33. For an albuminous seed with a well-developed embryo, the common Morning Glory (Ipomœa purpurea, Fig. [40-43]) is a convenient example, being easy and prompt to grow, and having all the parts well apparent. The seeds (duly soaked for examination) and the germination should be compared with those of Sugar and Red Maple ([19-21]). The only essential difference is that here the embryo is surrounded by and crumpled up in the albumen. This substance, which is pulpy or mucilaginous in fresh and young seeds, hardens as the seed ripens, but becomes again pulpy in germination; and, as it liquefies, the thin cotyledons absorb it by their whole surface. It supplements the nutritive matter contained in the embryo. Both together form no large store, but sufficient for establishing the seedling, with tiny root, stem, and pair of leaves for initiating its independent growth; which in due time proceeds as in Fig. [44, 45].
Fig. 44. Seedling of Morning Glory more advanced (root cut away); cotyledons well developed into foliage-leaves: succeeding internode and leaf well developed, and the next forming. 45. Seedling more advanced; reduced to much below natural size.
34. Smaller embryos, less developed in the seed, are more dependent upon the extraneous supply of food. The figures [46-53] illustrate four grades in this respect. The smallest, that of the Peony, is still large enough to be seen with a hand magnifying glass, and even its cotyledons may be discerned by the aid of a simple stage microscope.
35. The broad cotyledons of Mirabilis, or Four-o'clock (Fig. [52, 53]), with the slender caulicle almost encircle and enclose the floury albumen, instead of being enclosed in it, as in the other illustrations. Evidently here the germinating embryo is principally fed by one of the leaf-like cotyledons, the other being out of contact with the supply. In the embryo of Abronia (Fig. [54, 55]), a near relative of Mirabilis, there is a singular modification; one cotyledon is almost wanting, being reduced to a rudiment, leaving it for the other to do the work. This leads to the question of the
[36.] Number of Cotyledons. In all the preceding illustrations, the embryo, however different in shape and degree of development, is evidently constructed upon one and the same plan, namely, that of two leaves on a caulicle or initial stem,—a plan which is obvious even when one cotyledon becomes very much smaller than the other, as in the rare instance of Abronia (Fig. [54, 55]). In other words, the embryos so far examined are all
[37.] Dicotyledonous, that is, two-cotyledoned. Plants which are thus similar in the plan of the embryo agree likewise in the general structure of their stems, leaves, and blossoms; and thus form a class, named from their embryo Dicotyledones, or in English, Dicotyledonous Plants. So long a name being inconvenient, it may be shortened into Dicotyls.
