84 MOVEMENTS OF CURVATURE 



mechanical stimuli is not restricted to a single tentacle but spreads through 

 the leaf-lamina to neighbouring tentacles not directly excited. 



The small tentacles at the centre can perceive stimuli and transmit 

 them to the larger marginal ones which curve towards the centre of the 

 leaf, whereas the small central tentacles themselves do not curve. The 

 dorsi-ventrality of the stalk of the tentacle leads to the curvature being 

 always to the centre of the leaf, but does not prevent a slight lateral bend- 

 ing when a tentacle at the side is radiating a strong orienting stimulus. 

 By varying the intensity and duration of the stimulus, an excitation may 

 either be confined to the stimulated tentacle, or may be caused to spread 

 to neighbouring ones or even to all the tentacles on a leaf. Strong 

 stimulation, especially if chemical, may cause the lamina of Drosera 

 rotundifolia to become more or less concave, or may lead to an inrolling 

 of the margin of the elongated leaf of Drosera longifolia and D. intermedia, 

 which may sometimes be so pronounced as to completely enclose a captured 

 insect. 



The irritable movements of the tentacles and lamina of Drosera rotundifolia were 

 first noted by Roth \ Nitschke 2 gave an account of the movements and their 

 propagation which was in the main correct, but our knowledge was greatly increased 

 by the historical researches of Darwin. Among other important points Darwin 

 showed that the power of perception was localized in the heads of the tentacles. 

 Pfeffer established definitely the fact that the tentacles possessed contact-irritability, 

 although Darwin had previously shown that drops of rain did not act as stimuli, and 

 that a solid body only acts as a stimulus when it is pushed through the slimy 

 excretion into direct contact with the head of the tentacle. 



The sensitivity to contact-stimulation depends upon the stage of development 

 and other conditions, but under favourable circumstances is nearly as great as that 

 of the most sensitive tendrils, since Darwin found that a perceptible result was 

 produced by rubbing a hair weighing 0-000822 of a milligram upon the head of 

 a tentacle of Drosera rotundifoh'a. A single touch hardly produces any result, 

 whereas repeated strong contact causes a curvature to begin in 10 to 20 seconds, 

 and in 10 to 20 minutes the heads of the tentacles are pressed against the middle of 

 the leaf. 



The lamina of the leaves of Pinguicula vulgaris, P. lusitanica, and P. alpina 3 

 rolls inwards when subjected to mechanical and chemical stimuli. The excitation 

 spreads to a certain distance from the point of application of the stimulus, but the 

 leaf is only moderately sensitive, and it has not been determined whether the 

 stimulus is perceived by the lamina or by the heads of the numerous small stalked 

 glands present on the under surface. 



The mechanics of the movement. The curvature of a tendril produced by contact 



1 Roth, Beitrage z. Botanik, 1782, T. i, p. 60. 



2 Nitschke, Bot. Ztg., 1860, p. 229 ; 1861, pp. 224, 234, 253. 



3 Darwin, I.e., p. 374 ; Pfeffer, I.e., p. 516. Cf. also Klein, Cohn's Beitr. z. Biologic, 1883, 

 Bd. Ill, p. 163 ; Goebel, 1. c., p. 186. 



