Field and Woodland Plants

by William S. Furneaux

This being the case, we propose to devote a short chapter to a simple account of the uses of the parts of a flower, and to the various contrivances on the part of the plant to secure the surest and best means of perpetuating the species.

It has already been stated that the stamens produce pollen cells, and that the ovary contains one or more ovules. As soon as the anthers are mature, they open and set free the pollen cells they contained. A stigma is said to be mature when it exposes a sticky surface to which pollen cells may adhere, and on which these cells will grow. When a pollen cell has been transferred to such a stigma, it is nourished by the fluid secreted by the latter, and sends out a slender, hollow filament (the pollen tube) which immediately begins to descend through the stigma, and through the style, if any, till it reaches the ovary.

Pollen Cells throwing out their Tubes.

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.

It has already been hinted that the wonderful variety of form and colour exhibited by flowers has some connexion with this important matter of the transfer of pollen, and the reader who is really interested in the investigation of the significance of this great diversity will find it a most charming study to search into the advantages (to the flower) of the different peculiarities presented, especially if he endeavours to confirm his conclusions by direct observations of the methods by which the pollen cells are distributed to the stigmas.

Pollen cells are usually distributed either by the agency of the wind or by insects; and it is generally easy to determine, by the nature of the flower itself, which is the method peculiar to its species.