In many cases cross-pollination is insured because the _stamens and pistils are in different flowers_ (diclinous). Monœcious and diœcious plants may be pollinated by wind or insects, or other agents (Fig. 204). They are usually wind-pollinated, although willows are often, if not mostly, insect-pollinated. The Indian corn is a monœcious plant. The staminate flowers are in a terminal panicle (tassel). The pistillate flowers are in a dense spike (ear), inclosed in a sheath or husk. Each “silk” is a style. Each pistillate flower produces a kernel of corn. Sometimes a few pistillate flowers are borne in the tassel and a few staminate flowers on the tip of the ear. Is self-fertilization possible with the corn? Why does a “volunteer” stalk standing alone in a garden have only a few grains on the ear? What is the direction of the prevailing wind in summer? If only two or three rows of corn are planted in a garden where prevailing winds occur, in which direction would they better run? [Illustration: FIG. 205.--COMMON BLUE VIOLET. The familiar flowers are shown, natural size. The corolla is spurred. Late in the season, cleistogamous flowers are often borne on the surface of the ground. A small one is shown at _a_. A nearly mature pod is shown at _b_. Both _a_ and _b_ are one third natural size.] Although most flowers are of such character as to insure or increase the chances of cross-pollination, there are some _that absolutely forbid crossing_. These flowers are usually borne beneath or on the ground, and they lack showy colors and perfumes. They are known as =cleistogamous= flowers (meaning “hidden flowers”). The plant has normal showy flowers that may be insect-pollinated, and in addition is provided with these simplified flowers. Only a few plants bear cleistogamous flowers. Hog-peanut, common blue violet, fringed wintergreen, and dalibarda are the best subjects in the Northern states. Fig. 205 shows a cleistogamous flower of the blue violet at _a_. Above the true roots, slender stems bear these flowers, that are provided with a calyx, and a curving corolla which does not open. Inside are the stamens and pistils. Late in the season the cleistogamous flowers may be found just underneath the mold. They never rise above ground. The following summer one may find a seedling plant, in some kinds of plants, with the remains of the old cleistogamous flower still adhering to the root. Cleistogamous flowers usually appear after the showy flowers have passed. They seem to insure a crop of seed by a method that expends little of the plant’s energy. The pupil will be interested to work out the fruiting of the peanut (Fig. 206). Unbaked fresh peanuts grow readily and can easily be raised in the North in a warm sandy garden. [Illustration: FIG. 206.--PODS OF PEANUTS RIPENING UNDERGROUND.] [Illustration: FIG. 207.--STRUGGLE FOR EXISTENCE AMONG THE APPLE FLOWERS.]