The primrose and cowslip.—In the primrose we have flowers of a type differing from all those previously considered in this chapter. Not only are the sepals joined together to form a five-toothed calyx-tube, but the five petals are also joined together to form a corolla-tube, and the stamens are fixed on the corolla-tube. There are two kinds of primroses, known to country children as pin-eyed and thrum-eyed flowers respectively ([Fig. 82]). The two forms grow on separate plants. In a pin-eyed primrose the style is long, and the stigma—looking somewhat like the head of a pin—is at the top of the corolla-tube; while the stamens are halfway down. The thrum-eyed primroses have their stamens at the top, while the stigma of the pistil is halfway down the tube, exactly opposite the place where, in the pin-eyed form, the stamens are inserted. This curious state of things was a great puzzle to botanists until Darwin cleared up the mystery. A bee, thrusting its proboscis down a pin-eyed primrose in search of the nectar at the bottom, dusts it with pollen about halfway down—just in the place which will come in contact with the stigma when the animal visits a thrum-eyed flower. And the pollen from the thrum-eyed form adheres to the part of the bee which will presently touch the stigma of a long-styled flower. This beautiful and simple arrangement makes it practically certain that each primrose shall be fertilised by pollen from the other form. In the thrum-eyed form, however, it is possible for pollen to fall upon the stigma and produce self-fertilisation.
Fig. 82.—Chinese Primrose. L, long-styled (“pin-eyed”) flower;
K, short-styled (“thrum-eyed”) flower; G, stigma; S, anthers.
Fig. 83.—Cowslip. (× ⅙.)
It is obvious that the cowslip ([Fig. 83]) is closely related to the primrose. The difference lies chiefly in the character of the flower-stalk. In the cowslip this is long, and it bears at its top several stalklets, each of which ends in a flower. As in the primrose, cross-fertilisation is secured by some flowers being pin-eyed (long styled) and others thrum-eyed (short styled).