HABITATS. 941 
ments made by De Bary seem to prove that the spccies of 
Peronospera will only flourish on certain favoured plants, to the 
exclusion of all others. The non-parasitic moulds are scarcely 
exclusive. In Oidium some spccics are parasitic, but probably 
all the parasitic forms are states of EH, ysiphe, the non-parasitic 
alone being autonomous; of these one occurs on Porrigo lupi- 
nosa, 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 Bonduc, from India, to 
which there was no external opening visible, and which was 
broken with considerable difficulty. Several J/ucedines are 
developed on the dung of various animals, and seldom on any- 
thing else. 
The Physomycetes consist of two orders, Antennarici and Alu. 
corini, which differ from each other almost as much in habitat 
as in external appearance. The former, if represented by Anten- 
naria, runs over the green and fading leaves of plants, forming 
a dense black stratum, like a congested layer of soot; or in Zas- 
midium, the common cellar fungus, runs over the walls, bottles, 
corks, and other substances, like a thick sooty felt. In the dlw- 
corini, as in the Jlucedines, there is usually less restriction to 
any special substance. Jucor mucedo occurs on bread, paste, 
preserves, and various substances ; other species of AZuvor 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, Syzy- 
gites, &c., flourish 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 dis- 
tricts. The Perisporiacet are partly parasitical and partly not. 
The Erysiphet include those of the former which flourish at the 
expense of the green parts of roses, ops, maples, poplars, peas, 
and many other plants, both in Europe and in North America, 
