Onar. XVIII. RESERVOIRS FOR WATER. 4389 
the cavity beneath the bowed antenne, and then force 
their way through the little slit-like orifice between 
the valve and collar into the bladders filled with 
water, I cannot conjecture. 
Tubers—These organs, one of which is represented 
in a previous figure (fig. 26) of the natural size, 
deserve a few remarks. ‘l'wenty were found on the 
rhizomes of a single plant, but they cannot be strictly 
counted; for, besides the twenty, there were all pos- 
sible gradations between a short length of a rhizome 
just perceptibly swollen and one so 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 papilla. 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 con- 
29 
