lisation was effected only by the
aid of a distinct species. But, as we shall presently see, distinct
plants, raised from seed, of _Oncidium flexuosum_, and probably of the
other species, would have been perfectly capable of fertilising each
other, for this is the natural process. Again, Mr. Scott found that the
pollen of a plant of _O. microchilum_ was good, for with it he
fertilised two distinct species; he found its ovules good, for they
could be fertilised by the pollen of one of these species, and by the
pollen of a distinct plant of _O. microchilum_; but they could not be
fertilised by pollen of the same plant, though the pollen-tubes
penetrated the stigma. An analogous case has been recorded by M.
Riviere,[307] with two plants of _O. Cavendishianum_, which were both
self-sterile, but reciprocally fertilised each other. All these cases
refer to the genus Oncidium, but Mr. Scott found that _Maxillaria
atro-rubens_ was "totally insusceptible of fertilisation with its own
pollen," but fertilised, and was fertilised by, a widely distinct
species, viz. _M. squalens_.

As these orchids had grown under unnatural conditions, in {134}
hot-houses, I concluded without hesitation that their self-sterility
was due to this cause. But Fritz Mueller informs me that at Desterro,
in Brazil, he fertilised above one hundred flowers of the
above-mentioned _Oncidium flexuosum_, which is there endemic, with its
own pollen, and with that taken from distinct plants; all the former
were sterile, whilst those fertilised by pollen from any _other plant_
of the same species were fertile. During the first three days there was
no difference in the action of the two kinds of pollen: that placed on
the stigma of the same plant separated in the usual manner into grains,
and emitted tubes which penetrated the column, and the stigmatic
chamber shut itself; but the flowers alone which had been fertilised by
pollen taken from a distinct plant produced seed-capsules. On a
subsequent occasion these experiments were repeated on a large scale
with the same result. Fritz Mueller found that four other endemic
species of Oncidium were in like manner utterly sterile with their own
pollen, but fertile with that from any other plant: some of them
likewise produced seed-capsules when impregnated with pollen of widely
distinct