The ovule usually exists as a roundish or egg-shaped mass, with a small opening leading into the apex. This opening leads to a sac inside the ovule, which is filled with a soft protoplasmatic material and cells and is known as the embryo sac. Minute cells are found at the apex of the ovule from the development of which the embryo is produced, after the union of the pollen with a cell at the apex of the embryo sac of the ovule has taken place. [Illustration: CUT XLI. _Fertilization of the ovule._ st, stigma; p, pollen; pt, pollen-tube; o, ovary; e, embryo-sac. After Bergen.] _Fertilization._—As soon as the pollen grain lodges on the stigma, it begins to form into a pollen tube. In more or less time it makes its way through the style into the ovary. It then penetrates the opening at the apex of the ovule, reaches one of the cells and transfers its nucleus into an egg-cell. The latter begins at once to form cell-walls and increases by continued subdivision to the plant embryo. Only one pollen tube is necessary to fertilize each ovule. Still plants produce more pollen than ovules—the ratio is from 1:8 to 1:1000—because so many pollen are lost on their way to the ovules. The mechanism of the forwarding of the pollen to the ovule varies in different plants. In self-fertilization or in those plants in which the ovule may be impregnated by the pollen of the same flower, fertilization is comparatively easy. But in a great many plants cross-fertilization is the rule. In order to accomplish the most successful fertilization, the pollen must come from another plant of the same species. Here nature has devised different ways to carry the pollen from one plant to another. In the first place, there is the wind which accomplishes the task of forwarding the pollen. The wind-fertilized flowers have dry and powdery pollen, and the pistils are feathery, adapted to catch flying pollen grains. The flowers are characterized by their inconspicuousness. They are usually greenish, without any odor or nectar. Another device to forward the pollen is by way of insects. Most of the showy, sweet-scented or otherwise conspicuous kinds of flowers are entirely dependent for fertilization on the transference of pollen from one plant to another by insects. The showy colors and odors serve to attract insects to visit them for their nectar. Insects and flowers are interdependent upon one another. For many insects depend mainly upon the nectar and pollen of flowers for their food. These insects usually visit only one kind of flower during the day and thus carry only one kind of pollen. Butterflies, moths, and most of the bees go straight from one flower to another and carry a good deal of pollen entangled in the scales or hairs of their bodies. On its way, the insect leaves a good deal of the pollen on the stigma of the pistil and becomes dusted with new pollen to be carried to other flowers.