BULLETIN OF THE BUSSEY INSTITUTION. 327 
know, only on certain Solanacew, including the tomato. It does not, 
however, succeed well on the latter. The germinating power of the 
spores lasts for several weeks, but they do not germinate after a win- 
ter’s exposure. 
In brief, then, the mycelium of the Peronospora infestans, after 
making its way through the stem, leaves, and root of the potato, passes 
through the breathing-pores into the air, and there produces asexual 
spores. ‘These falling on the leaves of healthy plants, or reaching the 
tubers in the ground, spread the disease from plant to plant with 
greater or less rapidity, depending on the weather, until the frost 
destroys all except the tubers which have been gathered by the 
farmer. ‘The question arises, How is the disease propagated from 
year to year? Certainly not by the mycelium in the dead leaves 
and stalks, which cannot survive the cold of winter, or by the asexual 
spores which have been described. One way, and the only one, so 
far as has yet been proved, is by means of the mycelium in the tubers 
which have been gathered in the fall and planted the next spring. 
Of course, very rotten potatoes are not harvested, neither are those 
known to be rotten planted in the spring; but, nevertheless, as can 
be proved by microscopic examination, a certain amount of mycelium 
can often be detected in potatoes which appear sound, and it only needs 
a sufficiently damp season for it to produce disastrous results. 
The question whether the disease may not also be propagated from 
year to year in some way different from that just mentioned, requires 
farther consideration ; for, although such may be the case, we are not, 
as yet, in possession of a sufficient number of facts to warrant us in 
speaking with certainty, and it must be regarded as an open question 
which botanists are trying to answer. ‘There are, in the first place, 
theoretical grounds in favor of such a belief. The bodies called spores 
in fungi are some of them asexual, like those which we have seen in 
the potato rot, and others are the result of some sexual action, and 
are known as odspores. Any species of fungus may have both sexual 
and asexual spores, and, perhaps, several different kinds of the latter. 
Also, when a fungus is parasitic on different kinds of plants, the 
mycelium may, on one kind, bear only asexual, on another only sexual, — 
spores, or the two may be borne together. Although, in the case of a 
great many fungi, the odspores have never been found, all modern 
research renders it extremely probable that they always exist, aud we 
