22 Hooker: Physiological observations 



since it is a reaction in response to differences existing on the 

 opposite sides of the tentacle. The inflexion and expansion of a 

 Drosera tentacle present a remarkable instance of a practically 

 continuous reaction, produced by a single mechanism, but caused 

 by two stimuli of different nature, so that it is partly nastic and 

 partly tropic. 



iii. Growth and Turgidity 



Since the movement of a Drosera tentacle is the result of 

 growth, it is always longer at the end of a reaction than at the 

 beginning. The amount of elongation varies from o.i mm. to 

 0.5 mm. This suggests a simple demonstration to show that the 

 reaction is accompanied by growth. If a single tentacle on a 

 leaf is stimulated, it will project a noticeable distance beyond its 

 fellows after regaining its original position. In the field, a Drosera 

 leaf frequently has some tentacles longer than other ones in the 

 same row. These inequalities are the result of growth attendant 

 upon reaction, for all the tentacles in one row remain of equal 

 length if not stimulated. Sometimes a small insect is caught by 

 a single exterior tentacle, which carries it to the center of the leaf. 

 The insect is, however, too minute to cause a sufficient impulse to 

 be conducted from the discal to the other marginal tentacles. 

 They do not bend, so that after the reaction is completed they are 

 not as long as the tentacle which was inflected. 



Turgidity does not play a part in the movement of the ten- 

 tacles. This was proved by plasmolyzing bent tentacles in 20 

 per cent salt solution. The plasmolyzed tentacles remained bent. 



A comparison of FiGS. 4, 5 and 8 with graphs of Fitting, which 

 show the haptotropic reactions of tendrils of Sicyos angnlatus 

 (Fitting, '02, p. 378), of Fassiflora gracilis (Fitting, '03, p. 577, 

 fig. 6) and of Pilogyne suavis {ibid., p. 57S, fig. 7), shows that the 

 mechanics of movement in tendrils and tentacles are the same. 

 Similar figures are shown by Wiedersheim for photonastic reactions 

 of leaves of Impatiens parvifiora (Wiedersheim, '04, p. 241,^1^. /; 

 p. 2^2, fig. 2; p. 244, fig. 5), and for thermonastic reactions of the 

 perianth leaves of the tulip {ibid., p. 250, fig. 8) and the crocus 

 {ibid., p. 254, fig. 10; p. 255, fig. 12). It is clear that all these 

 reactions are produced by the same mechanism; rapid growth on 

 the convex side and in the median section with attendant contrac- 



