Fig. 784.—Male flower of nettle.
Fertilization is accomplished in two ways; (1) by the action of the wind, by which the pollen is carried away to other plants; and (2) by means of insects—the bee particularly. These flowers have distinctive qualities relatively. In the case of the pine the pollen is powdery; so those plants which are thus fertilized are the diæcious species, which include the poplar, the oak, and the birch, as well as the pines. These are all wind-carried pollen plants. The nettle is illustrated here with male and female flowers.
Fig. 785.—Female flower of nettle.
Plants fertilized by insects are visited by them, and they carry away upon their heads, or bodies, the pollen, which is then thrust into the stigma by the insect; or perhaps birds may carry the pollen in the same way after sipping the nectar, and thus playing an unconscious, but most important, part in the economy of nature.
Fig. 786.—Erect ovule.
We always find the ovule at the termination of an axis; it is unable to form a seed alone. The pollen grains must fertilize it, and in consequence many ovules come to nought. The ovule is produced in the pistil, which, as before stated consists essentially of two parts—the ovary and the stigma; the latter secretes a fluid to hold the pollen. We annex the representation of a highly-magnified pistil (vertical section, fig. 786a). The pollen grains are indicated by d, attached to stigma, c, projecting through the style, b, into the ovary, a, and passing through the ovules.