nd its ravages beyond that natural order,
whilst _Peronospora parasitica_ confines itself to cruciferous plants.
One species is restricted to the _Umbelliferae_, another, or perhaps
two, to the _Leguminosae_, another to _Rubiaceae_, two or three to
_Ranunculaceae_, and two or three to _Caryophyllaceae_. All the
experiments made by De Bary seem to prove that the species of
_Peronospora_ will only flourish on certain favoured plants, to the
exclusion of all others. The non-parasitic moulds are scarcely
exclusive. In _Oidium_ some species are parasitic, but probably all
the parasitic forms are states of _Erysiphe_, the non-parasitic
alone being autonomous; of these one occurs on _Porrigo lupinosa_,
others on putrefying oranges, pears, apples, plums, &c., and one
on honeycomb. _Acrospeira_ grows in the interior of sweet chestnuts,
and we have seen a species growing within the hard testa of the
seeds of _Guilandina Bondue_, from India, to which there was no
external opening visible, and which was broken with considerable
difficulty. Several _Mucedines_ are developed on the dung of
various animals, and seldom on anything else.

The _Physomycetes_ consist of two orders, _Antennariei_ and
_Mucorini_, which differ from each other almost as much in habitat as
in external appearance. The former, if represented by _Antennaria_,
runs over the green and fading leaves of plants, forming a dense black
stratum, like a congested layer of soot; or in _Zasmidium_, the common
cellar fungus, runs over the walls, bottles, corks, and other
substances, like a thick sooty felt. In the _Mucorini_, as in the
_Mucedines_, there is usually less restriction to any special
substance. _Mucor mucedo_ occurs on bread, paste, preserves, and
various substances; other species of _Mucor_ seem to have a preference
for dung, and some for decaying fungi, but rotting fruits are nearly
sure to support one or other of the species. The two known species of
the curious genus _Pilobolus_, as well as _Hydrophora_, are confined
to dung. _Sporodinia_, _Syzygites_, &c., nourish on rotten Agarics,
where they pass through their somewhat complicated existence.

The _Ascomycetes_ contain an immense number of species, and in general
terms we might say that they are found everywhere. The _Tuberacei_ are
subterraneous, with a preference for calcareous districts. The
_Perisporiacei_ are partly parasitical and partly not. The _Erysiphei_
include those of the former which flouri