DICOTYLEDONOUS EMBRYO. 165 In other monocotyledonous embryos the different parts are more manifest; thus, in many Grasses, as, for instance, the Oat, the cotyledon only partially incloses the plumule and radicle; and thus these parts may be readily observed in a hollow space on its surface. The inferior extremity of the radicle is usually rounded (Fig. 193, r), and it is through this point that the roots, r, burst in germination (Fig. 194; see page 67). The radicle is usually much shorter than the cotyledon, and generally thicker and denser in its nature; but in some embryos it is as long, or even longer, in which case the embryo is called macropodous. (b) The Dicotyledonous Embryo. - These embryos vary very much in form; most frequently they are more or less oval, as in the Bean and Almond (Fig. 195), where the embryo consists of two nearly equal cotyledons, c, between which is inclosed a small axis or tigellum, t, the upper part of which, g, is the plumule, and the lower, r, the radicle. The tigellum upon germination appears as a little stalk supporting the cotyledons. In by far the majority of cases the two cotyledons are nearly of equal size, as in the Pea; but in some embryos they are very unequal. Again, C FIG. 195. FIG. 193. while the cotyledons usually form the greater part of the embryo, in other instances they form but a small portion, or are sometimes altogether absent; while at other times their number is increased, and this may either occur as an irregular character, or as a regular condition, as in many Conifera where we frequently find six, nine, or even fifteen cotyledons; hence such embryos have been termed polycotyledonous. It is more probable, however, that this appearance of a larger number of cotyledons than is usual in Dicotyledonous plants arises from the normal number becoming divided down to their base into segments. In all cases where the number of cotyledons is thus increased, they are arranged in a whorl. The cotyledons are usually thick and fleshy, as those of the Bean and Almond (Fig. 195), in which case they are termed fleshy; at other times they are thin and leaf-like, as in the Lime (Fig. 196, c, c), when they are said to be foliaceous. The cotyledons are commonly sessile, and their margins are usually entire, but exceptions occur to both these characters. FIG. 196.-Embryo of the Lime-tree (Tilia europaa). c, c. Cotyledons, The cotyledons also vary in their relative positions to each other. Generally they are placed parallel, or face to face, as in the Almond (Fig. 195); but they frequently depart widely from such a relation, and assume others analogous to those already described in speaking of the vernation of leaves and the aestivation of the floral envelopes. Thus each of the cotyledons may be reclinate, conduplicate, circinate, equitant, or obvolute, etc. each with five lobes ar ranged in a palmate manner. r. Radicle. This position of the radicle in relation to the cotyledons is also liable to much variation. Thus, the radicle may follow the same direction as the cotyledons, or a different one. In the former case, if the embryo be straight, the radicle will be more or less continuous in a straight POSITION OF THE RADICLE. 167 line with the cotyledons (Fig. 197, r); if, on the contrary, the embryo is curved, the radicle will be curved also (Fig. 198), and sometimes the curvature is so great that a spiral is formed (Fig. 199). In the latter case, where the direction of the cotyledons and radicle is different, the latter may form an acute, obtuse, or right angle to them; or be folded back to such an extent as to lie parallel to the ch ro FIG. 198. FIG. 199. FIG. 200. FIG. 201. FIG. 197. FIG. 197.-Vertical section of the seed of the Pansy. h. Hilum. pl. Embryo with its radicle, r, and cotyledons, co. ch. Chalaza. al. Albumen. ra. Raphe. The embryo is erect or homotropous. FIG. 198.-Vertical section of the seed of the Poppy, with the embryo slightly curved in the axis of albumen. FIG. 199-Vertical section of the seed of Bunias, showing its spiral embryo. FIG. 200.-Embryo of the Woad (Isatis tinctoria). 1. Undivided. 2. Horizontal section. c. Cotyledons. r. Radicle. FIG. 201.-Embryo of the Wallflower. 1. Undivided. 