328 BULLETIN OF THE BUSSEY INSTITUTION. 
do not, at the present day, consider our knowledge of a fungus com- 
plete until we have found the odspores, and the organs which have 
produced them, the antheridia and odgonia. In the ease of Pero- 
nospora infestans, the odspores have never been discovered, and, con- 
sequently, the true systematic position of this fungus is uncertain, 
Judging from the mycelium and the asexual spores, it seems nearly 
related to Peronospora (Botrytis) gangliformis, Berk, and the true 
Peronospore, where both odspores and asexual spores are known. 
Reasoning by analogy, we should expect to find the odspores of the 
so-called Peronospora infestans like those of Peronospora gangli- 
formis, which causes the mould in lettuce, unfortunately common in 
this vicinity, and the source of considerable loss to market gardeners. 
Let us examine this plant, in passing, as it may help to a clearer con- 
ception of the potato disease. | 
The lettuce mould, like the potato rot, sends its myeelium through 
the foster plant, until it 
finally breaks through the 
breathing-pores and bears 
its asexual spores in the 
air, as shown in Fig. 3, 
which represents a portion 
of the epidermis of the let- 
tuce, witb a breathing-pore 
through which the myce- 
lium has grown. Thespores 
are more decidedly oval 
than in the potato rot, and 
are arranged star-fashion | 
on the swollen tips of the 
mycelium. ‘They germi- 
nate by direct germinal 
tubes, in the way shown 
in Fig. 2, 6, and these 
penetrate into the interior 
of the common groundsel 
Ries B (Senecio vulgaris), chic- 
cory, and sow-thistle, as well as of different species of lettuce. In 
the substance of the leaves of some of these plants, especially the 
: 
