I 20 
R. H. Compton. 
The Tubers. 
At the tip of a runner a swelling is frequently formed, and a 
small spherical tuber is produced (Fig. 6 ^.i.) When fully formed 
the tuber is about 0-5 mm. in diameter, and is opaque owing to the 
large quantity of storage material contained in its parenchymatous 
cells. The storage material is in the form of spherical grains of 
variable size, translucent in appearance, with a central nucleus, but 
gave a deep brown colouration with iodine ; some grains after a 
long time became tinged with violet, and there is little doubt that 
they consist of one of the starches, perhaps modified by long 
preservation in spirit. The stalk thickens its cell-walls close to the 
tuber on maturity, and the latter becomes detached (Fig. 7 A and B). 
The tuber—perhaps the smallest such structure possessed by 
any Phanerogam—acts as a means of vegetative propagation. At 
the distal end from the stalk is found a shallow depression in which 
new branches and leaves arise exogenously. The vascular tissue of 
these new structures becomes continuous with that present ab initio 
in the tuber. 
The germinating tuber produces from the concave depression a 
number of shoots. Young stages shew a single aerial leaf and a 
creeping branching axis bearing bladders (Fig. 7, A). Later the tuber 
increases greatly in size, reaching 3 mm. broad X 2 high, and new 
branches are produced ; so that when mature there are present an 
erect peduncle bearing a single flower and a small lanceolate 
bracteole, usually two aerial leaves, and a series of ramifying 
creeping axes bearing bladders and new tubers (Fig. 6, ^.ii.) 
At first opaque, on germination the tuber loses most of its 
storage material and becomes in consequence translucent in 
appearance. This condition persists after germination, and 
apparently the enlarged tuber comes to act as a water-storage body. 
The existence of such organs has long been known in U. inontana, 1 
where swellings are produced irregularly on the creeping axis: in 
the case of U. brachiata the water-storing function is a secondary 
application of the primarily reproductive starch tubers. 
The secondary distension of the tuber when it takes up the 
function of water-storage appears to be due to the mechanical 
stretching of the cell-walls. A similar phenomenon has been 
described in the water-tissue of the leaves of Rhizopliora, 
1 C. Darwin. Insectivorous Plants, 1893, p. 349. 
Hovelacque. Recherches dans l’Appareil Vegetale, 1888, p. 675, 
