246 Researches into the Nature of the Potato- Fungus. 
The oospores of Cystopus and Peronospora are formed in the 
interior of the tissue of the host, mostly between the cells, 
rarely in the interior of the cells, as, for instance, in Peronospora 
Sempervivi, discovered by Professor Schenk. They have their 
origin either in the same part of the host where the conidia 
occur, or their formation is limited to a particular spot. The 
species of Peronospora which inhabit different kinds of host- 
plants are found on all of them, or at least on several of them, 
with conidia as well as with oospores, e.g. P. Alsinearum on 
species of Stellaria and Cerasthim, P. Ficarice on several species 
of Ranunculus ; or they form conidia on all hosts, but oospores 
only on certain hosts, e.g. Cystopus cubicus. When, therefore, a . 
species of Peronospora is always found in a phanerogamic plant 
with conidia only and without oospores, it is fair to conjecture 
that the fungus inhabits a host plant of another species, in which 
the oospores must be sought. This conjecture must be based, 
not on a single instance, but on extended and long observation, 
for both the presence and the absence of oospores may be due 
to other causes, such as climate and season. I find P. Alsi- 
nearum in the country here on Stellaria media always with 
oospores and conidia in spring, while in autumn often with 
conidia, but never with oospores. In Cystopus candidus, the 
well-known rust-fungus of the Cruciferce, the oospores occur 
very seldom in North Germany, so far as my experience goes, 
but are common in South-west Germany in many species of 
host plants. We have not then absolute laws to deal with here, 
but laws which in special instances admit of exceptions. For 
example, Cystopus cubicus occurs very often on species of Tra- 
gopogon, Podospermum, and Scorzonera. I have found it almost 
always with conidia and oospores on Sc. hispanica, but for years 
as regularly without oospores on all other hosts. In one case, 
however, I found the oospores on the leaves of Tragopogon 
j)Orrifolius. 
When the oospores are mature, the part of the host which 
contains them soon dies. It then withers and falls to the ground ; 
the dead tissues, and often also the membrane of the oogonia, 
decay, and the oospores alone survive. In places where they 
exist in great numbers, they can be detected in the aground. 
A considerable period of apparent inactivity precedes their ger- 
mination, which does not, in the cases hitherto observed, take 
place till after the winter has passed, no matter whether the 
maturity had been arrived at in autumn or, as is mostly the 
case, in early summer. The germination of the oospore and 
the penetration of the young fungus into the host plant occur 
in the moist and warm season which succeeds the winter ; 
the renewal of summer vegetation is taken for granted. The 
