BULLETIN OF THE BUSSEY INSTITUTION. 421 
the changes which might occur from the end of the first hour until the 
beginning of the following quarter-hour; and almost punctually to 
the minute, the discharge of zodspores has begun. When the conidia 
sown are in good condition, the greater part of them emit their 
zoospores during an interval not greater than fifteen or twenty minutes ; 
and those which during this period do not discharge their contents in 
the form of zodspores do not do so at a later period, but abort. When 
the conidia are sown in the afternoon, however, it sometimes happens 
that a part of them do not germinate until the next morning. 
Odsrorrs. — We have found the odspores of FPeronospora viticola 
only on plants of Vitis estivalis, where they are apparently abundant, 
although not very easily seen. ‘They are found in the latter part of 
September and October, in the discolored, shrivelled parts of the 
leaves, and are most abundant just inside what are called the palisade 
cells of the upper surface. They are spherical, about .03 mm. in 
diameter, have a thick cell wall, which is smooth and slightly yellow 
in color, and almost completely fill the mother-cell, as shown in Plate 
IlI., Fig. 2. The dense structure and opacity of the upper surface of 
the leaf and the dense woolly covering of the lower surface, which can 
be entirely removed only with great difficulty, renders this species one 
of the least favorable of the genus for the study of the formation of the 
odspores. In dried specimens, the odspores can be well seen only after 
boiling in potash, afterwards treating with hydrochloric acid, and a 
careful dissection, a process, it will be easily imagined, likely to distort 
the parts. 
The odspores are generally set free by the cracking open, or, more 
slowly, by the rotting of the palisade cells, which allows the escape of 
the odspore with its tough covering. The germination of the odspores 
we have not been able to discover. 
The fungus we are now considering is very abundant on Vitis esti- 
valis, Michx., V. Labrusca, L., and all their cultivated varieties; on 
V. cordifolia, Michx.; on V. vulpina, L., and the cultivated Catawba 
grape ; and, in fact, on nearly all varieties of American grapes, although 
_ we have not as yet heard of its occurring on the Diana grape. It is 
probably found throughout the whole United States east of the Rocky 
Mountains, but it has not yet been reported from the west coast. It has 
been said not to occur on the smooth-leaved species, but its presence on 
V. cordifolia proves the contrary. It has also been said that it does not 
