14 LIGHT AND THE BEHAVIOR OF ORGANISMS 



is illuminated more strongly than the other, but that it is 

 rather the direction in which the ray of light passes through 

 the substance of the plant;" (1882, p. 851) . . . ''dass es 

 sich bei den heliotropischen Kriimmungen gar nicht darum 

 handle, dass die eine Seite des Pflanzentheils starker als die 

 andere beleuchtet sei, dass es vielmehr nur auf die Richtung 

 ankomme, in welcher der Lichtstrahl die Pflanzensubstanz 

 durchsetzt." 



It will thus clearly be seen that the term " ray direction," 

 so frequently used to characterize Sachs' view in opposition 

 to intensity difference, is confusing. It expresses the truth, 

 but not the whole truth. Sachs did not refer to ray direc- 

 tion in general but to ray direction through the tissue, nor 

 did he oppose intensity difference in general. He had 

 nothing to do with the view of Bert and Graber that varia- 

 tion in illumination of the field regulates reaction to light. 

 He opposed the view of De CandoUe who states explicitly 

 that it is difference of intensity on opposite sides of the 

 reacting organ which causes heliotropic curvatures. 



In the study of the reactions of sessile plants to light 

 there is but one phenomenon to consider — the turning of 

 the plant or some of its parts so as to assume a definite 

 position with reference to the source of light, i.e., orienta- 

 tion. In motile forms we have not only to deal with the 

 assumption of a definite axial position and movement but 

 we have also to deal with the phenomenon of aggregation. 

 How and why do certain unicellular organisms, for example, 

 collect in dense masses in certain regions of their environ- 

 ment? How is it that so many swarm spores, for instance, 

 collect on the side of the dish toward the source of light? 

 It was generally assumed that this phenomenon is due to 

 difference of intensity in the field, that these organisms in 

 some way select the illumination adapted to their needs 

 and remain there. But Nageli had observed as early as 

 i860 that flagellates and swarm spores collect at the side 

 of a porcelain dish nearest the window although the inten- 

 sity of light at this place is lower than elsewhere owing to 



