Spatially Determined Reactions 181 



magnet (290). Wager (751) maintains that geotropism. in 

 Euglena also is purely passive, due to the fact that the 

 hinder end of the animal is the heavier. Kanda (389) 

 has recently championed the "statocyst" theory of Lyon, 

 as against the mechanical theory, using Lyon's argument 

 that when Paramecia are rapidly rotated in an apparatus 

 called a " centrifuge, " their front ends are directed outward 

 by centrifugal force and therefore must be heavier, instead 

 of lighter, as the mechanical theory would require. Harper 

 had previously attempted to meet this objection by re- 

 garding such a position on the part of the centrifuged 

 animals as due not to centrifugal force but to compensatory 

 movements made actively by the animal. Both the 

 mechanical theory and the "statocyst" theory, then, seem 

 to be still on the field. 



It has been shown that the reactions of Paramecium to 

 gravity are modified by a variety of conditions. Negative 

 geotropism, in a sense its normal condition, is favored by 

 plentiful food supply and by an increase in temperature 

 within certain limits ; positive geotropism, movement down- 

 ward, may be brought about temporarily by mechanical 

 shock, by salts and alkalies, by temperature changes (503, 

 689) , to which, however, the animals may adapt themselves ; 

 with less constancy by increase in the density of the fluid 

 containing them, and with lasting effect by lack of food. 

 It has been suggested that the downward movement under 

 these circumstances is protective, since it shields the animals 

 from surface agitation of the water, from surface ice, and 

 from failure of the surface food supply (500). We shall 

 see that similar conditions often change the direction of an 

 animal's response to light. 





