THE GEOTROPISM OF FRESHWATER SNAILS. 



95 



TABLE VII. 



GEOTROPISM OF Physa AT THE DIFFERENT ANGLES OF INCLINATION OF A GROUND 



GLASS PLATE IN AIR IN TOTAL DARKNESS. 



At beginning of experiment each head placed upward. Table shows results 

 after one minute. 



Physa evidently could stick on the ground-glass plate better 

 than on the plain glass plate, as would be expected. Experi- 

 ments, therefore, could be carried on even at an angle of 90. 

 Here again, it is noticeable that there is a decrease of positive geo- 

 tropism at this angle. The optimum inclination in this case is 

 between the angles of 67^ and 



5. Summation of Gravity and Light Stimuli. 



Physa being positive to gravity and negative to light, as 

 already seen, it w r ould be expected to crawl downward even at a 

 small angle of inclination, if it were placed in a strong light. This 

 is just what happened. One morning the rays of sunlight were 

 falling at an angle of about n%. The angle of the rays of 

 sunlight was nearly constant during the experiments. Ten 

 selected individuals were carefully placed with their heads down 

 on a moist plain glass plate. The plate was then reversed and 

 put on the rack whose angle of inclination was 11%- They 

 all oriented themselves away from the rays of sunlight, that is, 

 downward, and crawled in that direction. Ten trials were made 

 and there was no exception. 



Besides the above, observations on exclusion of the air were 

 attempted, but the results were not satisfactory. Generally 

 speaking however, Dawson's observations seem to be right, 

 although the writer observed one individual "drop" once. 



