The seed of the broad bean.—The seed of the broad bean (Fig. 1) is large, having a diameter of perhaps an inch and a half, and a thickness of half an inch. In shape it is oval, but at one region the edge is indented, and a black scar (st. sc.) runs from the indentation along the edge for a distance of about three-quarters of an inch. This scar is the place of attachment of the stalk which formerly carried the seed in the bean-fruit (pod). It may be called the stalk-scar. If a soaked bean is wiped dry and then gently squeezed, a small drop of water escapes from the end of the stalk-scar [Pg 6] nearest the indentation. The hole out of which the water comes is very small and difficult to see, but its position is thus made clear. This hole (m) is called the micropyle,—a word meaning the “little gate.” The bean seed is covered by a tough brown skin, the seed-coat (Fig. 2, s.c.), a funnel-shaped depression in which leads to the micropyle (m). The depression is occupied by a part of the seed which is shaped like a conical peg and called the radicle (R); the point of the radicle is directed toward the micropyle. The great body of the seed is composed of two fleshy, cream-coloured lobes, easily wedged apart by inserting a knife-blade between them; these fleshy lobes are the cotyledons (Cot.). Between them, and continuous with the radicle, is a small yellow body, the plumule (pl.). The relations of the radicle, plumule and cotyledons are best seen by removing one cotyledon (Fig. 2). Fig. 2.—Broad-Bean seed, seen from the inside, after the removal of half the seed-coat and one cotyledon. Cot., the inner face of remaining cotyledon; C′, area of attachment of other cotyledon; m, micropyle; pl, plumule; R, radicle; S.c., seed-coat; st. sc., stalk-scar. (× 1.) A scraped cotyledon at once turns blue when a drop of dilute iodine solution is poured on it, thus showing the presence of starch. We shall see in Chapter II. what use the growing seedling makes of the starchy food which is stored in its cotyledons. [Pg 7] The seed of the pea.—Except in size and shape the seed of the pea is very similar to the bean seed. Its form is spherical, and the scar left by the stalk which formerly attached it to the wall of the pea-pod (Fig. 3) is plainly to be seen. Pointing towards the micropyle is the peg-like radicle; the plumule lies between the hemispherical cotyledons. As before, the cotyledons can be proved to contain starch, by the blue colour which is formed when a drop of iodine solution is poured on the scraped surface. Fig. 3.—Pods and Seeds of Pea. (× ½.)