The albumen of seeds often affords good specimens of secondary deposit, the cells being almost entirely filled with it. An example may be found in a section of vegetable ivory, of which ornaments are sometimes made; its structure resembles essentially that of the plum-stone. In other instances the cells contain secreted matters, as starch, oil, &c.; and sometimes the cotyledons also contain starch and oil. An example of the former exists in the albumen of wheat; and of the latter, in the horse-chestnut, the filbert, and mustard-seed. The albumen and cotyledons serve to supply the embryo with nutriment until the roots have grown sufficiently to enable them to absorb it from the soil; the cotyledons also serve as temporary leaves. The form and relative position of the radicle and cotyledons serve to distinguish certain groups of plants. This may be illustrated by the natural order Cruciferæ, or that containing the mustard, wall-flower, &c. Thus, in one group, which may be represented by the wall-flower, the cotyledons are flat or plane (Pl. I. figs. 43 & 44), the radicle being applied to their edges. This is best seen in a transverse section (fig. 43). They are then called _accum´bent_ (_accumbo_, to lie against); and the botanical sign is O=. In the second group, the cotyledons are plane (Pl. I. fig. 38), with the radicle applied to the back of one of them, as in the seed of the common shepherd’s purse (_Capsel´la bur´sa pastóris_) (Pl. VII. fig. 19). They are then termed _in´cumbent_ (_incumbo_, to lie upon), and the sign is O||. While in the third group the cotyledons are folded in the middle, like the leaves of a book (Pl. I. figs. 49 & 50), and the radicle is enclosed between them, as in the white mustard (_Sina´pis alba_). The cotyledons are then called _condu´plicate_ (_conduplico_, to fold); and their sign is O> >. The plants above-mentioned are evidently all Dicotyledonous, or their seeds have two cotyledons; and they contain no albumen. In the Monocotyledonous division, which may be represented by a grain of wheat (Pl. I. fig. 53), the single cotyledon forms a minute sheath (_a_), enclosing the plumule (_b_), the radicle (_c_) being here but little developed at first, the greater part of the grain consisting of the albumen (_d_); the grain should be softened in water before examination. In the germinated grain the cotyledon appears as a pale sheath, surrounding the convolute green leaves of the plumule; which may be best seen in a transverse section (Pl. I. fig. 48). _Fertilization._--A few words must now be said regarding the formation of seeds, and the action of the pollen-tubes in the process of fertilization.