Should the reader desire to watch the growth of the pollen tubes, he can easily do so by shaking some pollen cells (preferably large ones, such as those of some lilies) on to a solution of sugar, and watching them at intervals with the aid of a lens. In the course of a few hours the pollen tubes will be seen to protrude, and these eventually grow to a considerable length. In order that the ovules of a flower may develop into seeds, it is necessary that they become impregnated by pollen from the anthers of the same species, and this is brought about in the following manner: The pollen cells having been transferred by some means to the mature stigma, they adhere to the surface of the latter, and, deriving their nourishment from the secretion of the stigmatic cells, as above described, proceed to throw out their tubes. These tubes force their way between the cells of the stigma and style, and enter the ovary. Each tube then finds its way to one of the ovules, which it enters by means of a minute opening in its double coat called the micropyle, penetrates the embryo-sac, and reaches the ovum or egg-cell. The ovule is now impregnated or fertilised, and the result is that the ovum divides and subdivides into more and more cells till at last an embryo plant is built up. The ovule has thus become a seed, and its further development into a mature plant depends on its being transferred to a suitable soil, with proper conditions as to heat and moisture. If the flower concerned is a perfect one, and the ovules are impregnated by pollen from its own anthers, it is said to be self-fertilised; but if the pollen cells that fertilise the ovules have been transferred from a distinct flower, it is said to be cross-fertilised. Now, it has been observed that although self-fertilisation will give rise to satisfactory results in some instances, producing seeds which develop into strong offspring, cross-fertilisation will, as a rule, produce better seeds. In fact, self-fertilisation is not at all common among flowers, and the pollen has frequently no effect unless it has been transferred from another flower. In a few cases it has been found that the pollen even acts as a poison when it is deposited on the stigma of the same flower, causing it to shrivel up and die. In many instances the structure and growth of the flower is such that self-pollination is absolutely impossible; and where it is possible the seedlings resulting from the process are often very weak.