CHAPTER XXXII
THE GRAIN OF WHEAT
“Now turn your attention to this picture of an ivy seed cut through lengthwise. Where is the germ or little plant in its egg? It is that little white thing, rather long and narrow, embedded in the substance of the seed at one end. A fine line marks the division of the two cotyledons, which are now pressed close together. Next to them comes the tigella, or little stalk, ending in the radicle, or rootlet. Notice, my friends, how small these cotyledons are, how different from the enormous nursing-leaves of the almond, acorn, broad bean, kidney bean, and pea. These poor little plant-udders must soon get dried up, and if there were no other resources available at the time of germination the ivy would speedily starve to death.
Longitudinal Section of Ivy Seed
“But look: under the skin of the seed we find a goodly store of farinaceous matter, in which the germ is embedded. Almost the whole of the seed consists of this accumulation of flour. So here we have the food-supply that will supplement that contained in the cotyledons, a very insufficient provision in itself. This granary of plenty within which the germ is lodged, this storehouse of food is called the [[156]]perisperm. The almond, acorn, pea, bean, with a host of others, are quite lacking in anything of the sort, having under the skin only the germ and nothing more, absolutely nothing. The reason for this difference is plain enough. The almond, bean, pea, acorn, with their big cotyledons bursting with nutritive matter, do not need a supplementary ration; the germ will be sufficiently suckled by the udders nature has provided in the form of these cotyledons. But the ivy, with its poor little cotyledons, calls for help, and finds it in the farinaceous storehouse of the perisperm.
“Thus a seed may have a double supply of nourishment to meet the needs of the young plant: that contained in the cotyledons and that stored up in the perisperm. Cotyledons are never lacking, but the perisperm is not found in all seeds. There is none in the almond, acorn, chestnut, apricot, bean, or pea; but to make up for this lack their cotyledons are of considerable size. On the other hand, buckwheat, chickweed, and ivy, whose cotyledons are small, are provided with a perisperm. All this may be reduced to one general rule. Cotyledons and perisperm play similar parts: they both help to nourish the little plant in its infancy. So, generally speaking, the seed with large cotyledons has no perisperm, while the seed with small cotyledons has one.
Longitudinal Section of a Grain of Wheat