fference of
their caudicles, which, of course, undergo no change. Fritz Mueller
has, moreover, made a large number of crosses between orchids belonging
to distinct species and genera, and he finds that in all cases when the
flowers are not fertilised their footstalks first begin to wither; and
the withering slowly spreads upwards until the germens fall off, after
an interval of one or two weeks, and in one instance of between six and
seven weeks; but even in this latter case, and in most other cases, the
pollen and stigma remained in appearance fresh. Occasionally, however,
the pollen becomes brownish, generally on the external surface, and not
in contact with the stigma, as is invariably the case when the plant's
own pollen is applied.

Fritz Mueller observed the poisonous action of the plant's own pollen
in the above-mentioned _Oncidium flexuosum_, _O. unicorne, pubes_
(_?_), and in two other unnamed species. Also in two species of
Rodriguezia, in two of Notylia, in one of Burlingtonia, and of a fourth
genus in the same group. In all these cases, except the last, it was
proved that the flowers were, as might have been expected, fertile with
pollen from a distinct plant of the same species. Numerous flowers of
one species of Notylia were fertilized with pollen from the same
raceme; in two days' time they all withered, the germens began to
shrink, the pollen-masses became dark brown, and not one pollen-grain
emitted a tube. So that in this orchid the injurious action of the
plant's own pollen is more rapid than with _Oncidium flexuosum_. Eight
other flowers on the same raceme were fertilized with pollen from a
distinct plant of the same species: two of these were dissected, and
their stigmas were found to be penetrated by numberless pollen-tubes;
and the germens of the other six flowers became well developed. On a
subsequent occasion many other flowers were fertilized with their own
pollen, and all fell off dead in a few days; whilst some flowers on the
same raceme which had been left simply unfertilised adhered and long
remained fresh. We have seen that in cross-unions between extremely
distinct orchids the pollen long remains undecayed; but Notylia behaved
in this respect differently; for when its pollen was placed on the
stigma of _Oncidium flexuosum_, both the stigma and pollen quickl