the fall of the vine leaves. As long as the frost does not set
in, new ones continually spring up, and each one attains to ripeness
in a few days. If frost appears, it can lie dry a whole year, without
losing its power of development. This latter commences when the
sclerotium is brought into contact with damp ground during the usual
temperature of our warmer seasons. If this occur soon, at the latest
some weeks after it is ripe, new vegetation grows very quickly,
generally after a few days; in several parts the colourless filaments
of the inner tissue begin to send out clusters of strong branches,
which, breaking through the black rind, stretch themselves up
perpendicularly towards the surface, separate from one another, and
then take all the characteristics of the conidia-bearers. Many such
clusters can be produced on one sclerotium, so that soon the greater
part of the surface is covered by filamentous conidia-bearers with
their panicles. The colourless tissue of the sclerotium disappears