The ovules are the rudimentary seeds, situated in a case at the base of the pistils, each consisting of a central portion, called the nucleus, which is surrounded by two coats, the inner called the secundine, the outer the primine. When the hairlike tube of the pollen-grain passes through the orifice in the coatings of the ovule, and reaches the nucleus, or embryo sack, it is supposed to emit a spermatic or plantlet germ, which passes through the wall of the embryo sack and enters the germinal vesicle contained in it. The vesicle corresponds to the vesicle, or germinal spot, in the eggs of birds, and ovum of mammiferous animals. The germ remains in the vesicle, and finally becomes the embryo, fully developed into a plantlet, as may be seen in many seeds. Flowers of plants are called perfect when the stamens and pistils are in the same flower, as the apple; mon[oe]cious, when in different flowers and on the same plant, as the white oak; and di[oe]cious, when in different flowers and on different plants, as in the hemp. In that class of plants in which the stamens, or males, are on one plant, and the pistils, or females, on another, the males of course must always remain barren; and the pistilates, to be fruitful, must have the pollen from the anthers of the staminate brought in contact with its stigma by wind, insects, or other means. In plants with perfect flower, the stamens are generally situated around and above the pistil, so that the pollen falls upon the stigma by mere force of gravity. In the potato, the pollen is conveyed from the anthers to the stigma by actual contact of the two organs. Cross-breeding in plants consists in fertilizing one variety with the pollen of another variety of the same species. The offspring is called a cross-breed, or variety. The process of cross-breeding consists in taking the pollen of one variety and applying it to the stigma of another variety, in such a way as to effect its fertilization. This is done by cutting away (with scissors) the stamens of the flower to be fertilized, a short time before they arrive at maturity, and taking a flower in which the pollen is ripe, dry, and powdery, from the stalk of the variety wished for the male parent; and holding it in the right hand, and then striking it on the finger of the left, held near the flower, thus scattering the pollen on the stigma of the pistil of the flower to be fertilized. The utmost care should be taken to apply the pollen when the flower is in its greatest vigor, and the stigma is covered with the necessary coating of mucus to insure a perfect connection of the pollen with the pistil, and make the fertilization perfect. All flowers not wanted in the experiment should be removed before any pollen is formed.