in a rhizome, but this sometimes decays and drops off . They do not
contain any air, and they sink in water; their surfaces are covered
with the usual papillae. The bundle of vessels which runs up each
rhizome, as soon as it enters the tuber, separates into three distinct
bundles, which reunite at the opposite end. A rather thick slice of a
tuber is almost as translucent as glass, and is seen to consist of
large angular cells, full of water and not containing starch or any
other solid matter. Some slices were left in alcohol for several days,
but only a few extremely minute granules of matter were precipitated on
the walls of the cells; and these were much smaller and fewer than
those precipitated on the cell-walls of the rhizomes and bladders. We
may therefore con- [page 440] clude that the tuber do not serve as
reservoirs for food, but for water during the dry season to which the
plant is probably exposed. The many little bladders filled with water
would aid towards the same end.
To test the correctness of this view, a small plant, growing in light
peaty earth in a pot (only 4 1/2 by 4 1/2 inches outside measure) was
copiously watered, and then kept without a drop of water in the
hothouse. Two of the upper tubers were beforehand uncovered and
measured, and then loosely covered up again. In a fortnight s time the
earth in the pot appeared extremely dry; but not until the thirty-fifth
day were the leaves in the least affected; they then became slightly
reflexed, though still soft and green. This plant, which bore only ten
tubers, would no doubt have resisted the drought for even a longer
time, had I not previously removed three of the tubers and cut off
several long rhizomes. When, on the thirty-fifth day, the earth in the
pot was turned out, it appeared as dry as the dust on a road. All the
tubers had their surfaces much wrinkled, instead of being smooth and
tense. They had all shrunk, but I cannot say accurately how much; for
as they were at first symmetrically oval, I measured only their length
and thickness; but they contracted in a transverse line much more in
one direction than in another, so as to become greatly flattened. One
of the two tubers which had been measured was now three-fourths of its
original length, and two-thirds of its original thickness in the
direction in which it had been measured, but in another direction only
one- third of its former thickness. The other tuber was one-fourth
shorter, one-eighth less thick in the direction in which it had been
measured, and only half as thick in another direction.
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