THE SEED’S FOOD-SUPPLY
“When it has leaves and roots that are sufficiently developed, the
little almond-tree will nourish itself by drawing what it needs from
the earth and air. But until then it must live, it must grow stronger,
and it must increase a little in size. As nothing can come from
nothing, the germinating seed must find somewhere the material for its
first growth. This cannot be in the soil so long as the radicle is
nothing but a point, incapable of any work; neither can it be in the
air so long as the little leaf-bud has not unfolded and developed into
foliage. The seed, then, must have a certain supply of nutriment stored
up within itself. Let us turn our attention to this prepared stock of
food.
“In the almond we have studied the gemmule or leaf-bud, the radicle,
and the tigella; but there still remain two large pieces, easily
separable from each other, and constituting by themselves alone almost
the entire bulk of the seed. These two pieces are the plant’s first
pair of leaves, but leaves of a peculiar structure, being very thick,
fleshy, and relatively enormous in size. They are the alimentary
reservoirs, the storehouses of food from which in its beginning the
young plant must draw its sustenance. When germination begins, these
two large leaves, swollen with nutritive matter, yield little by little
a part of their substance to the tiny plant and suckle it, as it were.
They might therefore be called vegetable udders, nursing-leaves, but
science calls them cotyledons. The unhatched chick in its shell has the
yolk of the egg to furnish substance for its growth, the young lamb has
its mother’s milk, the germ of the plant has the juice of the
cotyledons.
“The same structure, with two cotyledons of great size and easy to
observe, may be found in the broad bean, pea, kidney bean, and acorn,
and in the stones of the peach, apricot, and plum. It would also be
found in the pips of pears and apples as well as in the seeds of most
of our cultivated plants, but more difficult to distinguish in
proportion to the smallness of the seed. In every instance the seed
would be found to have two cotyledons as food-storehouses, and also a
gemmule and a radicle united by the tigella. Other plants, on the
contrary, like maize, wheat, and the other cereals, as also the lily,
tulip, and iris, have but one cotyledon, one nursing-leaf for the new
vegetable organism.
“It is not always easy, especially when the seed is very small, to
ascertain whether it has two cotyledons or only one; but as soon as
germination has begun, this difficulty disappears. Then the seed with
two cotyledons is seen to push up two leaves, the very first to appear,
situated opposite each other, and very often differing in shape from
those that come later. In the radish, for example, they are
heart-shaped; in the carrot, long and narrow like little tongues. These
two leaves that precede the others are known as seminal leaves. They
come from the two cotyledons, which generally open in the air and grow
green while nourishing the young plant with a part of their substance;
but sometimes, as in the acorn, they remain hidden underground. On the
other hand, seeds having but one cotyledon come up with only one
seminal leaf, generally narrow and long. This is what we observe if we
watch the germination of a grain of wheat.
“A simpler and quicker method may be used for ascertaining how many
cotyledons a seed has. Hold a leaf up to the light and you will see its
texture traversed by a multitude of little cords which serve it as a
kind of framework. These cords are called veins or nerves. Now then, if
you compare the leaf of a pear-tree with a blade of wheat, or reed, you
will see that in the former the veins are more and more subdivided and
ramified, joining one another and thus forming a network with irregular
meshes, while in the latter the veins do not branch, but run in
parallel lines without forming meshes. We should find the same
difference of framework between the leaves of the elm, poplar, and
plane-tree, on the one hand, and those of the iris, narcissus, and
tulip, on the other. This difference being established, I will add that
with few exceptions, of no interest to us here, every plant with
netted-veined leaves has two cotyledons in its seed, and that every
plant with parallel-veined leaves has but one. Consequently it is only
necessary to glance at the foliage in order to know whether the seed
has two cotyledons or only one. I will say further that pines, firs,
and the other resinous trees have as many as ten cotyledons, which show
themselves as a delicate tuft of leaves when the little plant comes out
of the ground.”
Uncle Paul then led the children into the garden to fix in their minds
by observation the lesson they had just learned. “Gather haphazard,”
said he, “the first leaves you come to; then examine them and tell me
how many cotyledons the seed must contain. First, here is the iris,
with large blue flowers and sword-shaped leaves.”
“I see,” said Jules, “veins running in regular lines side by side,
without ever joining one another. Since these veins are parallel the
iris seed has only one cotyledon.”
“And this blade of grass, this also that I pick from a corn-stalk?”
asked his uncle.
“They, too, have parallel veins, both of them; and so their seeds must
have only one cotyledon.”
“And this grape-leaf, this leaf of the cherry tree?”
“It’s my turn now,” Emile hastened to interpose. “The veins form a sort
of lace with very fine meshes. The grape and the cherry have two
cotyledons.”
“It is as easy as that, my friends. The leaf with its arrangement of
veins shows us the fundamental characteristics of the plant. It tells
us whether the germ is fed by one nursing-leaf or two, whether the
young plant comes up with one seminal leaf or two.”