§4] PHOTOTAXIS AND PHOTOPATHY 181 



1. False and True Phototaxis. — It must certainly be a very 

 old observation that wlien small organisms are placed in a 

 vessel in front of a window, they are soon found arranged with 

 reference to the window ; some lying on the nearer side, some 

 on the further side, and others swimming indifferently back 

 and forth through the vessel. The conclusion is near at hand 

 that this arrangement of the organisms is determined by the 

 light. This conclusion is, however, not necessarily correct. 

 Thus, Sachs ('76) showed that, under certain conditions, 

 wholly passive substances — oil drops, in a mixture of water 

 and alcohol — might exhibit a similar aggregation towards the 

 window or away from it. These conditions are that the vessel 

 should be cooler next the window. Then, on the cooler side, 

 there will be a descending current ; on the warmer side, an 

 ascending current ; on the surface, a current towards the win- 



a 



Fig. 51. — Vertical section through a dish showing distribution in water of passively 

 suspended bodies, as a result ol difference of temperature at the two sides of the 

 vessel. A, warmer side ; B, cooler side. Arrows show the direction of movement 

 of currents in the water. The objects lighter than water are grouped at 6 ; those 

 heavier than water, at a. 



dow ; and on the bottom, a current from the window. If 

 the passive bodies are such as float, they will thus be carried 

 towards the window, and will exhibit a false phototaxis (in 

 the positive sense); if, on the contrary, they tend to sink, they 

 will be carried from the window, and show false negative pho- 

 totaxis. Now this appearance, due to passive transportation 

 by currents, may likewise, under the given conditions, be 

 exhibited by organisms — but the phenomenon is not due to 

 light (Fig. 51). 



There is at least one other kind of pseudophototaxis. This 



( — ) photopathy ; and correspondingly we can in this second case speak of the 

 organisms themselves as photophil or photophob. In this nomenclature I foUovr 

 Graber. Stkasburgek used photometry for what I here call photopathy, but 

 Oltmanns ('92, p. 206) has employed photometry to indicate the capacity of 

 organisms to perceive different degrees of Intensity of light. So, perhaps, the 

 terminology here employed may lead to the least confusion. 



