DISPERSAL IN CUP FUNGI 293
they bear are carried far and wide. The thickness of
the layer of non-turbulent air varies with atmospheric
conditions, but it is commonly only a few millimetres
thick.
A spore gun can have two functions. First, it may simply
free the spores from immediate contact with the parent
tissue. Secondly, it may shoot the spores through the layer
of static or laminar air into the turbulent zone above. The
ascus has both these functions—the basidium only the first.
Most Basidiomycetes have contrived to bear their basidia
on a toadstool-type of fruit body so that the spores dis-
charged from the basidia fall into the turbulent air below
the cap. In Ascomycetes, although the fruit-body is some-
times provided with a stalk which is sometimes quite long,
it is often more or less sessile and relies on the force of ascus
discharge to carry the ascospores into the turbulent air
(Fig. 2).
The explosive ascus is a turgid living cell. It is usually
long and cylindrical. Its wall is a thin, transparent, elastic
membrane. Lining this is a thin layer of protoplasm sur-
rounding a large central vacuole of sap in which the spores
occur near the apical end. Eventually the ascus bursts
either by a hinged lid or by the development of a minute
pore in the apex. When this happens, the stretched wall
contracts and the contents of the ascus, including the spores,
are squirted into the air. Usually the asci occur packed
together like posts in a palisade with filamentous packing
hyphae (paraphyses) in between them (Fig 3).
The feature that distinguishes the Cup Fungi—the
Discomycetes—from other Ascomycetes is that the ascus-
bearing layer or hymenium is exposed so that more than
one ascus can discharge its spores at the same time. In
many Cup Fungi the phenomenon of "puffing" occurs.
This may be illustrated by considering one of the most
characteristic genera of Discomycetes, namely, Peziza
(Fig. 3). During periods when an apothecium is left un-
disturbed, great numbers of asci become ripe and are then
in a condition of unstable equilibrium. When such a
specimen is touched or even breathed upon, these ripe asci
burst simultaneously, sending into the air to a distance of