from the act of crossing.

Gaertner experimented on two plants of _Lobelia fulgens_, brought from
separate places, and found[308] that their pollen was good, for he
fertilised with it _L. cardinalis_ and _syphilitica_; their ovules were
likewise good, for they were fertilised by the pollen of these same two
species; but these two plants of _L. fulgens_ could not be fertilised
by their own pollen, as can generally be effected with perfect ease
with this species. Again, the pollen of a plant of _Verbascum nigrum_
grown in a pot was found by Gaertner[309] capable of fertilising _V.
lychnitis_ and _V. Austriacum_; the ovules could be fertilised by the
pollen of _V. thapsus_; but the flowers could not be fertilised by
their own pollen. Koelreuter, also,[310] gives the case of three {137}
garden plants of _Verbascum phoeniceum_, which bore during two years
many flowers; these he successfully fertilised by the pollen of no less
than four distinct species, but they produced not a seed with their own
apparently good pollen; subsequently these same plants, and others
raised from seed, assumed a strangely fluctuating condition, being
temporarily sterile on the male or female side, or on both sides, and
sometimes fertile on both sides; but two of the plants were perfectly
fertile throughout the summer.

It appears[311] that certain flowers on certain plants of _Lilium
candidum_ can be fertilised more easily by pollen from a distinct
individual than by their own. So, again, with the varieties of the
potato. Tinzmann,[312] who made many trials with this plant, says that
pollen from another variety sometimes "exerts a powerful influence, and
I have found sorts of potatoes which would not bear seed from
impregnation with the pollen of their own flowers, would bear it when
impregnated with other pollen." It does not, however, appear to have
been proved that the pollen which failed to act on the flower's own
stigma was in itself good.

In the genus Passiflora it has long been known that several species do
not produce fruit, unless fertilised by pollen taken from distinct
species: thus, Mr. Mowbray[313] found that he could not get fruit from
_P. alata_ and _racemosa_ except by reciprocally fertilising them with
each other's pollen. Similar facts have been observed in Germany and
France;[314] and I