1883.] 
THE AMERICAN VINE MILDEW. 
19 
wherever it appeared. In 1882 it had invaded 
the German empire, at Alsace and in Bavaria. 
By reason of its rapid dispersion the danger 
has therefore come very near to our shores. 
FIG. 1. PERONOSPOEA VITICOL1 ON VINE LEAF. 
We owe to the researches of many botanists, 
amongst others to those of De Bary and Mil- 
lardet, our knowledge of the life history of this 
destructive cryptogam. In its attacks on the 
vine it appears chiefly on the leaves, but only 
on the under, never on the upper surface ; 
more rarely it attacks the young stems and the 
incipient bunches. On the leaves it originates 
mostly on the projecting nerves, and spreads 
from them over the whole surface, as repre¬ 
sented in fig. 1, which shows at a a cross 
section of the leaf, with the thick growth of 
the conidia-bearing branches of the fungus 
forming a kind of down on the under surface, 
and at b a similar section showing the leaf and 
the nerve both dried up through the effects of 
the fungus growth. Fig. 2 shows a portion 
of the Peronospora, more highly magnified, A 
representing a conidium fallen off, b one more 
highly magnified, c, d, oosporangia with at¬ 
tached antheridia, e a ripe oosporangium, and 
f an oospore therefrom—all after Millardet. 
The American Mildew differs from the 
ordinary Vine Mildew, which is white, in being 
of a grey colour with a dingy green tinge, and 
of a downy character, the tree-like ramified 
FIG. 3. GRAPES AFFECTED BY PERONOSPORA. 
threads emerging from the stomates, and ter¬ 
minating in a unicellular conidia, of which 
a very large number are developed ; they are 
pear-shaped attached by the narrow end, and 
when ripe fall off, when, if they come within 
a short time in contact with a drop of water 
they split up and each portion becomes con¬ 
verted into a ciliated antheridium or swarm 
&pore, which within some four or five hours 
reaches its resting or mature stage, and then 
immediately germinates, and penetrates the 
tissue of the vine leaf, growing up between the 
cells, and forming fresh fungus threads which 
send into the cells minute suctorial processes. 
From these fungus threads proceed the con- 
idium-bearing branches, which emerge through 
the stomates of the under side of the leaf, and 
