Researches into the Nature of the Potato-Fungus. 247 
oospores therefore act as the organs of hibernation in the life of 
Peronosporeoe ; and for the species which grow only during the 
summer, the oospores are the only means for carrying them 
through the winter ; others may, as has been shown above, 
hibernate by means of perennial mycelium. 
All the foregoing statements have been confirmed by direct 
observation made in special cases. In no particular do they rest 
on mere conjecture. It will be apparent then that, as regards 
the whole life-history of the Peronosporece, there exists a general 
plan, which, however, presents numerous variations in detail 
according to the species. This must be kept clearly in mind 
in the investigation of every case which occurs in practice, so 
that while we follow the general well-established rule, we must, 
nevertheless, not expect identical phenomena in all cases. 
Of the SaprolegniecB, which must be considered in connection 
with the question before us, only a. few species of Pythium are 
parasitical. Most of the species live in dead organic bodies. 
What we know of the species parasitical on living plants corre- 
sponds with the known phenomena of the Peronosporece, to 
such an extent, at any rate, that it is unnecessary to describe 
them here minutely. One fact only is important, viz. that the 
oospores of the Saprolegniece often live through the winter, or, 
at least, are able to remain a considerable time in apparent 
inactivity ; while, on the other hand, it has been observed that 
they are capable of germinating within a few weeks, or even a 
few days, from the time they attain maturity. From this it is 
evident that oospores, even of closely related forms, do not 
always pass through the same series of changes ; and, since the 
oospores of some species of Peronospora present great similarity 
of structure to those of Saprolegniecp, it may be regarded as 
possible that such species would exhibit the same phenomena 
in their history. 
There is yet another circumstance which must here be re- 
ferred to. There are parasites which cannot complete their 
entire development in one host-plant, but require two different 
species of hosts ; in the one they complete a distinct stage of 
their development ; in the other, the second stage, and then 
return to the first host to begin again. This necessary change 
of the host is called heteroecia or metcecia. We have familiar 
examples of this among animals in the tapeworms, and among 
fungi in many of the rusts. It should be remarked here that 
metcecia has not been observed, and is not even likely, to occur 
in any Peronosporea or Saprolegniea ; all the species belonging 
to these families, which have been thoroughly investigated, are 
known to have only a single host on which they complete 
without change their entire development, — they require no 
