Te inh J oo ae ec RN eae ea hb aes 
189 
in thickness finally takes place mainly on the back and sides, 
where the strains are greatest and vary most, less on the con- 
cave surface in contact, where the cambium is directly subject 
to considerable pressure. There is, however, a difference in the 
response according to the nature of the stimulus applied. If the 
object clasped is free and light, so that the hook is excited by 
rubbing, but subjected to little or no strain, the main growth 
takes place on the concave surface, which is the one directly 
irritated. The stimulus of contact is the first to act, and the 
response is mainly by growth at the surface in contact; but 
contact is not finally, nearly so potent a factor in determining 
the amount of growth in thickness of the hook, as are the 
stresses and strains, to which the hook is subject, when firmly 
attached. 
In Artabotrys and Ancistrocladus a slight additional curva- 
ture of 10° to 60° may be produced as the result of contact, 
and also rarely in Uncaria (10° to 20°). In Olaz the additional 
curvature may amount to 1’), turns, and in Strychnos from 
11), to 3 turns. The epidermal surfaces of the hooks of Uncaria 
and Artabotrys are hairy, but those of the other hooks are smooth. 
Uncaria. 
In Uncaria sclerophylla the hooks are short, sickle shaped, and 
backwardly recurved, a pair of lateral ones alternating regularly 
with a single ventral one. The stem is rather thin and is square 
in transverse section. The leaves arise in pairs, right and left, and 
ventral and dorsal, the stalks of the latter twisting so that the 
laminas become horizontal. The hooks arise in the axils of 
the leaves. That, in the axis of the dorsal leaf, remains rudi- 
mentary, whereas the ventral one becomes larger and thicker, 
than the paired hooks do. Thus one such attained, near the 
base, without contact, an antero-posterior diameter (A—P) of 
610, and a lateral diameter (S —S) of 270, 100% of a mm. 
In taking experimental measurements, a part of the hook 3 on 
from the base is marked with a minute dot of indian ink 
and the antero-posterior (A — P) and lateral (S—S) diameters 
