Fig. 91.
Garden bean.
m, micropyle;
h, hilum or scar;
r, raphe;
c, point where
chalaza lies.
Fig. 92.
Bean seed split open
to show plantlet.
211. The castor-oil bean.—This is not a true bean, since it belongs to a very different family of plants (Euphorbiaceæ). In the germination of this seed a very interesting comparison can be made with that of the garden bean. As the “bean” swells the very hard outer coat generally breaks open at the free end and slips off at the stem end. The next coat within, which is also hard and shining black, splits open at the opposite end, that is at the stem end. It usually splits open in the form of three ribs. Next within the inner coat is a very thin, whitish film (the remains of the nucellus, and corresponding to the perisperm) which shrivels up and loosens from the white mass, the endosperm, within. In the castor-oil bean, then, the endosperm is not all absorbed by the embryo during the formation of the seed. As the plant becomes older we should note that the fleshy endosperm becomes thinner and thinner, and at last there is nothing but a thin, whitish film covering the green faces of the cotyledons. The endosperm has been gradually absorbed by the germinating plant through its cotyledons and used for food.
Fig. 93.
How the garden bean comes out of the ground. First the looped
hypocotyl, then the cotyledons pulled out, next casting off the
seed coat, last the plant erect, bearing thick cotyledons,
the expanding leaves, and the plumule between them.
Arisæma triphyllum.[15]
212. Germination of seeds of jack-in-the-pulpit.—The ovaries of jack-in-the-pulpit form large, bright red berries with a soft pulp enclosing one to several large seeds. The seeds are oval in form. Their germination is interesting, and illustrates one type of germination of seeds common among monocotyledonous plants. If the seeds are covered with sand, and kept in a moist place, they will germinate readily.
Fig. 94.
Germination of castor-oil bean.