212 
s—s A—P 
thickest point, to 210. 235. 100%» mm.; whilst a loop, exerting 
slight pressure on all sides, caused a similar curvature, the 
increase in thickness, however, being from 100 ; 100 to 940 ; 255, 
In the first case, the stimulus, acting only on the concave sur- 
face, causes the main growth to be in the antero-posterior 
plane; whilst, in the second, with pressure equally applied to 
all sides, the antero-posterior diameter still becomes greater 
than the lateral, but the difference between the two is not 
nearly so marked. 
The basal half or peduncle, and the extreme apex, do not 
perceptibly respond to contact stimuli, with, or without, marked 
pressure. Between these regions and the most irritable part is 
a short intermediate zone, in which a moderate thickening, and 
curvature of relatively slow development, are produced by con- 
tact and pressure. 
The irritability of the concave surface of the hook is only 
slight, as compared with that of the concave. Owing to the 
flexible nature of the stalk, it is impossible to obtain any mar- 
ked pressure between the convex surface and the body in con- 
tact; in contact with a rough surface, however, an average 
increase in the antero-posterior diameter of 58 . 100% mm., but 
barely any in the lateral diameter , as compared with untou- 
ched hook-tendrils, may be noticed. In one case, a slight increase 
of the original curvature took place, the bending being most 
pronounced at the point where contact was applied , and being 
probably due to a localized hypertrophy, on the convex surface, 
caused by the stimulus. As a general rule, however, when 002 
tact is applied to the convex surface , the original curvature 
remains unaltered, or may even become slightly lessened. 
The coiling and thickening is more rapid, when the suppot 
is rough. than when it is smooth; quicker when a large area 
of the irritable concave surface is in contact, than when the 
area in contact is small, whilst the amount and rapidity of 
the responding growth is proportional to the pressure; @ mode- 
rate pressure over a large surface being much more effective, 
