[Illustration: Fig. 21. Fruit (one key) of Silver Maple, Acer dasycarpum, of natural size, the seed-bearing portion divided to show the seed. 22. Embryo of the seed taken out. 23. Same opened out, to show the thick cotyledons and the little plumule or bud between them. 24. Germination of Silver Maple, natural size; merely the base of the fruit, containing the seed, is shown. 25. Embryo of same, taken out of the husk; upper part of growing stem cut off, for want of room.] 24. =Examples of Embryos with thickened Cotyledons.= In the Pumpkin and Squash (Fig. 26, 27), the cotyledons are well supplied with nourishing matter, as their sweet taste demonstrates. Still, they are flat and not very thick. In germination this store is promptly utilized in the development of the caulicle to twenty or thirty times its length in the seed, and to corresponding thickness, in the formation of a cluster of roots at its lower end, and the early production of the incipient plumule; also in their own growth into efficient green leaves. The case of our common Bean (Phaseolus vulgaris, Fig. 28-30) is nearly the same, except that the cotyledons are much more gorged; so that, although carried up into the air and light upon the lengthening caulicle, and there acquiring a green color, they never expand into useful leaves. Instead of this, they nourish into rapid growth the plumule, which is plainly visible in the seed, as a pair of incipient leaves; and these form the first actual foliage. 25. Very similar is the germination of the Beech (Fig. 31-33), except that the caulicle lengthens less, hardly raising the cotyledons out of the ground. Nothing would be gained by elevating them, as they never grow out into efficient leaves; but the joint of stem belonging to the plumule lengthens well, carrying up its pair of real foliage-leaves. 26. It is nearly the same in the Bean of the Old World (Vicia Faba, here called Horse Bean and Windsor Bean): the caulicle lengthens very little, does not undertake to elevate the heavy seed, which is left below or upon the surface of the soil, the flat but thick cotyledons remaining in it, and supplying food for the growth of the root below and the plumule above. In its near relative, the Pea (Fig. 34, 35), this use of cotyledons for storage only is most completely carried out. For they are thickened to the utmost, even into hemispheres; the caulicle does not lengthen at all; merely sends out roots from the lower end, and develops its strong plumule from the upper, the seed remaining unmoved underground. That is, in technical language, the germination is _hypogæous_. [Illustration: Fig. 26. Embryo of Pumpkin-seed, partly opened. 27. Young seedling of same.]