The young ovule (_I_, _J_) consists of a central, elongated body (nucellus), having a single layer of cells enclosing a large central cell (the macrospore or embryo sac) (_sp._). The base of the nucellus is surrounded by two circular ridges (i, ii) of which the inner is at first higher than the outer one, but later (_K_, _L_), the latter grows up above it and completely conceals it as well as the nucellus. One side of the ovule grows much faster than the other, so that it is completely bent upon itself, and the opening between the integuments is brought close to the base of the ovule (Fig. 95, _L_). This opening is called the "micropyle," and allows the pollen tube to enter. The full-grown embryo sac shows the same structure as that already described in _Monotropa_ (page 276), but as the walls of the full-grown ovule are thicker here, its structure is rather difficult to make out. The ripe stigma is covered with little papillæ (Fig. 95, _F_) that hold the pollen spores which may be found here sending out the pollen tube. By carefully opening the ovary and slightly crushing it in a drop of water, the pollen tube may sometimes be seen growing along the stalk of the ovule until it reaches and enters the micropyle. To study the embryo a series of young fruits should be selected, and the ovules carefully dissected out and mounted in water, to which a little caustic potash has been added. The ovule will be thus rendered transparent, and by pressing gently on the cover glass with a needle so as to flatten the ovule slightly, there is usually no trouble in seeing the embryo lying in the upper part of the embryo sac, and by pressing more firmly it can often be forced out upon the slide. The potash should now be removed as completely as possible with blotting paper, and pure water run under the cover glass. The fertilized egg cell first secretes a membrane, and then divides into a row of cells (_N_) of which the one nearest the micropyle is often much enlarged. The cell at the other end next enlarges and becomes divided by walls at right angles to each other into eight cells. This globular mass of cells, together with the cell next to it, is the embryo plant, the row of cells to which it is attached taking no further part in the process, and being known as the "suspensor." Later the embryo becomes indented above and forms two lobes (_Q_), which are the beginnings of the cotyledons. The first root and the stem arise from the cells next the suspensor. CHAPTER XVIII. CLASSIFICATION OF DICOTYLEDONS. DIVISION I.--_Choripetalæ_. Nearly all of the dicotyledons may be placed in one of two great divisions distinguished by the character of the petals. In the first group, called _Choripetalæ_, the petals are separate, or in some degenerate forms entirely absent. As familiar examples of this group, we may select the buttercup, rose, pink, and many others.