228 TROPIC MOVEMENTS 



a difference in the intensity of illumination, but also upon the direction 

 of the light rays, owing to the fact that it is only those rays of light which 

 penetrate the cells and tissues which operate as stimuli. Hence a beam 

 of light must exert a lessened stimulatory action when it falls at an 

 oblique angle to the surface of the plant, since less light will penetrate 

 and more be reflected. It is, however, impossible to say whether the 

 sensitive cells respond to the direction of the light rays or to their relative 

 intensities on different surfaces. In the case of tendrils the directive action 

 of the support is the result of the tendril's own activity in bringing fresh 

 surfaces into contact, while chemotropic, osmotropic, and possibly also 

 galvanotropic stimulation depend upon the distribution of differences of con- 

 centration rather than upon the direction of diffusion of stimulatory materials. 

 Sachs and MUller 1 concluded that the light rays acted as stimuli 

 in virtue of their direction, but without bringing any definite proof 

 forward 2 . The arguments of Darwin, Wiesner, and Oltmanns do not, 

 however, definitely show that only differences in the intensity of the 

 illumination act as stimuli 3 . The results obtained by using angular prisms 

 filled with humic acid 4 , or indian ink and glycerine gelatine 5 , are incon- 

 clusive. A beam of light falling upon the plant after passing through 

 a prism so arranged that the intensity of the light is diminished at right- 

 angles to the direction of propagation has only to induce a slight curvature 

 of the plant to produce the same intensity of illumination on both sides 6 . 

 If the beam falls on the plant at an acute angle, a phototropic movement 

 occurs even when the prism is so placed that the plant must curve towards 

 the less bright portion of the beam. The same occurs in the case of 

 freely motile organisms, whose direction of locomotion is in fact determined 

 by the phototropic orientation of the body. Strasburger performed ex- 

 periments of this kind with zoospores, and Oltmanns with freely motile 

 and rooted plants. Similar results are obtained with organisms creeping 

 on a substratum and exposed to an oblique beam. Many motile lower 

 animals can also be induced in the same way to move towards regions 

 where the illumination is feebler if the beam is so arranged that the 

 feebler portion of the beam is towards its source 7 . If a plant is directed 

 towards a strong source of illumination, feeble light falling at right angles 



1 Sachs, Arb. d. hot. Inst. in Wiirzburg, 1880, Bd. II, p. 487; H. Miiller, Flora, 1876, p. 92. 



2 Cf. Pfeffer, Osmotische Untersuchungen, 1877, p. 213; Unters. a. d. hot. Inst. zu Tubingen, 

 1884, Bd. I, p. 478. 



3 Darwin, The Power of Movement in Plants, iSSi, p. 398; Wiesner, Bot. Ztg., 1880, p. 456; 

 Oltmanns, Flora, 1892, p. 183. 



4 Cf. Strasburger, Wirkung d. Lichtes uncl der Warme auf Schwarmsporen, 1878, p. 35. 



5 Oltmanns, Jahrb. f. wiss. Bot., 1892, Bd. xxm, p. 416; Flora, 1892, p. 183. 



6 Cf. Pfeffer, Pflanzenphysiol., i. Aufl., 1881, Bd. n, p. 373; Elfving, Die photometr. Bewe- 

 gungen d. Pflanzen, 1901 (Ofvertryck af Finska Vet. Soc. Forhandlingar, Bd. XLIII). 



7 Cf. Nagel, Bot. Ztg., 1901, Abth. ii, p. 289, and the literature there quoted. 



