TUBERS—STARCH
“There are buds that, though called to an independent existence, do
not, before separating from the mother plant, store up provisions nor
thicken their scales; but the plant itself is charged with feeding
them. When it is intended that the stem or branch shall itself maintain
the buds it bears, then, instead of coming out into the open air where
it would speedily cover itself with foliage and flowers, it remains
underground and has for leaves only rudimentary scales. It grows so
corpulent and deformed as to cease to bear the name of branch and to
take instead that of tuber. As soon as necessary supplies have been
stored up, the tuber detaches itself from the mother plant, and
thenceforth the buds it bears find in it abundant nourishment for their
separate existence. A tuber, then, is an underground branch swollen
with nutritive material and having undeveloped scales in place of
leaves, and it is also dotted here and there with buds which it must
feed.
“Let us now look at a potato. What do we see on the surface? Certain
small cavities or eyes; that is to say, so many buds, for these eyes
develop into branches if the potato is placed in favorable conditions.
On old potatoes, late in the season, the buds are seen to send forth
sprouts which need only a little sunshine to turn green and become
stalks. Agriculture makes good use of this peculiarity: to propagate
the plant it is customary to put into the ground, not the seeds, which
would yield no harvest before the lapse of several years, but the
tubers, which produce abundantly the same year. Or else the potato is
cut into pieces and each piece, planted in the ground, sends up a new
plant on condition that it has at least one eye; if it has none it rots
without producing anything.
“Furthermore, you can see on the eyes tiny little scales, which are
leaves modified to adapt them to an underground life, leaves with the
same right to the name as the tough scales of an ordinary bud. Since it
has leaves and buds the potato is therefore a branch. Should there
remain any lingering doubts on this subject, it might be added that by
earthing up the plant, that is to say by heaping soil around the stalk,
the young branches thus buried can be converted into potato-bearers;
and it might also be added that in rainy and cloudy seasons it is not
rare to see some of the ordinary branches thicken and swell up in the
open air, and thus produce potatoes more or less perfect. Accordingly
the potato is to be regarded as an underground branch swollen with
nourishment—in short, a tuber.
“Many other plants produce similar branches that grow under ground. In
this number is the Jerusalem artichoke, the tubers of which have buds
arranged two by two on opposite swellings, from front to back and from
right to left in turn, exactly as are leaves and buds on the stem.
“The potato feeds it buds on a farinaceous substance called fecula or,
in less learned language, starch. It is the very material that makes
the vegetable so rich in nutriment for us. We turn to our own account
what the plant has stored up for its young shoots. Starch is contained
in the extremely small cavities with which the flesh of the tuber is
all riddled. These cavities are called cells. They are microscopic sacs
made of a fine membrane and having no opening. Crammed full of starch
grains and crowded one against another, they compose the fleshy
substance of the potato. But these cavities are so small that a person
would strain his eyes in vain in any attempt to see them in the
cross-section of a potato. A magnifying glass is necessary. So minute
are the cells that in a piece of potato no larger than a pin’s head
there is room for dozens and dozens of them. This picture shows you,
but much larger than in nature, a potato cell with the grains of starch
it encloses.”
“How beautifully,” exclaimed Emile, “those grains of starch are
arranged in their little cubby-hole! They might be taken for a nest of
eggs. And you say there are heaps and heaps of these little starch
cells?”
“Yes, my boy; in a medium-sized potato they could be counted by
millions and millions.”
“It must be rather a curious sight to look at a little piece of potato
through a powerful magnifying-glass.”
“It is indeed one of the most curious sights, this countless multitude
of starch grains, all the same shape, all white as snow, gathered
together by tens, dozens, scores, and even more, in their delicate
little box-like cells.
“Let us perform an experiment not beyond our means; let us remove the
starch from a potato. All we need to do is to tear open the cells in
order to liberate the starch grains, and then filter them out. Watch me
do it. With a kitchen grater I reduce the potato to pulp and thus tear
the cells open. Now I put the pulp on a piece of linen over a large
glass and pour a little water through it with one hand while with the
other I keep stirring the pulp. The grains of starch from the ruptured
cells are washed away by the water and carried through the meshes of
the fabric, while the remnants of the cell-walls, being too large to
pass through, stay behind in the filter.
“Thus I obtain a glassful of turbid water. Look at it under a bright
sun. In the water a multitude of white satiny specks are falling like
so much snow and piling up on the bottom. In a few moments the deposit
has settled. I then throw away the clear water above it and have left a
powdery substance, magnificently white, which if pressed between the
fingers creaks like fine sand. It is the starch of the potato, and is
made up of such fine grains that it would take from one hundred and
fifty to two hundred to equal the head of a pin in size. Nevertheless
these grains, minute though they are, have a very complicated
structure, each one of them being composed of a large number of tiny
leaflets folded one over another. The picture I showed you just now
will serve to give you an idea of these superposed leaflets that go to
make, all together, a single grain. Now if some of this starch is
boiled in a little water, the successive leaflets of the grain open and
separate, and the whole becomes an unctuous jelly far exceeding in
volume that of the starch used.”
To prove this assertion, Uncle Paul proceeded to heat in a little water
the starch taken from the potato, and soon the powdery matter was
reduced to a beautiful pellucid jelly.