of polymorphism.

The form of the threads, and the form and disposition of the spores,
vary according to the genera of which this order is composed. In
_Oidium_ the mostly simple threads break up into joints. Many of the
former species are now recognized as conditions of _Erysiphe_. In
_Aspergillus_, the threads are simple and erect, with a globose head,
around which are clustered chains of simple spores. In _Penicillium_,
the lower portion of the threads is simple, but they are shortly
branched at the apex, the branches being terminated by necklaces of
minute spores. In _Dactylium_, the threads are branched, but the
spores are collected in clusters usually, and are moreover septate. In
other genera similar distinctions prevail. These two groups of black
moulds and white moulds are the noblest, and contain the largest
number of genera and species amongst the _Hyphomycetes_. There is,
however, the small group of _Isariacei_, in which the threads are
compacted, and a semblance of such hymenomycetal forms as _Clavaria_
and _Pterula_ is the result, but it is doubtful if this group contains
many autonomous species. In another small group, the _Stilbacei_,
there is a composite character in the head, or receptacle,[r] and in
the stem when the latter is present. Many of these, again, as
_Tubercularia_, _Volutella_, _Fusarium_, &c., contain doubtful
species. In _Sepedoniei_ and _Trichodermacei_, the threads are reduced
to a minimum, and the spores are such a distinctive element that
through these groups the _Hyphomycetes_ are linked with the
_Coniomycetes_. These groups, however, are not of sufficient size or
importance to demand from us, in a work of this character, anything
more than the passing allusion which we have given to them.

[Illustration: FIG. 30.--_Penicillium chartarum_, Cooke.]

We come now to consider the structure in the Sporidiifera, in which
the fructifying corpuscles or germs, whether called spores or
sporidia, are generated within certain privileged cysts, usually in
definite numbers. In systematic works, these are included under two
orders, the _Physomycetes_ and the _Ascomycetes_. The former of these
consists of cyst-bearing moulds, and from their nearest affinity to
the foregoing will occupy the first place.

[Illustration: FIG. 31.--_Mucor mucedo_, with three sporangia. _a._
Portion of frill with sporangiola.]

PHYSOMYCETES include, especially amongst the _Mucorini_, many most
interesting and inst