6o LIGHT AND THE BEHAVIOR OF ORGANISMS 



ent upon the direction in which the rays pass through the 

 tissue or upon difference of intensity on opposite sides of 

 the reacting organ. Sachs (see p. 13) originated the former 

 view and Miiller and others supported it, while Darwin, 

 Wiesner and Oltmanns were prominent champions of the 

 latter. Darwin also emphasized in particular the impor- 

 tance of change in intensity. Pfeffer (1906, p. 228) says 

 that the experimental results and the arguments offered in 

 support of either view are not conclusive. 



Darwin exposed monocot plumules (stems of young 

 seedlings) with one side covered with India ink in front of 

 a window and found that they did not bend straight toward 

 the window, but deflected toward the uncovered side. This 

 result seems to indicate that the curvature is due to differ- 

 ence in light intensity on the surfaces. Pfeffer (1906, pp. 3, 

 229), however, considers it inconclusive, largely on account 

 of the possible effect of the India ink on transpiration 

 (evaporation). Oltmanns studied the curvature of plants 

 grown behind a hollow prism containing India ink and 

 glycerine gelatine so arranged that the light intensity 

 decreased from right to left, and found that they deflected 

 toward the brighter end of the field. He therefore con- 

 cluded in favor of difference of intensity as the controlling 

 factor in orientation. His results, however, are not con- 

 clusive, owing to the diffusion of light by the particles of 

 India ink in suspension (see p. 40). 



b. Apparatus. — In the following work the objections 

 to the experiments of Darwin and Oltmanns were elimi- 

 nated by the use of an apparatus known as the light grader 

 modified to suit the conditions of the experiments. The 

 important features in the construction of this apparatus 

 will be understood readily by referring to Fig. 4. The 

 walls of the apparatus are all light-proof and dead black 

 inside, so as to prevent reflection. The outline of a cross 

 section at any point is square. The upper portion of the 

 front wall of the vertical part of the apparatus is hung on 

 hinges forming a door. From the bottom of this door is 



