View A43-1112-1979-eng.pdf

EDIBLE AND POISONOUS MUSHROOMS OF CANADA
whereas those in which the asci are arranged in a layer exposed to the air are
known as Discomycetes (Figure 43, p. 9). The Ascomycetes as a whole com-
prise many thousands of species including yeasts, molds, powdery mildews,
ergots, numerous leaf-spotting and wood-staining fungi and many others. In
this book only a very few of the larger fleshy Discomycetes, and one Pyreno-
mycete, are discussed'.
The third subclass is called the Basidiomycetes. In this group the hyphae
also have cross walls and the spores are produced on a speciahzed cell known
as the basidium (Figure 37, p. 9). In the basidium two nuclei fuse and then
typically divide twice, giving rise to four nuclei, but in contrast to the Ascomycetes
where the spores mature within the ascus and are discharged when
mature, in the Basidiomycetes the nuclei migrate to the tips of little stalks and
the spores develop outside the basidium itself and are forcibly shot off" the
stalks when they are mature. Most of the fungi described in this book belong
in the Basidiomycetes.
The fourth subclass is known as the Fungi Imperfecti. This group is not
strictly comparable to the other three since it comprises those forms in which a
perfect or sexual state is unknown or lacking and reproduction takes place by
vegetative means, usually by some form of asexual spore. These asexual
spores are often called conidia and many forms that used to be grouped with
the Fungi Imperfecti are now known to be conidial states of Ascomycetes,
Phycomycetes, or Basidiomycetes. However there are still a great many whose
perfect or sexual state is unknown and it is possible that some of these forms
reproduce so efficiently with conidia alone that they have lost the ability to
develop the sexual state. Hence it is necessary to retain this subclass, although
it does not represent a natural grouping. None of the Fungi Imperfecti are
discussed in this book.
BASIDIOMYCETES
The mushrooms and most of the larger fungi that will be found by the
amateur collector belong in the subclass Basidiomycetes and it is necessary to
consider the classification of this group in a little more detail. As has been
noted, the fundamental character of the group is that the spores are produced
outside the mother cell rather than within it. The typical basidium is a more or
less club-shaped single cell with four Httle stalks or sterigmata arising at the
apex and the spores develop on the tips of these stalks (Figure 37, p. 9).
When mature, the spores are forcibly discharged from the sterigmata by the
pressure of surface tension of a drop of water that is excreted from the tip of
the sterigma below the spore. This is the type of basidium found in most of the
larger fleshy fungi such as the agarics, boletes, hydnums, clavarias, and
polypores.
The basidia are arranged in a definite layer in the fruiting body in such a
way that the spores can be discharged into the open air. This layer is called the
hymenium and these fungi are further divided into families based on the shape
22