28 JACQUES LOEB AND HARDOLPH WASTENEYS 



and after the proper time (two hours or more) the number of the 

 polyps which had bent to the Ught in various parts of the spectrum 

 was ascertained. As the reader will notice, the bending of the 

 polyps took place after they had been put back into the dark. 

 This corresponds with the method followed by Blaauw in his 

 experiments on plants and of Loeb and Ewald in their experi- 

 ments on the applicability of the law of Bunsen and Roscoe to 

 heliotropic reactions. 



We have also made experiments in which the stems were ex- 

 posed long enough to the spectrum to enable them to 'form their 

 polyps while still exposed to the Hght. The determination of 

 the wave length to which each stem was exposed was rendered 

 possible through the use of the absorption bands of a solution 

 of didymium nitrate. With the aid of these bands, we could 

 define accurately the position of each stem and polyp in the 

 spectrum. We are indebted to Dr. E. Butterfield for the exact 

 location of these bands in the spectrum. 



In noting the result the reader must keep the following facts 

 in mind : When a short exposure to light influences the orienta- 

 tion of the polyps of a stem, this will show itself in the fact 

 that the majority of the polyps of that stem will bend straight 

 to the hght. If there is no effect of light, the polyps will grow 

 in any direction but it may happen according to the laws of 

 probability that one or the other may bend to the light. In 

 order to be sure that the light influences the direction in which 

 the polyps bend we must require that so great a percentage 

 bend toward the Ught that chance may be excluded, before we 

 draw the conclusion that we are dealing with a heliotropic effect. 

 We considered it a positive result when 50 per cent or more of 

 the polyps bent to the Ught. That they should all bend to the 

 light cannot well be expected, especially in cases of short dur- 

 ation of exposure. The new polyps are extremely delicate and 

 they are not all healthy or strong. Moreover, certain polyps 

 will be partly screened from the light by the stems. Blaauw, 

 as well as Loeb and Ewald, had to use the bending of 50 per 

 cent of the specimens as a criterion of a positive result. The 

 fact that each stem has only a limited number of polyps creates 



