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.
“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
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.
“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 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 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.”