20 HooKER: PHYSIOLOGICAL OBSERVATIONS 
cave side becomes shorter, probably as a result of compression. — 
The acceleration is greatest on the convex surface and diminishes _ 
toward the concave. This difference in the rate of growth pro- 
duces the bending when the acceleration extends all the way 
through the pedicel and includes the concave surface. If a neutral 
line is present, it presumably serves as a brace against which the — 
elongation of the convex side acts. . 
The increase in the rate of growth begins in a part of the pedicel 
at or near its base, whence it moves toward the gland, widening 
its scope until one to two thirds of the pedicel is involved. The 
gland and the adjacent portion of the pedicel do not grow. CG : 
Darwin (’08, p. 9) states that the distal half remains straight in all | 
cases; but growth is not always as confined as he indicates, for in 
one case (TABLE VIII) 70 per cent of the tentacle grew, and 1 
another (TABLE I) 73 per cent. If the place where the reaction 
Whe ig A a a, Te 
base (cf. C. Darwin, ’08, p. 206). : This is particularly character 
istic of those tentacles which have the gland imbedded on the 
upper side of the pedicel, instead of at the apex. 
The amount of bending is determined by two factors; the 
difference between the lengths of the convex and concave sides, 
and the thickness of the bending region. An increase in the 
former augments, in the latter diminishes the amount of bending. 
Darwin (08, p. 9) describes an instance of movement through 
variable. It may be nearly six hours (TaBLE II, Fic. 5), or only 
twenty minutes (TaBLE III, Fics. 6, 7 and 8). C. Darwin (’ 
p- 23) once saw inflexion finished in 17 minutes, 30 seconds. 
