26 



JACQUES LOEB AND HARDOLPH WASTENEYS 



TABLE 1 



good for the heliotropic reactions of animals. But these experi- 

 ments are not adequate to decide the question of perfect identity 

 of the photochemical substances in all cases. For this purpose 

 experiments with spectral colors are required. The most re- 

 liable experiments made on plants are apparently those of Blaauw 

 on the spore bearers of Phycomyces and the seedlings of A vena. 



Blaauw proceeded in the following way: He exposed a row of 

 seedhngs of Avena to a carbon arc spectrum for a certain time. 

 The seedlings were than placed in the dark and after the proper 

 time it was ascertained which part of the spectrum had induced 

 heliotropic curvatures. By varying the duration of time of 

 exposure to the spectrum it was found that with a minimal 

 time of exposure only certain blue rays, namely, those of a wave 

 length of 478 mm, caused heliotropic bending, while with 

 longer exposure longer waves also became efficient. In this way 

 the minimum duration of exposure for various parts of the spec- 

 trum was ascertained. Table 1 gives his result. 



The red and yellow parts of the spectrum were ineffective for 

 the intensity and time hmits used and the optimum of efficiency 

 was in the blue, in the region between 466 and 478mm. 



A shorter series of experiments was made on the fruit bearers 

 of Phycomyces, with the following results : 



44 to 47 per cent of the Phycomyces showed heliotropic curvatures 

 after 192 seconds of illumination at 615 mx 

 after 192 seconds of illumination at 550 fxn 

 after 16 seconds of illumination at 495 yu^i 

 after 32 seconds of illumination at 450 mm 

 after 64 seconds of illumination at 420 mm 



