Insects visit buttercups for the sake of nectar and pollen, and whilst creeping about the flower transfer pollen to the stigmas of the carpels. The insects may have brought some of this pollen from other flowers, and then cross-fertilisation is caused. If the pollen is derived from the same flower self-fertilisation is the result. After fertilisation the ovules become seeds; the sepals, petals, and stamens drop off; and the carpels swell up, forming a dry compound fruit (Fig. 66), consisting of several nutlets. Two other common plants of this family are the anemone (Fig. 67) and marsh marigold (Fig. 68). In neither of these cases has the [Pg 99] flower any petals, but the calyx has taken on the appearance of a corolla. In the marsh marigold it is large and yellow; in the anemone it is white or purple. The three green leaves immediately beneath the flower of the anemone are called bracts. They should not be mistaken for sepals. Fig. 68.—Marsh Marigold. (× ⅙.) The plants of the buttercup family are as generally poisonous as the crucifers are wholesome. Monkshood is especially poisonous, and its root has been mistaken, with fatal results, for that of horse-radish. It is often noticed that grazing cattle avoid the buttercups in a field. The bitter and disagreeable taste of the leaves is of course a valuable protection to the plant. 18. THE PEA FAMILY. 1. The garden pea.—Notice again the habit of the plant: its compound, net-veined leaves with large stipules, and its method of climbing by tendrils which are modified leaflets. Examine the flower and make out (a) the calyx of five united sepals; (b) the curiously shaped corolla. The large upper petal is called the [Pg 100] standard, the two at the side are the wings, and the lowest (really two locked together) is the keel; (c) the ten stamens. One (opposite the standard) is separate; the remaining nine have the lower parts of their filaments united to form a tube. Slit open the filament-tube and remove the stamens, noticing how they are attached to the other parts of the flower; (d) the pistil. Slit open the ovary and see the ovules in it. Watch the various stages of the formation of the fruit (pod). Fig. 69.—Vetch. (× ⅓.) Watch bees visiting the flowers. The insect alights on the wings, and its weight pulls them down and lowers the keel, bringing the stamens against the bee’s body. Cut a complete flower down the middle with a sharp knife, and notice that calyx, corolla, and stamens seem not to be inserted separately on the receptacle, but to spring from a common base.