326 BULLETIN OF THE BUSSEY INSTITUTION. 
bodies elongate at one end into a tube, which soon begins to branch 
just like the original Peronospora mycelium. ‘The germination by 
.means of zodspores, although the most frequent, is not the only 
method. Sometimes, the contents of the spore are discharged in one 
mass, and from this mass zodspores are produced as before. A third 
and more simple way is shown in Fig. 2, 6. Here the germinal tube 
grows directly from the smaller end of the spore without the inter- 
vention of zoéspores. It is not very well understood why the spores of 
the same fungus should be subject to such variations in their mode of 
germinating, but it would seem to depend partly on the influence 
of light; the production of zodspores proceeding more favorably in 
the dark, whereas the direct production of a germinal tube takes 
place more frequently in the light. I have, however, repeatedly sown 
spores of Peronospora infestans in watch glasses, and both modes of 
germination were seen in the same watch glass. The germination by 
zoospores is the more dangerous, of course, because each spore can 
then reproduce from six to fifteen new Peronospora plants instead 
of only one, and those who are fond of figures can easily make most 
appalling calculations to show what would be the result if all these 
germinal tubes grew. 
Fortunately, the circumstances under which the tubes will grow are 
limited, as the group of fungi to which the Peronospora infestans is, 
or until recently has been, supposed, at least, to belong, will grow only 
on a single species, or a few nearly related species of plants. Conse- 
quently, although the Peronospora spores will germinate anywhere 
if there is only moisture and warmth, the germinal tubes will all die | 
unless the spores have fallen upon, or near, some potato plant, or some 
species of plant botanically closely allied. Some spores, of course, 
when ripe, easily fall upon, or are carried by the wind to other pota- 
toes, as yet unaffected by the rot. Others fall to the ground and germi- 
nate there, and seem to be able, without great trouble, to penetrate 
to the tubers. De Bary found the ground under infected plants full 
of the germinating spores. If the germinal tubes come in contact 
with a potato leaf, stalk, or tuber, they push their way directly 
through the epidermis, without, apparently, being obliged to enter by 
the breathing-pores. Once inside, the mycelium grows at the expense 
of the potato cells through all parts of the plant, as we have already 
seen. ‘The potato rot fungus will grow, so far as we yet absolutely 
