[Illustration: Fig. 454. Viola cucullata; blue flowers above, cleistogamous flowers smaller and curved below. Section of pistil at right.] =848.= But there are many flowers in which both stamens and pistils are present, and yet in which cross pollination is accomplished through the agency of insects. =859. Pollination of the bluet.=—In the pretty bluet the stamens and styles of the flowers are of different length as shown in figures 455, 456. The stamens of the long-styled flower are at about the same level as the stigma of the short-styled flower, while the stamens of the latter are on about the same level as the stigma of the former. What does this interesting relation of the stamens and pistils in the two different flowers mean? As the butterfly thrusts its “tongue” down into the tube of the long-styled flower for the nectar, some of the pollen will be rubbed off and adhere to it. When now the butterfly visits a short-styled flower this pollen will be in the right position to be rubbed off onto the stigma of the short style. The positions of the long stamens and long style are such that a similar cross pollination will be effected. [Illustration: Fig. 455. Dichogamous flower of the bluet (Houstonia cœrulea), the long-styled form.] [Illustration: Fig. 456. Dichogamous flower of bluet (Houstonia cœrulea), the short-styled form.] =850. Pollination of the primrose.=—In the primroses, of which we have examples growing in conservatories, that blossom during the winter, we have almost identical examples of the beautiful adaptations for cross pollination by insects found in the bluet. The general shape of the corolla is the same, but the parts of the flower are in fives, instead of in fours as in the bluet. While the pollen of the short-styled primulas sometimes must fall on the stigma of the same flower, Darwin has found that such pollen is not so potent on the stigma of its own flower as on that of another, an additional provision which tends to necessitate cross pollination. [Illustration: Fig. 457. Dichogamous flowers of primula.] In the case of some varieties of pear trees, as the Bartlett, it has been found that the flowers remain largely sterile not only to their own pollen, or pollen of the flowers on the same tree, but to all flowers of that variety. However, they become fertile if cross pollinated from a different variety of pear. =851. Pollination of the skunk’s cabbage.=—In many other flowers cross pollination is brought about through the agency of insects, where there is a difference in time of the maturing of the stamens and pistils of the same flower. The skunk’s cabbage (Spathyema fœtida), though repulsive on account of its fetid odor, is nevertheless a very interesting plant to study for several reasons. Early in the spring, before the leaves appear, and in many cases as soon as the frost is out of the hard ground, the hooked beak of the large fleshy spathe of this plant pushes its way through the soil.