In some few cases the special movements of certain organs almost ensure pollen being carried from plant to plant.  Thus with many orchids, the pollen-masses after becoming attached to the head or proboscis of an insect do not move into the proper position for striking the stigma, until ample time has elapsed for the insect to fly to another plant.  With Spiranthes autumnalis, the pollen-masses cannot be applied to the stigma until the labellum and rostellum have moved apart, and this movement is very slow. (10/35.  ’The Various Contrivances by which British and Foreign Orchids are fertilised’ first edition page 128.) With Posoqueria fragrans (one of the Rubiaceae) the same end is gained by the movement of a specially constructed stamen, as described by Fritz Muller.

We now come to a far more general and therefore more important means by which the mutual fertilisation of distinct plants is effected, namely, the fertilising power of pollen from another variety or individual being greater than that of a plant’s own pollen.  The simplest and best known case of prepotent action in pollen, though it does not bear directly on our present subject, is that of a plant’s own pollen over that from a distinct species.  If pollen from a distinct species be placed on the stigma of a castrated flower, and then after the interval of several hours, pollen from the same species be placed on the stigma, the effects of the former are wholly obliterated, excepting in some rare cases.  If two varieties are treated in the same manner, the result is analogous, though of directly opposite nature; for pollen from any other variety is often or generally prepotent over that from the same flower.  I will give some instances:  the pollen of Mimulus luteus regularly falls on the stigma of its own flower, for the plant is highly fertile when insects are excluded.  Now several flowers on a remarkably constant whitish variety were fertilised without being castrated with pollen from a yellowish variety; and of the twenty-eight seedlings thus raised, every one bore yellowish flowers, so that the pollen of the yellow variety completely overwhelmed that of the mother-plant.  Again, Iberis umbellata is spontaneously self-fertile, and I saw an abundance of pollen from their own flowers on the stigmas; nevertheless, of thirty seedlings raised from non-castrated fflowers of a crimson variety crossed with pollen from a pink variety, twenty-four bore pink flowers, like those of the male or pollen-bearing parent.

In these two cases flowers were fertilised with pollen from a distinct variety, and this was shown to be prepotent by the character of the offspring.  Nearly similar results often follow when two or more self-fertile varieties are allowed to grow near one another and are visited by insects.  The common cabbage produces a large number of flowers on the same stalk, and when insects are excluded these set many capsules, moderately rich in seeds.  I planted a white Kohl-rabi, a purple Kohl-rabi, a Portsmouth