There are two distinct forms of the primrose flower, often called the pin-eyed and the thrum-eyed, the two forms growing on different plants. The former has its stamens at a contracted portion of the tube, about half way down, and a style so long that the stigma is visible at the top of the tube. The latter has its stamens at the contracted throat of the tube, while the style is so short that the stigma is half-way down.
The Bugle.
These two forms may be termed the long-styled and the short-styled primrose, respectively, and the difference is of great importance, inasmuch as it helps to bring about the cross-fertilisation of the flower.
The Broad-Leaved Garlic.
The principal agents concerned in the transfer of pollen from one flower to another are the wind and insects, but it is evident that the work is done, in the case of the primrose, by insects; for not only do we find that the anthers and the stigma are protected from the wind, being more or less hidden in the tube of the corolla, but the showy corolla, the delicate scent emitted by the flower, and the nectar produced at the base of the tube all combine to encourage nectar-loving insects whose proboscis is long enough to reach the sweets.
While such an insect is sucking the nectar from a short-styled primrose, the base of its proboscis is rubbing pollen from the anthers at the top of the tube, and the removal of the pollen is assisted by the contracted throat of the corolla in this kind of flower. Should that insect then visit a long-styled flower, the base of the proboscis, now dusted with pollen, will transfer some of the pollen cells to the stigma. In the same way pollen will be transferred from the anthers of the long-styled to the short-styled flower, since the stamens and stigma respectively occupy corresponding positions in the tubes of the corollas.