176 TROPIC MOVEMENTS 



secondary maximum in the ultra-red l . It can, therefore, readily be under- 

 stood why, when the light is feeble, a perceptible reaction may only be 

 produced by the more refrangible rays. It is, however, possible that in 

 some cases only these rays are able to excite a heliotropic response. 



Many researches have been performed by various authors on the heliotropic 

 action of different rays 2 . Guillemin worked with especial care and showed that, 

 owing to the varying absorption and dispersion of the different rays, the position 

 of the heliotropic maximum varied according to whether prisms of quartz, rock-salt, 

 or flint-glass were used. For these reasons it is easy to understand why the curve 

 obtained by Guillemin, Wiesner, and other authors do not always precisely agree. 

 The fact that Sachs could detect no heliotropic action under a solution of potassium 

 bichromate was probably the result of feeble intensity of the light used, or of the 

 special properties of the experimental material. Wiesner 3 found that the heliotropic 

 action of the red and orange rays was weakened by the admixture of yellow rays. 

 Gardner, Guillemin, and Wiesner all observed that the plants did not always set 

 themselves precisely parallel to the incident rays, but curved somewhat towards the 

 more active regions of the spectrum, a result only to be expected. 



Polarized light acts, according to Guillemin and Askenasy, in the same way as 

 ordinary light 4 . The non-luminous ultra-violet rays exercise a strong and the ultra- 

 red rays a feeble phototropic action. Rontgen rays appear to exert mainly injurious 

 actions 6 , for Schober was unable to detect any tropic action of these rays on 

 seedlings, although Joseph and Prowazek found that Paramoecium and Daphnia 

 showed a negatively tactic reaction. The Becquerel and radium rays exercise a 

 certain injurious action, but have no tropic influence, as far as is known 6 . 



SECTION 38. Thermotropism. 



In addition to the action of the ultra-red rays which are associated 

 with the visible part of the spectrum, dark heat rays of still greater wave- 

 length as well as differences of temperature may produce a thermotropic 

 curvature in certain cases. As far as our present knowledge goes, however, 



1 According to Wiesner (Die heliotropischen Erscheinungen, 1878, I, p. 461, the ultra-red rays 

 which pass through a solution of iodine in carbon bisulphide also act in this way. 



2 Pggili ( J 8i7) ; Zantedeschi, Bot. Ztg., 1843, p. 620; Payer, Ann. d. sci. nat., 1844, y ser., 

 T. II, p. 99; Dutrochet, ibid., 1843, 2 e ser, T. XX, p. 329; Gardner, London, Edinburgh, and 

 Dublin Phil. Mag., 1844, Vol. XXIV, p. 7 ; Guillemin, Ann. d. sci. nat., 1857, 4* ser., T. VII, p. 154; 

 Sachs, Bot. Ztg., 1864, p. 361 ; N. J. C. Miiller, Bot. Unters., 1872, Bd. I, p. 57; G. Kraus, 1876, 

 I.e. ; Wiesner, Die heliotropischen Erscheinungen, 1878, I, p. 44; 1880, II, pp. 10, 87, 89. Wiesner 

 gives a full account of the literature and also of the methods. 



3 L. c., II, p. 50. 



4 Guillemin, 1. c., p. 172 ; Askenasy, Bot. Ztg., 1874, p. 237. 



5 Cf. Seckt, Ber. d. bot. Ges., 1902, p. 87; Joseph and Prowazek, Zeitschrift f. allgem. 

 Physiologic, 1902, Bd. I, p. 143. 



6 A summary of all that is known in regard to the physiological action of these rays is given by 

 K. Hoffmann, Die radioaktiven Stoffe, 1903, p. 21. See also Bohn, Compt. rend., 1903, T. CXXXVI, 

 p. 1012. 



