197 
diameter, whereas, with strong pressure, the hook grows more 
rapidly laterally, i.e. at right angles to the pressure, and may 
finally, owing partly to the further growth of the concave 
surface being mechanically prevented, become thicker from 
side to side, than it is from back to front. 
The question at once arises, as to whether the stimulus of 
- contact alone, without any pressure setting up a strain in the 
hook being applied, is sufficient to cause a marked response. 
In young hooks, rubbed along the concave surface and sides, 
with a roughened piece of bamboo, at short intervals of time 
for a few minutes every morning, during a period of 3 weeks, 
no perceptible increase in the rapidity of growth, as compared 
with other untouched hooks of the same age, could be detected , 
provided no excoriation of the epidermis, or permanent injury, 
was inflicted on the rubbed hooks. On, however, repeating the 
experiment, and rubbing the hooks at intervals, during a quarter 
of an hour, three times daily, a slightly increased growth of 
the rubbed hooks was perceptible. 
Uncaria sclerophylla. 
Average of 4 experiments. At first. After 3 weeks. After 5 weeks. 
5.9: Ao? s—S A—P 38 A—? 
Rubbed hooks. 85 . 190 130 . 280 140 . 320 
Untouched. : 80 . 195 110 . 225 130 . 255 
Though the differences obtained by these means are not very 
marked, still they are large and constant enough to make 
certain the fact, that the hooks are, when young, sensitive to 
the stimulus of contact alone, though as they become older 
their sensitivity to contact becomes less and less and finally 
disappears. With Ancistrocladus the difference obtained between 
the rubbed and untouched hooks is still less marked, but in 
Artabotrys is greater, the hooks of the latter plant being inter- 
mediate, in sensitivity to intermittent contact, between Uncaria 
and Strychnos, 
It is possible, that though intermittent contact acts as a 
relatively feeble stimulus to the hooks of Uncaria, continuous, 
