inthosporium Smithii_, the
spores much exceed the dimensions of the threads;[m] in other species
they are smaller. In _Dendryphium_, the threads and spores are very
similar, except that the threads are branched at their apex, and the
spores are often produced one at the end of another in a short
chain.[n] In _Septosporium_ again, the threads and spores are similar,
but the spores are pedicellate, and attached at or near the base;
whilst in _Acrothecium_, with similar threads and spores, the latter
are clustered together at the apex of the threads. In _Triposporium_,
the threads are similar, but the spores are tri-radiate; and in
_Helicoma_, the spores are twisted spirally. Thus, we might pass
through all the genera to illustrate this chief feature of coloured,
septate, rather rigid, and mostly erect threads, bearing at some point
spores, which in most instances are elongated, coloured, and septate.

[Illustration: FIG. 26.--_Acrothecium simplex._]

[Illustration: FIG. 27.--_Peronospora Arenariae._]

MUCEDINES.--Here, on the other hand, the threads, if coloured at all,
are still delicate, more flexuous, with much thinner walls, and never
invested with an external cortical layer. One of the most important
and highly developed genera is _Peronospora_, the members of which are
parasitic upon and destructive of living vegetables. It is to this
genus that the mould of the too famous potato disease belongs.
Professor De Bary has done more than any other mycologist in the
investigation and elucidation of this genus; and his monograph is a
masterpiece in its way.[o] He was, however, preceded by Mr. Berkeley,
and more especially by Dr. Montagne, by many years in elucidation of
the structure of the flocci and conidia in a number of species.[p] In
this genus, there is a delicate mycelium, which penetrates the
intercellular passages of living plants, giving rise to erect branched
threads, which bear at the tips of their ultimate ramuli, sub-globose,
ovate, or elliptic spores, or, as De Bary terms them--conidia. Deeply
seated on the mycelium, within the substance of the foster plant,
other reproductive bodies, called oogonia, originate. These are
spherical, more or less warted and brownish, the contents of which
become differentiated into vivacious zoospores, capable, when
expelled, of moving in water by the aid of vibratile cilia. A similar
structure has already been indicated in _Cystopus_, otherwise it is
rare in fungi, if the