24 Hooker: Physiological observations 



of all the tentacles. Those which remain inactive at one time, 

 might bend on a subsequent occasion. However this may be, no 

 tentacle has been observed to react the fourth time. 



The second and third reactions of a tentacle are essentially 

 similar to the first. After two reactions a tentacle is much less 

 sensitive than before, as is shown by the increased reaction time. 

 Each subsequent bending takes longer (cf. C. Darwin, '08, p. 19), 

 but the original position is regained in approximately the same 

 time. The amount of growth does not vary according to any 

 definite rule. It depends rather on external conditions and is 

 apparently uninfluenced by the age of the tentacle. If the 

 external conditions are constant, the amount of growth remains 

 the same. In one example (Table VIII) this was approximately 

 the case, the increase in length for the first, second and third 

 reactions being 0.16 mm., 0.18 mm. and 0.14 mm. respectively. 



The region of growth is however more restricted in each 

 subsequent reaction, and is confined to more basal portions of the 

 tentacle. Table VII shows that 57 per cent of the tentacle grew 

 in length during the first inflexion, 50 per cent during the second, 

 and 32 per cent during the third. In another instance (Table 

 VIII) the regions of growth were successively 70, 53, and 27 

 per cent of the length of the tentacle. In each reaction a certain 

 portion of the tentacle becomes fully grown. A Drosera tentacle 

 is therefore characterized by intercalary, basipetal growth. Be- 

 cause of this, the number of inflexions which a tentacle can make 

 must necessarily be limited. It is therefore highly probable that 

 a tentacle would bend a fourth time only under very exceptional 

 circumstances. 



Since growth is limited to more basal regions, the thickness of 

 the reacting portion is greater at each successive inflexion. More- 

 over the greatest difference between the lengths of the opposite 

 sides tends to decrease. Both of these factors cooperate in reduc- 

 ing the amount of bending, so that the gland is moved through 

 successively smaller angles, as shown in Fig. 1 1 . 



vi. N as tic reactions 



In all the experiments described, the glands of the marginal 

 tentacles were stimulated directly, but reactions follow likewise 



