THE WHITE 11UST, ETC. 
481 
tenuated, as in the berberry mildew; but divided into two tootli- 
like processes. The conceptacles in this species contain from 
four to eight sporangia, each of which has four or five spores. 
In England, the leaves of the guelder-rose, and in France 
(perhaps also in this country) those of the alder, nourish a 
parasite belonging to this division. This “ blight ” possesses 
so much in common with others to which allusion has been 
made, that it will scarcely be necessary to describe it in detail. 
A figure of the tip of one of the appendages of the variety 
found on the alder (fig. 19), is given in the plate. 
Since the illustrations to this paper were engraved, we have 
found another species which had not been before noticed in this 
country (M. Hedivigii, Lev.), on the leaves of the mealy guelder- 
rose in the vicinity of Darenth wood, near Dartford, in Kent. 
The mealy character of the leaves of this plant, and the minute 
size of the conceptacles of the parasite, render it difficult to 
find ; indeed, it could not be noticed unless it were sought for, 
as we sought it, lens in hand. It only occurs on the under 
surface of the leaves : the mycelium is very web-like and fuga- 
cious, the conceptacles minute, globose, and scattered. Four 
sporangia, each containing but four spores, are enclosed in each 
conceptacle, which is surrounded by a few appendages (about 
six) thrice dichotomous, and thickened at the tips of the 
ultimate branches, which are incurved. 
The species of true Erysiphe are distinguished botanically 
from the foregoing by the floccose character of the appendages, 
in which feature they accord with the species found on the 
rose and the hop, but from which they differ in the concep- 
tacles containing numerous sporangia instead of only one as 
in those species. 
One of the most common and conspicuous of these is found 
on the leaves, and leaf-like stipules, of the garden pea. Every 
leaf in a crop will sometimes suffer, and the gardener, to his 
great mortification, finds that the mildew is more prolific than 
his peas. The leaves become sickly and yellow as the myce- 
lium of the fungus spreads over them, when they present a 
peculiar appearance, as if growing beside a chalky road in dry 
dusty weather and had become covered with comminuted 
chalk. Soon the conceptacles appear, profusely scattered over 
the white threads, like grains of gunpowder (fig. 22), and after 
a brief struggle for existence the pea and its parasite die to- 
gether. In this species ( Erysiphe Martii , Lev.), the appen- 
dages are nearly transparent, short, and much interwoven with 
the mycelium (fig. 23), the sporangia globose containing from 
four to eight spores (fig. 24). It is not confined to peas, 
although that habitat has been here given for it, because it is 
so common upon them. Beans, melilot, St. John's Wort, some 
