A TOADSTOOL—ITS FUNCTION AND FORM 7
Toadstools with their uniformity of general shape and with
their considerable range in size, from minute species of
Marasmius to horse mushrooms (Psalliota arvensis), offer
an inviting opportunity of testing the principle of similitude.
I originally* analysed the agarics with central stipes using
the data of fruit-body size given in Ramsbottom's Handbook
of the Larger British Fungi. The picture which developed
from this analysis was quite clear and showed the expected
tendency for form to vary with size.
The crude data I had used, however, were merely a taxono-
mist's subjective estimates of fruit-body size. More recently
Dr. T. E. T. Bond of the University of Bristol has tested my
thesis† using the beautiful illustrations in Lange's Agaricina
Danica. Each picture in that great work is an accurate draw-
ing of an actual specimen. Bond determined the cap diameter,
and the diameter of the stipe near the middle. 1,166 specimens
were thus measured representing nearly a thousand species.
He found the average cap diameter of all the specimens for
each width of stipe (0-1 mm., 1-2 mm., 2-3 mm. . . . etc.) and
then plotted the diameter of cap against that of the stipe (Fig.
4). Here it is quite clear that the cap diameter does not vary
directly as the diameter of the stipe, but that the cube of the
cap diameter varies as the square of the stipe thickness. The
relationship between cap and stipe diameter is logarithmic and
not linear.
It would seem that we have here a fine example of the prin-
ciple of similitude in nature. Form changes as a consequence
of change in size. Large toadstools tend to have relatively
stout stalks and small ones relatively slender stalks. It is the
elephant and the deer all over again.
Let us now turn our attention to another feature of toadstool
geometry—the arrangement of the gills. In this connection
let me remind you that gill surfaces must be a certain distance
apart.
In many very small agarics all the gills are of equal length,
but in larger species (most Russula spp. forming a conspicuous
exception) there is normally an equal number of shorter gills
* Ingold, G. T. Trans. Brit. Mycol. Soc. 29, 108, 1946.
† Bond, T. B. T. Trans. Brit. Mycol. Soc. 35, 190. 1952