38 LIGHT AND THE BEHAVIOR OF ORGANISMS 



than on the other and that this causes them to beat more 

 or less effectively until the organism becomes directed 

 toward or from the source of stimulation, a direction it must 

 retain. 



By careful reading of Verworn's theory, quoted above, 

 one is led to infer that he considered the flagella or cilia to 

 be stimulated directly. This, however, is not an essential 

 part of the theory. The essential point is that there is a 

 difference in the effect of the beat of the cilia on opposite 

 sides when these sides are differently illuminated. It does 

 not matter whether this is caused directly by the effect of 

 the stimulating agent on the cilia or indirectly through 

 impulses transmitted to the cilia from the body protoplasm. 

 An organism once oriented in accord with this theory must 

 remain oriented unless it is thrown out of orientation by 

 some other agent than that which has caused the orienta- 

 tion. Orientation according to this theory is direct. Light 

 acts constantly as a directive stimulus. Difference of in- 

 tensity on opposite sides of the organism causes unequal 

 action of the cilia on the two sides. Symmetrical location 

 of organs is essential in the organism. 



It will thus be seen that Verworn's theory of tropisms 

 agrees with the theories of Loeb, especially the more recent, 

 in all essential points. These two authors, however, opposed 

 each other from the beginning. Loeb argued in favor of 

 ray direction, Verworn in favor of intensity difference; 

 neither seems to have known precisely what the other 

 meant. Verworn gives the following statement (1899, 

 p. 450) : " From the preceding consideration and by analogy 

 with the directive effects of other stimuli it is evident that 

 only the difference in the intensity of the light upon differ- 

 ent parts of the body can produce a directive effect; where 

 the stimulus acts upon the surface of the body from all 

 sides with equal intensity, the reason for a definite axial 

 position disappears, as is to be observed most clearly in the 

 action of chemical stimuli upon all sides. Although this is 

 obvious, some investigators, such as Sachs and Loeb, have 



