92 SAKYO KANDA. 



3. Geotropism of Physa with Lung Empty and Filled with Air, in 



Presence of Food. 



Food being one of the strongest of forces in determining 

 behavior, it was thought advisable to observe its effect on 

 Physa. Green algae were carefully placed on the bottom of the 

 beaker in which there were five selected individuals of Physa. 

 It was surprising to find that with food present Physa seldom 

 crawled up to the surface for the air supply. After obtaining 

 air moreover most of them crawled down just as precisely as 

 they crawled up. It must also be added that they were not in a 

 starving condition previous to these experiments. 



It should be remembered, however, that in this case three 

 forces, that is to say, (i) light, to which Physa is negative, 

 (2) gravity, to which it is positive after taking in air, and (3) 

 food, to which it is presumably positive, were here combined. 

 The result of the combination of these three forces is the acceler- 

 ation of positive geotropism. Negative geotropism, on the 

 other hand, is retarded, even though it is very important for the 

 air supply. 



4. Geotropism of Physa at the Different Angles of Inclination of the 



Supports in the Air and in Total Darkness. 



Imagining that negative and positive geotropisms of Physa 

 and others are due only to respiratory phenomena, Walter claims 

 that "so far as gravity alone is concerned, they should show no 

 response at all" (10, p. 26). According to Dawson, geotropism 

 (negative) is only possible "when their lungs are empty, but when 

 they have sufficient air they become indifferent to gravity and 

 crawl in all directions" (2, p. 93). In a certain sense, therefore, 

 Walter and Dawson agree on this point. This contention will 

 be experimentally examined in this section. 



(a) Experiments with a Plain Glass Plate. After a few trials 

 it was found that Physa was positively geotropic even at a slight 

 inclination of a plain glass plate in the air and in total darkness. 

 The following method of experimentation was therefore adopted. 

 The well-moistened glass plate being held slightly inclined, the 

 animal was placed upon it with its head down. When five se- 

 lected individuals had been so placed, the plate was reversed, 



