52 MOVEMENTS OF CURVATURE 



In all these cases the attaching organs are stimulated by contact with 

 or rubbing against any solid body. Contact with air or liquids such as 

 a stream of water or mercury produces no effect, whereas in Mimosa and 

 similar plants any shaking or disturbance may act as a stimulus if 

 sufficiently intense. This difference is due to the existence of a special 

 contact irritability in the attaching organs, which may also be termed 

 haptotropism or thigmotropism, and which is excited by differences of 

 pressure or variations of pressure in contiguous or neighbouring regions 1 . 

 Hence smearing a tendril with stiff gum-arabic exercises no stimulatory 

 effect, and similarly a glass rod covered with moist but solid 10 per cent, 

 gelatine produces no excitation even when strongly pressed and rubbed 

 against the most sensitive tendrils. Coated glass rods may therefore be 

 used to handle tendrils without stimulating them, or the tendril may 

 be placed upon a glass dish coated with the solidified gelatine. Naturally 

 contact with a rough body exerts a greater stimulus than contact with 

 a very smooth one. Hence smooth and slender tendrils, since they can 

 exert but little pressure on one another, and usually remain in contact for 

 a short time only, rarely coil around each other 2 . Stouter and stiffer 

 tendrils like those of Bauhinia and Smilax naturally respond to self-contact 

 more readily. The absence of any response to wind and rain is obviously 

 of great use to the plant. 



The tendrils of Sicyos angnlatus, Cyclanthera pedata, and Passiflora 

 gracilis are especially sensitive, the tendril of the first-named plant being 

 perceptibly stimulated by the contact of a thread of cotton weighing 

 0-00025 f a milligram laid upon the tendril 3 . The tendril is therefore 

 more sensitive than the human skin, which receives no impression when 

 a thread of this weight moves gently upon it 4 . A worsted thread of i to 

 10 mgm. weight stimulates the less sensitive tendrils as well as many 

 irritable petioles, but a stronger stimulus is required for the tendrils of 

 Vitis. A bamboo fibre i mm. diameter and weighing o-i gram is just 

 able to produce a curvature and slight but perceptible thickening in the 

 pulvinar tendril of Dalbergia linga and in the hook tendril of Stryc/mos, 



1 Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1885, Bd. i, p. 483. A detailed list of cases in 

 which contact irritability has been established is given here. This form of irritability was later 

 detected by Peirce (1. c., p. 66) in Cuscnta, and by Ewart (1. c., pp. 196, 203) in the irritable hooks 

 of tropical climbers, although in these the stresses and strains set up in the attached hook influence 

 the ultimate amount of thickening. 



2 Pfeffer, I.e., p. 495. 



3 Id., I.e., p. 506; Darwin, I.e., pp. no, 131, 405 ; Climbing Plants, 1875, pp. 153, 171, 197. 

 * Exact determination is difficult, since the excitation depends upon the extent of surface in 



contact, the degree of roughness, and the rapidity of movement. Cf. Frey u. Kiesow, Zeitschr. f. 

 Psychologic u. Physiologic der Sinnesorgane, 1899, Bd. xx, p. 153. Kemmler (Hermann's Handbuch 

 d. Physiologic, 1888, Bd. ill, Kap. 2, p. 325) states that the minimal stimulus for sensitive skin is 

 that due to the gentle movement of a weight of 0-002 of a milligram. 



