BULLETIN OF THE BUSSEY INSTITUTION. 41T 
have already said, it makes its appearance first on the under surface of 
the leaves, most abundantly on the veins near the petioles, and after- 
wards in spots all over the under surface. It is most easily recognized 
on Vitis cordifolia, where the under surface of the leaves is smooth, and 
where the frost-like substance of the fungus is in strong contrast with 
the green leaf on which it grows. When growing on Vitis Labrusca, 
V. estivalis, or cultivated varieties of those species, it appears in the form 
of spots, at first pure white, afterwards rusty, slightly raised above the 
level of the hairs with which the lower surface of the leaves is clothed. 
Sometimes, and, except in Vitis cordifolia, we have not found it to 
occur at all frequently, the fungus invades the petioles of the younger 
leaves and the stems which swell to considerably more than their 
usual dimensions. As the disease advances, the fungus spreads over 
the whole of the lower surface of the leaves, until, as not unfrequently 
happens, scarcely a healthy leaf remains; red spots appear, at first 
small, afterwards larger; the leaf becomes dark brown, shrivels up, 
and becomes very brittle, but it does not fall from its attachment at 
once, as we should expect. The fungus, like all the other species of the 
genus, flourishes best in moist warm weather, but seems more tolerant 
of dryness than any other Peronospora with which we are acquainted. 
We have allowed leaves to remain exposed for some days on a table 
in the dry atmosphere of a laboratory, and fresh conidia were pro- 
duced for several days even when the leaf seemed quite dry. During 
August, the disease, in the region of Boston, advances gradually until, 
towards the middle of September, almost every leaf is affected and 
hangs dead upon the branches. 
Myce.ivum. — A microscopic examination of the leaves and stems 
shows an abundance of mycelial threads or hyphae. They are from 
.008-.0122 mm. in diameter in the stem and petioles, but are 
generally smaller in the leaves. In the stem, their general course is 
up and down, and they are found in all parts, except the wood proper 
where they do not penetrate. The contents of the hyphe are granular 
and somewhat oily, and there are but rarely any cross partitions. 
They force their way in all directions between the parenchymatous 
cells, and into them by means of haustoria, Plate II., Fig. 3, a, which 
are abundant, especially in the stem. ‘The haustoria are usually not 
more than half the diameter of the hyphz, and resemble strongly those 
of Oystopus candidus, being spherical and connected with the hyphz 
VOL. I. 53 
