A TOADSTOOL—ITS FUNCTION AND FORM 3
with it (Fig. 2). However, we are still very uncertain of the
exact mechanism by which the spore is actually discharged.
Fig.'2. Panaeolus campanulatus. Part of a vertical section of a gill show-
ing spore discharge from basidia. The lines with arrows show the trajectories
of spores. Reproduced, by permission, from Buller's Research on Fungi.
The distance of discharge varies from 0.1 mm. to 0.4 mm.
in different species, but in any one species it is very constant.
I want to stress this point—the basidium is a spore-gun with
a very short range.
There are certain major consequences for fruit-body archi-
tecture which follow from the short range of the basidium..
Firstly, for a fruit-body to be reasonably efficient an hymenium
of basidia should not face upwards. If it did spores shot into
the air to only a fraction of a millimetre would almost at once
fall back on the parent tissue and stick fast with little chance
of subsequent dispersal. It is, in fact, very rare in Hymeno-
mycetes to find hymenial surfaces facing upwards. They nor-
mally either face downwards or are vertical or are at some angle
between these two directions. In the great majority of
Hymenomycetes the hymenial surfaces are more or less verti-
cal so that most of the basidia are in the horizontal position.
Secondly, and this is a point we shall return to later, because
of the short distance of discharge, opposite hymenial surfaces
can be arranged quite close to one another provided that they
are vertical.