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

“I have just told you that many plants have only one cotyledon. I will add that this cotyledon is usually very small. It is especially in these plants that the perisperm is present. The grain of wheat offers [157]a notable illustration of this truth. Cut lengthwise and looked at through a magnifying-glass, this seed would reveal to us what is represented in the picture I now show you. At the bottom and toward one side is the germ, forming but a very small part of the seed. At c is the single cotyledon, whence will come the first leaf, the seminal leaf. At e is the gemmule, which will furnish the next leaves. At the opposite end is a short nipple, the radicle, whence the root will spring. Now compare the tiny cotyledon of the wheat with the two voluminous ones of the almond. The latter, with their rich store of nourishment, will easily be able to feed the young plant until it has vigorous roots; but the cotyledon of the wheat, so poor and slender—can it nourish the young plant? Certainly not. Then the wheat germ must without fail have a storehouse of provisions. This storehouse is the perisperm (pr), a farinaceous mass constituting nearly the whole of the seed. This same perisperm, the first food of the wheat’s first shoot, is also the chief food of man; it is what, under the millstone, becomes flour, of which bread is made. But how can the farinaceous substance of the perisperm nourish the plant? A very simple experiment will show us. Put some wheat in a saucer and keep it slightly moist. In a short time the seed will germinate. As soon as the young sprouts show their green points [158]take one of the grains: you will find it softened all through. You can crush it between your fingers and squeeze out a white fluid, very sweet to the taste and much resembling some sort of milk. What has taken place ought not to be beyond your power to surmise from the account I gave you of the wonderful change starch may undergo. The perisperm of the wheat-grain consists chiefly of starch. During germination this accumulation of starch is converted into a sugary substance, into glucose in fact. Thence comes the sort of plant-milk with which the seed is now swollen. The germ is immersed in this sweet liquid; it imbibes it, soaks it up almost as a fine sponge would; and with the matter thus absorbed it augments its own substance, which lengthens into root, stem, and leaves. With what furnishes us bread the grain of wheat suckles the starting wheat-stalk.”