240 Researches into the Nature of the Potato-Fungus. 
The known forms of mycelium present great similarity of 
structure, notwithstanding some peculiarities which supply 
valuable characteristics for distinguishing species. 
The characters derived from the structure and growth of the 
conidiophores divide the FcronosporecB generally into two, per- 
haps better into three, genera, Cystopus, Peronospora, and a third, 
which may be called Phytophthora. 
The conidiophores of Cystopus, the fungus of the "[white rust " 
of plants, are short, club-shaped, terminal branches of the myce- 
J Hum, which bear on their expanded apex a 
' single row of conidia, developed in basipeta! 
order. The conidiophores grow in large 
bunches (see Fig. 1). In Peronospora (see Fig. ■ 
2) the conidiophores occur singly, or in small 
bunches, and are tree-like or dichotomously 
branching tubes. On the sharp end of each 
one of these branches a solitary conidium is 
formed, and this is never followed by a second. 
Phytophthora (see Fig. 3) is distinguished 
from Peronospora in having not one but several 
conidia successively formed at the end of each 
branch of the tree-like conidiophore. When 
the first conidium is ripe, it is pushed to the 
side by an unequal swelling of the point to 
which it is attached. The top of this swollen 
portion then begins to grow in the original 
direction of the branch into a new conical 
point ; and when this has reached a length 
equal to that of a conidium, or a conidium 
and a half, a new conidium is produced at its 
apex. The same process may be repeated in 
vigorous specimens from 10 to 15 times. After 
the falling of the very easily shed conidia, as 
many swellings remain on each branch of the 
conidiophore as there had been conidia ; such 
swellings do not occur in the stems of the true 
Peronospora, but are a sure empirical character of the Phyto- 
phtliora. Among the well-known forms of the family, the potato- 
fungus, Peronospora infestans, Mont., is the only one which has 
this peculiarity. It may therefore be distinguished as Phyto- 
phthora infestans. 
The further development of the mature conidia is the same in 
Cystopus, Phytophthora, and many species of Pcroyiospora (see 
Fig. 4). If fresh ripe conidia are placed in water, their proto- 
plasm, separated into 3 to 8 or more portions, issues through the 
swelling gelatinous membrane of the tops of the conidia and 
Branch of the mycelium 
of Cystopus jmrtulaca. 
Lev., with conidiophores. 
MugD. 390 dium. 
