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x distinct flowers, then termed unisexual. They are either upon the same plant, or monœcious, or upon different plants, when they are styled diœcious.

The manner in which the fertilization of the ovules takes place is still involved in much obscurity. It is, however, certain that the contact of the pollen with the stigma is necessary for its accomplishment; and this end is attained by Nature in various ways. In some instances the filaments of the stamens are endowed with a peculiar irritability, which causes them, when touched by insects or otherwise, to bend towards the stigma and discharge the contents of the anthers; this is the case with the flowers of the Barberry. In the Stinging Nettle the filaments are curved spirally within the calyx, and, when the latter expands, spring up suddenly and scatter the fertilizing grains over the adjacent pistils. In many plants the elastic coat of the anther itself performs a similar office, and the liberated pollen floats in clouds and becomes deposited upon the stigmas; in Pine woods, during the flowering season, the air is frequently filled with the minute yellow grains of pollen thus discharged. That of monœcious and diœcious plants is often conveyed to the pistils by the bees and flies attracted by the honey in the flowers, and whose bodies are usually furnished with hairs, to which the grains readily adhere. The pollen-grains, however, conveyed to the stigma, are retained upon that organ by means of a viscid fluid secreted upon its surface, and, after remaining there a short time, send forth tubes, which, piercing the substance of the pistil and penetrating into the ovary, impregnate the ovules therein contained.

The numerous hybrids of some flowers so abundantly raised by the florist are produced by the contact of the pollen with the stigmas of distinct species of the same genus. Similar varieties are not uncommon in Nature; but they rarely produce seed, and consequently these abnormal forms seldom become permanent.

The ovary, when matured, becomes the fruit of the plant, and