244 Researches into the Nature of the Potato-Fungus. 
Cystopus, produce zoospores from their conidia, present also the 
same phenomenon in connection with the oospores. 
Up to this time the sexual organs have not been observed in 
Phytophthora, the potato-fungus. What is known of it in other 
respects corresponds so well with Peronospora, that there is doubt- 
less every reason to expect exactly the same sexual organs in 
the one as in the other. Still this is not certain ; and, in par- 
ticular, stress must be laid on the fact that there is another group 
of fungi to which, as far as our knowledge goes, the potato- 
fungus might with equal reason be compared ; I refer to the 
family of Saprolegniece, fungi which for the most part live in 
water ; sotne grow in living plants. Their mycelium and the 
mode of formation of their zoospores essentially agree with 
what is observed in Peronospora, but the form of the zoosporangia 
is often different, though not always. The principal difference 
of the two families consists in the mode of formation of the 
oospores, for in Saprolegniece the entire protoplasm of the 
oogonium goes to form the oospores, — forming either one by 
separating itself entirely from the oogonium and surrounding 
itself with the endospore and epispore, or forming several by 
separating into a number of parts, each of which possesses the 
structure of an oospore. The shape of all these organs is 
always very like that of the same organs in Peronosporeai, and 
the method of germination in the oospores is also similar ; but 
the oospores themselves are usually smaller and more slender. 
Should, therefore, Phgtophthoi'a come to be classed with the 
Saprolegniece, the mode of formation of the oospores would be 
expected to be similar to what is known to take place among its 
allies, just as when it is placed, as it usually is, with the Pero- 
nosporece, we ascribe to it the same method of reproduction as 
we find in that family. 
2. The phenomena connected with the growth of the Perono- 
sporece are tolerably uniform ; all are typical parasites in living 
plants containing chlorophyll. Their complete development is 
dependent on their finding the living organism, with its chemical 
and physical properties, which will afford it a suitable host ; and 
most species are so restricted in this respect that they can 
only grow in certain species or groups of species of plants but 
not in others, a condition of things which holds good for para- 
sites generally. But this does not exclude the possibility of 
bringing a parasite more or less forward in its development by 
means of artificial nutriment. 
That the species of the Pcronosporece, as parasites, do more or 
less interfere with the life of their host and produce disease in it 
must be obvious ; and the circumstances under which this occurs 
is too generally known to require detailed explanation here. 
