ycelium, in the substance of the plant, destroying
the tissues with which it comes in contact. As these processes of
multiplication take place very rapidly, millions of spores are soon set
free from a single infested plant; and, from their minuteness, they are
readily transported by the gentlest breeze. Since, again, the zoospores
set free from each spore, in virtue of their powers of locomotion,
swiftly disperse themselves over the surface, it is no wonder that the
infection, once started, soon spreads from field to field, and extends
its ravages over a whole country.

However, it does not enter into my present plan to treat of the potato
disease, instructively as its history bears upon that of other epidemics;
and I have selected the case of the _Peroganspora_ simply because it
affords an example of an organism, which, in one stage of its existence,
is truly a "Monad," indistinguishable by any important character from our
_Heteromita_, and extraordinarily like it in some respects. And yet this
"Monad" can be traced, step by step, through the series of metamorphoses
which I have described, until it assumes the features of an organism,
which is as much a plant as is an oak or an elm.

Moreover, it would be possible to pursue the analogy farther. Under
certain circumstances, a process of conjugation takes place in the
_Peronospora_. Two separate portions of its protoplasm become fused
together, surround themselves with a thick coat and give rise to a sort
of vegetable egg called an _oospore_. After a period of rest, the
contents of the oospore break up into a number of zoospores like those
already described, each of which, after a period of activity, germinates
in the ordinary way. This process obviously corresponds with the
conjugation and subsequent setting free of germs in the _Heteromita_.

But it may be said that the _Peronospora_ is, after all, a questionable
sort of plant; that it seems to be wanting in the manufacturing power,
selected as the main distinctive character of vegetable life; or, at any
rate, that there is no proof that it does not get its protein matter
ready made from the potato plant.

Let us, therefore, take a case which is not open to these objections.

There are some small plants known to botanists as members of the genus
_Colcochaete_, which, without being truly parasitic, grow upon certain
water-weeds, as lichens grow upon trees. The little plant has the form of
an elegant green star, the