195 
is, 3 X x 13 = 54.6 metres of stem, or allowing for the pos- 
sible additional strain, caused by wind or rain, the usual engi- 
neering margin of safety of 5/,, the length of stem, which can 
safely be supported by a single hook, =9 metres. When ex- 
posed to the full force of a tropical storm of wind and rain, 
even the strongest single hook will be unable to support so 
great a length of stem as this. As a matter of fact, in nature, 
attached hooks commonly occur at very much shorter intervals. 
In the whiplike, hook bearing, petiolar prolongations of Ca/asum 
(rattan) leaves, the longitudinal breaking strain is from 13 to 
16 kilo’s, whilst the largest hooks, which it bears, break off 
with a strain of from 10 to 12 kilo’s, and always at the point 
of attachment to the parent stem. This difference is in correlation 
with the difference of function. In the case of Uncaria many 
hooks may be attached, whilst the strain upon the stem is 
permanent and may be, at a given cross section, considerably 
ereater, than any individual hook is capable of bearing. In the 
rattan, the organ bearing hooks is developed for defensive 
purposes and not for climbing, the strain upon the central axis , 
when anything is caught, being only temporary, and rarely 
acting on more than one or two hooks at the same time. 
The purpose of the thickening of the hooks, not only of 
Uncaria, but also of the other hook climbers as well, when in 
contact, is twofold. It gives that increased strength and stiff- 
ness to the hook, which is especially necessary in the curved 
clasping portion, and also, owing to the growth taking place 
on the concave surface, as well as on the convex surface and 
sides, a support at first only loosely clasped, is, if not too thin , 
or too soft and yielding, finally held very firmly. The growth 
on the inner concave surface, when in contact, is at first more 
active than it is elsewhere, but, as soon as the support is 
clasped tightly, the increasing pressure between the two surfaces 
hinders more and more the further growth in this direction, so 
that the formerly rounded concave surface becomes flattened , 
whilst, the sides and back of the hook, on which the full stimulus 
