“Most flowers have both stamens and pistils; but there are plants that have their stamens and pistils in separate flowers. Sometimes the flowers with stamens only and those with pistils only are found on the same plant; sometimes they are found on separate plants. Plants having flowers with stamens only and flowers with pistils only on the same stock are called monœcious plants. This expression means ‘living in one house.’ The flowers with stamens and those with pistils do indeed live together in the same house, since they are found on the same plant. The pumpkin, cucumber, melon, hazel-nut-tree are monœcious plants. “Where flowers with stamens and those with pistils are found on different stocks, the plants are termed diœcious; that is to say, they are double-house plants. Hemp, the locust-tree, and the date-tree are diœcious. “It is especially in monœcious and diœcious plants that the pollen’s indispensability is plainly manifest on account of the natural separation of the stamens and pistils. Let us take for example the locust, a tree of extreme southern France, bearing seeds in pods similar to those of the pea, but brown, very long, and very wide, and containing in addition to the seeds a sugary pulp. Supposing we took a notion, if the climate permitted, to grow locust seeds in our garden, what locust tree must we plant? Evidently the one with pistils, because it alone produces the ovaries that become fruit. But that is not enough. Planted by itself, the locust tree with pistils will indeed blossom profusely every year, but will never in all eternity bear any seeds, for its flowers will fall without leaving a single ovary on the branches. What is wanting? The action of the pollen. Near the locust with pistils let us plant one with stamens. Now fructification proceeds as we wish. Puffs of wind, insects that pilfer from one flower and carry to another—these convey the pollen from the stamens to the stigmas, the torpid ovaries spring to life, and the locust pods grow and ripen perfectly. With pollen, seeds; without pollen, no seeds. “Another example. In spots of fertile land in Northern Africa, spots of land called oases, the Arabs cultivate numerous date-trees which provide them with dates, their principal food. Date-trees, too, like the locust, are diœcious. Now, in the country of the date-tree, a sandy plain parched by the sun, spots of watered and fertile land are rare and have to be turned to the utmost possible account. Accordingly the Arabs plant only date-trees with pistils, the only ones that will produce dates. But when they are in flower, the Arabs go long distances to fetch bunches of blossoms with stamens from wild date trees in order to shake the pollen on the trees they have planted. Without this precaution there is no harvest.