Fig. 19.—Germination of Bean.
Germination of Bean.—The common bean, as we have seen (Fig. [15]), has cotyledons that occupy all the space inside the seed-coats. When the hypocotyl, or elongated caulicle, emerges, the plumule leaves have begun to enlarge, and to unfold (Fig. [18]). The hypocotyl elongates rapidly. One end of it is held by the roots. The other is held by the seed-coats in the soil. It therefore takes the form of a loop, and the central part of the loop “comes up” first (a, Fig. [19]). Presently the cotyledons come out of the seed-coats, and the plant straightens and the cotyledons expand. These cotyledons, or “halves of the bean,” persist for some time (b, Fig. [19]). They often become green and probably perform some function of foliage. Because of its large size, the Lima bean shows all these parts well.
Fig. 20.—Sprouting of Castor Bean.
Germination of Castor Bean.—In the castor bean the hilum and micropyle are at the smaller end (Fig. [20]). The bean “comes up” with a loop, which indicates that the hypocotyl greatly elongates. On examining germinating seed, however, it will be found that the cotyledons are contained inside a fleshy body, or sac (a, Fig. [21]). This sac is the endosperm. Against its inner surface the thin, veiny cotyledons are very closely pressed, absorbing its substance (Fig. [22]). The cotyledons increase in size as they reach the air (Fig. [23]), and become functional leaves.
| Fig. 21.—Germination
of Castor Bean. Endosperm at a. | Fig. 22.—Castor
Bean. Endosperm at a, a; cotyledons at b. | Fig. 23.—Germination Complete in Castor Bean. |
Germination of Monocotyledons.—Thus far we have studied dicotyledonous seeds; we may now consider the monocotyledonous group. Soak kernels of corn. Note that the micropyle and hilum are at the smaller end (Fig. [24]). Make a longitudinal section through the narrow diameter; Fig. [25] shows it. The single cotyledon is at a, the caulicle at b, the plumule at p. The cotyledon remains in the seed. The food is stored both in the cotyledon and as endosperm, chiefly the latter. The emerging shoot is the plumule, with a sheathing leaf (p, Fig. [26]). The root is emitted from the tip of the caulicle, c. The caulicle is held in a sheath (formed mostly from the seed-coats), and some of the roots escape through the upper end of this sheath (m, Fig. [26]). The epicotyl elongates, particularly if the seed is planted deep or if it is kept for a time confined. In Fig. [27] the epicotyl has elongated from n to p. The true plumule-leaf is at o, but other leaves grow from its sheath. In Fig. [28] the roots are seen emerging from the two ends of the caulicle-sheath, c, m; the epicotyl has grown to p; the first plumule-leaf is at o.
| Fig. 24.—Sprouting
Indian Corn. Hilum at h; micropyle at d. |
Fig. 25.—Kernel of Indian Corn.
Caulicle at b; cotyledon at a; plumule at p. | Fig. 26.—Indian Corn. Caulicle at c; roots emerging at m; plumule at p. |