4 THE ESSEX NATURALIST
In toadstools the basidia occur in the hymenium covering the
surface of the gills, or covering the teeth in Hydnum or lining
the pores in a Boletus and are elongated at right angles to the
surface. In some, perhaps the majority of species, it seems
likely that the hymenium consists only of basidia in various
stages of development. Most mycologists have, however,
usually regarded the hymenium as composed of basidia inter-
mixed with sterile paraphyses. It is a difficult point to prove,
but I suspect that what have passed for paraphyses are merely
very young basidia. However, in the ink-cap fungi belonging
to the genus Coprinus, paraphyses are clearly differentiated
cells with the important function of spacing the basidia
sufficiently far apart. In addition the hymenium may contain
conspicuous cells known as cystidia, but these are few compared
with the number of basidia. Sometimes, as in Coprinus
atramentarius they obviously serve to keep the tightly packed
gills sufficiently apart during development, but in most toad-
stools, if they have a function, it is by no means obvious.
Now let us turn our attention to the fruit-body itself. Con-
sider the stipe. The first striking point is its relative stoutness.
If we compare a toadstool with an annual herb or even with
a tree the thickness of its support in relation to what is sup-
ported is considerable. The second point is that, like a tree
trunk, the stipe is vertical. This vertical habit is due to nega-
tive geotropism of the stipe. In some toadstools, such as the
mushroom, Psalliota campestris, the only response of the stipe
is to gravity. However, in many others, especially in Pholiota
spp. and Armillaria mucida which grow on trees, and in the
coprophilus agarics, the stipe is at first positively phototropic
and grows towards light. Later, however, this response to
light seems to disappear and the stipe becomes negatively
geotropic.
Let us now consider the cap—the pileus. This type of struc-
ture—a disc supported on a central stalk—is rather rare in the
plant kingdom outside the fungi, but does occur in the well-
known tropical seaweed Acetabularia and in the antheridio-
phore of the liverwort Marchantia.
The majority of toadstools have gills which hang down from
the underside of the cap. The plane of these gills must be verti-
cal. This follows if we consider the process of spore discharge.