BENDING OF HIGHER PLANTS TOWARD THE LIGHT 65 



dot of ink in line with these on the plate by sighting through 

 a small circular hole in a piece of opaque paper. The 

 horizontal movements of the tip of the radicle could thus 

 be quite accurately recorded by making dots on the plate 

 in line with the pointer and the tip of the radicle, at any 



-> 



n 



-> 



■> 



13 12 



Fig. 5. Tracings of shadow of a plumule of corn showing its reaction in light of 

 graded intensity, three-fourths natural size. I. Cross section of two beams of 

 light as used in the experiment; intensity at a and a', zero; at b and b', 100 ca. m.; 

 I, 2, 3, 4, 5, 6, 7, successive positions of plumule at intervals of 60 minutes, right 

 side more highly illuminated than left; 8, 9, 10, 11, 12, 13, same with left side more 

 highly illuminated than right. It will be seen that the plumules deflect slightly 

 toward the more highly illuminated side under both conditions. 



II. Side view of plumule showing amount of curvature toward source of light at 

 close of experiment, n, direction of light. 



desired intervals, and connecting them with a line. The 

 records thus made represent the movement of the radicle 

 magnified ten times. The direction of the rays was recorded 

 by tracing the edge of a ruler placed on the glass plate in 

 such a position that the edge was in line with the shadow 

 of the plumule cast on a white surface temporarily arranged 

 for the purpose (Fig. 6). 



The intensity of light to which the plumules were ex- 

 posed varied from about 2 to 14 ca. m. In most of the 

 experiments they were exposed to the lowest intensity, the 

 edge of the plumule at the beginning of the experiment 

 jDeing in close contact with that side of the beam of light 

 which had the lowest intensity (see Fig. 5). 