2. Horizontal section. r. Radicle. C. Cotyledons. cotyledons, in which case the radicle may be either applied to their margins (Fig. 201, r), when the cotyledons are said to be accumbent; or against the back of one of them (Fig. 200, r), when the cotyledons are incumbent. Relation of the Embryo to the other Parts of the Seed, and to the Fruit.-In the first place, with regard to the albumen. It must necessarily happen that when the albumen is present, the size of the embryo will be in the inverse proportion to it; thus in Grasses we have a large deposit of albumen and but a small embryo, while in the Nettle (Fig. 202) the embryo is large and the albumen very small. The embryo may be either external to the albumen, and thus in contact with the integuments, as in Grasses, in which case it is described as external; or it may be surrounded by the albumen on all sides, except on its radicular extremity, as in the Pansy (Fig. 197), when it is internal. The embryo is said to be axile or axial when it has the same direction as the axis of the seed (Fig. 197, pl); or, when this condition is not complied with, it is abaxile or eccentric (Fig. 203, pl). mic In FIG. 203. FIG. 204. FIG. 202.-Vertical section of the achanium of the Nettle, containing a single seed. t. Integuments of the seed. pl. Plar. Radicle. st. Stigma. FIG. 203.-Vertical section centa. of the fruit and solitary erect orthotropous seed of the Dock (Rumex). ov. Pericarp. mic. Micropyle. pl. Embryo, (Fig. 204,emb), when it is de scribed as peripherical. As a gen which is inverted or antitropous, and turned toward one side of the albumen, alb. ch. Chalaza. r. Radicle. FIG. 204.-Vertical section of the seed of Lychnis dioica. te. Integuments. emb. Embryo on the outside of the albumen, eral character alb. The embryo is amphitropous. the radicle is turned toward the micropyle (Fig. 203, r), in which case. it is said to be homoblastic, and the cotyledonary extremity to the chalaza, ch. Some apparent exceptions to these relative positions occur in the Euphorbiaceæ, and a few other plants, when the radicle is described as enantioblastic; but such are merely accidental deviations arising from certain trifling irregularities in the course of the development of the parts of the seed. While the relation of the radicle and cotyledonous portion is thus seen to be generally constant, it must necessarily happen, from the varying relation which the hilum bears to the micropyle and chalaza, that its relation to the radicle and cotyledonary portion of the embryo must also vary in like manner. Thus, in an orthotropous seed (Fig. 203), the chalaza and hilum coincide with each MORPHOLOGY OF THE FLOWER. 169 other, and the radicle is then turned toward the apex of the seed, and the cotyledonary portion to the chalaza and hilum; in this case the embryo is said to be antitropous or inverted. In an anatropous seed (Fig. 197), where the micropyle is contiguous to the hilum, h, and the chalaza, ch, at the opposite extremity, the radicle, r, will point toward the hilum or base of the seed, and then the embryo is said to be erect or homotropous. In a campylotropous seed, where the chalaza and micropyle are both near to the hilum (Fig. 154), the two extremities of the embryo, which in such cases is generally peripherical, become also approximated, and it is said to be amphitropous. We have already stated that the terms used in defining the position of the seed to the same cavity are employed in the same sense as previously mentioned when speaking of the ovule (page 154). But as regards the radicle, this is said to be superior or ascending (Figs. 202, r, and 203, r), when it is directed toward the apex of the cell; inferior or descending when it points to the base; centripetal if turned inward toward the axis or center; and centrifugal when it is turned toward the sides. SECTION 7. THEORETICAL STRUCTURE, Or General MORPHOLOGY OF THE FLOWER. We are now in a position to examine in detail the theory that the organs of the Flower are all modifications of one common type-the leaf; or, in other words, that they are homologous parts, and that they owe their differences to special causes connected with the functions which they have severally to perform. It is evident that the bract is closely allied to the leaf, from its structure, figure, form, and color, and from the ordinary development of one or more buds in its axil. But in order to be perfectly convinced of this analogy, let any one examine the Foxglove or the Peony, and then it |