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. C. 

 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 in 

 another (Table I) 73 per cent. If the place where the reaction 

 begins is not the most basal portion of the pedicel, the acceleration 

 of the rate of growth extends also a short distance in the direction 

 of the leaf-blade (Table I). Those regions where the reaction 

 commences have the highest rate of growth and are involved in 

 the reaction for the greatest length of time: consequently they 

 grow most, so that the amount of elongation is greatest at the 

 base and decreases toward the apex. Ordinarily, bending is 

 confined to one place, but in some of the long marginal tentacles 

 the pedicel curves in two places^ — in the middle and again at the 

 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. 

 The longest tentacles bend most, since their growing region has 

 the largest dimensions. The gland may be moved through a 

 wide angle, for example 215 degrees as shown in FiG. 2. C. 

 Darwin ('08, p. 9) describes an instance of movement through 

 an angle of 270 degrees. The time required for bending is very 

 variable. It may be nearly six hours (Table II, Fig. 5), or only 

 twenty minutes (Table III, Figs. 6, 7 and 8). C. Darwin ('08, 

 p. 23) once saw inflexion finished in 17 minutes, 30 seconds. 



