Home › O › Ovule (Part of the Ovule where)

Ovule (Part of the Ovule where)

The part of the Ovule where the coasts are grown to the nucellus is the chalaza. At this part, separation takes place when the seed is ripe, and falls from the stalk; and a scar is left, known as the hilum. In the nucellus, a change goes on which results in the formation of a cell-the embryo-sac--very much larger than the others. It lies near the micropyle, from which, in most Ovules, it is separated by one or two layers of cells of the nucellus. It may be almost as long as the nucellus, and may, in course of growth, crush the latter, until all its tissue disappears.

This cell is full of protoplasm; and in the latter may be seen several bodies, as follows At the end next the micropyle are three cells, viz., two rather long ones, known as helper cells; and, at their lower end, a round cell--the embryonal vesicle--from which the embryo is afterwards developed. At the opposite end of the embryo-sac lie three small bodies--the antipodal cells--the use of which is unknown. They are believed to represent a structure that plays an important part in the development of Ferns and their allies, but that has become superseded in the higher plants. In the protoplasm between them lie one or two nuclei, which have to assist in the formation of the endosperm while the embryo, or young plant, is being formed. A brief account of the development of the Ovule into the seed will help to make these various structures more intelligible.

When the stigma is ready to receive the pollen grains, and one of these is placed upon it, the grain very soon, in most plants, pushes a slender pollen tube between the cells of the stigma, down the loose "conducting tissue" of the style, and into the cavity of the ovary, where it finds its way to the micropyle of an Ovule. Passing down this, it pushes the thin coat of the embryosac before it, presses between the helper cells, and reaches the embryonal vesicle. It is believed that a part of the protoplasm passes from the tube into one of the helper cells, which then acts in turn upon the vesicle. The helper cells, and, probably, the antipodal cells, disappear; and the embryonal vesicle begins to grow, and to be divided by cell-walls, so that a rounded mass of tissue is formed, attached above to a row of cells, or to a mass in some plants, called the suspensor. The lower rounded mass is the embryo, which increases in size, and shows beginnings of the axis and leaves, or cotyledons. These parts can easily be made out in most plants. In the meanwhile, after fertilisation has been effected, the nuclei in the protoplasm begin to divide, and to from new nuclei, around which cells from, and the embryo-sac becomes filled, more or less completely, with cellular endosperm. This may disappear in the ripening of the seed (e.g., in Beans); or it may remain, and may form a large part of the seed (e.g., in Wheat), in which case it is often called the albumen of the seed.

DEVIATIONS FROM THE ABOVE TYPE OF STRUCTURE AND DEVELOPMENT. Structure. In many plants (e.g., Actaea, Compositae, &c.) there is only one coat on the Ovule in a few (e.g., Mistletoe) there is no coat, the nucellus being exposed in the cavity of the ovary. The funiculus varies much; in some Ovules it is very long while others do not possess it. Most Ovules have it closely adherent to one side, the Ovule being inverted, or anatropous, as in Actaea, so as to bring the micropyle close to the placenta, while the Ovule itself remains straight other Ovules are curved or campylotropous as in the Pea and in Lychnis; so that in them also the micropyle is brought near the placenta, but the funiculus is only adherent slightly to one side of the