INof the great pea-and-bean family, the five-petal arrangement—exemplified in its simplest form by the Dog and the Buttercup—is not obvious, at first glance. The largest of the petals is called the ‘standard,’ and stands erect as a kind of background. Below, and on each side of this, are two smaller petals known as the ‘wings,’ between which is the ‘keel.’ The latter is composed of the two remaining petals, united to form a tapering, bag-like structure, with a small opening at its tip. This general description will be found to apply to the of the , the Broom, the Bird’s-foot Trefoil, and many allied plants.
The essential organs are completely enclosed by the keel, which serves also as an alighting place for bees. Also, the ripe pollen is discharged by the anthers into the keel, and kept there in reserve, until a bee comes to probe for nectar through openings at the keel’s base. When this happens, the insect’s weight forces the keel downwards in such a way that the stamens within literally pump out some of the pollen from the tip, thus applying it to the underside of the insect’s body. The stigma (which in this instance is receptive only when it is rubbed) is also forced out, and will thus receive any pollen which the bee may have brought from another flower.
Theillustrates a method by which self- , if not entirely eliminated, is rendered unlikely. If a number of Primrose plants are examined, it will be found that the flowers are not always quite alike. There are two forms, in one of which—known popularly as ‘thrum-eyed ‘—the pistil has a short style or stalk connecting it with the ovary, while the stamens are situated at the mouth of the corolla tube. In the other form, called ‘pin-eyed,’ the style is long, and its globular stigma almost fills the entrance of the tube, while the anthers are low down—being on about the same level as the stigma in the corresponding ‘thrum-eyed ‘flowei
Long-tongued flies, and occasionally bees, visit Primroses in search of their nectar, which is secreted low down at the base of the tube. When an insect probes a long-styled flower, its proboscis is dusted with pollen at a point which, if it
subsequently visits a short-styled flower, is brought into contact with its stigma; and vice versa. As the contrivance is not absolutely fool-proof, Primroses are sometimes self-fertilised. But experiment has shown that the best production ofresults from cross- ; and this can only be secured by a transfer of pollen from one type of flower to the other by the method that has been described.