Cuar. XVIII] RESERVOIRS FOR WATER. 305 
much swollen that it might be doubtfully called a tuber. 
When well developed, they are oval and symmetrical, more 
so than appears in the figure. The largest which I saw was 
1 inch (25°4 mm.) in length and *45 inch (11-43 mm.) in 
breadth. They commonly lie near the surface, but some 
are buried at the depth of 2 inches. The buried ones are 
dirty white, but those partly exposed to the light become 
greenish from the development of chlorophyll in their 
superficial cells. They terminate 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 transparent 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 conclude that 
the tubers 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 44 by 4} 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 the 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 
242 
