The Elements of Botany, For Beginners and For Schools

by Asa Gray

Fig. 46. Section of a seed of a Peony, showing a very small embryo in the albumen, near one end. 47. This embryo detached, and more magnified.

Fig. 48. Section of a seed of Barberry, showing the straight embryo in the middle of the albumen. 49. Its embryo detached.

Fig. 50. Section of a Potato seed, showing the embryo coiled in the albumen. 51. Its embryo detached.

Fig. 52. Section of the seed of Mirabilis or Four-o'clock, showing the embryo coiled round the outside of the albumen. 53. Embryo detached; showing the very broad and leaf-like cotyledons, applied face to face, and the pair incurved.

Fig. 54. Embryo of Abronia umbellata; one of the cotyledons very small. 55. Same straightened out.

[38.] Polycotyledonous is a name employed for the less usual case in which there are more than two cotyledons. The Pine is the most familiar case. This occurs in all Pines, the number of cotyledons varying from three to twelve; in Fig. [56, 57] they are six. Note that they are all on the same level, that is, belong to the same node, so as to form a circle or whorl at the summit of the caulicle. When there are only three cotyledons, they divide the space equally, are one third of the circle apart. When only two they are 180° apart, that is, are opposite.

39. The case of three or more cotyledons, which is constant in Pines and in some of their relatives (but not in all of them), is occasional among Dicotyls. And the polycotyledonous is only a variation of the dicotyledonous type,—a difference in the number of leaves in the whorl; for a pair is a whorl reduced to two members. Some suppose that there are really only two cotyledons even in a Pine embryo, but these divided or split up congenitally so as to imitate a greater number. But as leaves are often in whorls on ordinary stems, they may be so at the very beginning.

Fig. 56. Section of a Pine-seed, showing its polycotyledonous embryo in the centre of the albumen, moderately magnified. 57. Seedling of same, showing the freshly expanded six cotyledons in a whorl, and the plumule just appearing.