A TOADSTOOL—ITS FUNCTION AND FORM 5
The sticky spores are shot to a distance of about 0.2 mm. If
the gills were not vertical discharged spores would either fall
back on to the gill surface from which they were shot, or on to
the opposite gill surface. The vertical position is the only one
that allows free escape of the spores into the air below the
pileus.
The verticality of the gills is secured by a system of coarse
and fine adjustments. The negative geotropism of the stipe
gives a rough vertical orientation to the pendent gills. But in
addition each individual gill is capable of growth near its in-
sertion on the cap, so that if it is slightly out of the vertical
differential growth in this region occurs until it is again in the
vertical plane.
There is clearly a minimum distance which must separate
opposing gill surfaces if efficient spore discharge is to occur.
This minimum must not be less than the distance (about 0.2
mm.) of actual discharge for, if it were, the spores would
simply be shot on to the opposite gill. Further a certain mar-
gin of safety is necessary and the amount of this will be deter-
mined largely by the general rigidity of the fruit-body,
especially the rigidity of the stipe.
This brings us back to the stipe and we may now summarize
its importance to the fruit-body. Firstly its early phototropism
may bring the young developing pileus into the open and this
is particularly important when the surface of the substratum
is irregular. Secondly, its later-developed negative geotropism
raises the cap into the air and roughly orientates the gills.
Further the considerable rigidity of the stipe prevents the cap
waving in the wind. Above all, however, the stipe provides a
space between the cap and the ground so that the spores emerg-
ing from its under side fall normally into turbulent air and can
be carried away by eddy diffusion.
A few words must be said about spore output. It is enormous.
A toadstool or mushroom 5in. across may liberate spores at the
rate of half a million a minute and maintain that rate through-
out the several days of its spore-fall period.
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There is something very geometrical about a toadstool and
now I want to talk about the geometry of fruit-bodies.