y
became dark brown, in the same manner as if the plant's own pollen had
been applied.

Fritz Mueller suggests that, as in all these cases the plant's own
pollen is not only impotent (thus effectually preventing
self-fertilization), but likewise prevents, as was ascertained in the
case of the Notylia and _Oncidium flexuosum_, the action of
subsequently applied pollen from a distinct individual, it would be an
advantage to the plant to have its own pollen rendered more and more
deleterious; for the germens would thus quickly be killed, and,
dropping off, there would be no further waste in nourishing a part
which ultimately could be of no avail. Fritz Mueller's discovery that a
plant's own pollen and stigma in some cases act on each other as if
mutually poisonous, is certainly most remarkable.

We now come to cases closely analogous with those just {136} given, but
different, inasmuch as individual plants alone of the species are
self-impotent. This self-impotence does not depend on the pollen or ovules
being in a state unfit for fertilisation, for both have been found
effective in union with other plants of the same or of a distinct species.
The fact of these plants having spontaneously acquired so peculiar a
constitution, that they can be fertilised more readily by the pollen of a
distinct species than by their own, is remarkable. These abnormal cases, as
well as the foregoing normal cases, in which certain orchids, for instance,
can be much more easily fertilised by the pollen of a distinct species than
by their own, are exactly the reverse of what occurs with all ordinary
species. For in these latter the two sexual elements of the same individual
plant are capable of freely acting on each other; but are so constituted
that they are more or less impotent when brought into union with the sexual
elements of a distinct species, and produce more or less sterile hybrids.
It would appear that the pollen or ovules, or both, of the individual
plants which are in this abnormal state, have been affected in some strange
manner by the conditions to which they themselves or their parents have
been exposed; but whilst thus rendered self-sterile, they have retained the
capacity common to most species of partially fertilizing and being
partially fertilized by allied forms. However this may be, the subject, to
a certain extent, is related to our general conclusion that good is derived