The organs concerned in the production of seed by which flowering plants are propagated, are the stamens, actively, and pistils, passively.
Stamens produce in their anthers pollen grains, which must be carried to the pistils, either those of the same flower or of another. This process is called pollination. "Cross-pollination," the transference of pollen to a different flower, gives seed greater potency, so that stronger and hardier plants result from it than when the pollen falls upon its own pistil. How is cross-pollination to be obtained? Not by any movements of the flower itself. The aid of some foreign agent must be invoked.
In the evolution of plants, flowers and insects began to appear upon the earth about the same time. At first the colors of all flowers were dull green, greenish white, or white. The food of most insects is nectar, a sweet secretion found in the very heart of flowers, which by them is converted into honey. Insects fly to flowers, dive into them in their efforts to secure the coveted nectar, and, in so doing, necessarily rub their bodies against the upright stamens. If the anthers are ripe and the pollen grains are free, they are caught in their hairy bodies or limbs. The insects then fly to another flower and leave the pollen upon its stigma, provided that is ripe and ready. Many flowers, if neglected by insects, cannot produce seed at all. If the stamens and pistils mature at different times, or if they are so situated that they cannot reach one another, self-pollination cannot take place. Insects must be attracted to such flowers.
That insects are not color-blind may be seen from the fact that they will follow to their undoing "nectar paths," streaks or lines of color found on the stems of some insect-devouring plants which lead straight to the smooth, fatal rim of the cup.