196 
is now acting, continue their growth without hindrance, until 
the full size of the hook and limit of possible growth, under 
the given stimulation, is reached. 
If a young attached hook is subjected to strain, it generally 
bends to one side, and allows the support to escape; or, if the 
strain acts exactly in the plane of curvature of the hook, the 
latter may break. Old attached hooks, however the strain 
may be directed, do not let the support escape, until they 
break, which takes place in Uncaria always at a point between 
9 
: and = of the length of the hook, from the base. 
The purpose of the hooks is to enable the plant to climb, but 
if a branch is bent downwards, and the hooks attached to fixed 
supports, they remain clasped and increase in thickness. The 
growth is slower, and the amount of thickening less, however, 
than when the stem is directed upwards, the difference being 
probably due to the more shaded position and downward in- 
clination, by lessening both the transpiration and assimilation 
of the neighbouring leaves, causing a more or less marked 
interference with the nutrition of the hooks. 
The following experiments show, that the nature of the sup- 
port is of some importance; ceteris paribus, a rough support 
being a more effective stimulus than a smooth one. 
Uncaria sclerophylla. Young hooks after 8 weeks. 
Attached to 
Rough pieces of bamboo. 6 grm. De® with glass fixed wood Hooks 
weight and 6 mm. diam. inserted Pier ldary Support. without 
through saived hooks. Pressure slight. yb ¢ dia Fairly strong contact 
: : strain. 
wor Pees (ar Eee ASP g-3 A—P 
Average of 15 pairs 193 . 384 | 185 . 356 305 . 438 156 . 343 
ses hook. Pressure 
n Ant. Post. plane. 225 . 500 215 . 465 390 . 560 175 . 410 
hao hook. Pressure 
partly oblique. 240 . 380 235 . 360 445 . 570 165 . 415 
When without contact, or, when subjected to only slight pres- 
sure, the greatest growth takes place along the antero-posterior 
AS), can Withee care ene 
