1919] Esterly: Reactions of Various Plankton Animals 73 



DOES THE GEOTROPISM CHANGE AS THE INTENSITY OF 

 LIGHT VARIES? 



It was attempted, in the experiments that have been described, to 

 ascertain whether the direction of movement in a vertical container 

 is different in diffuse light and in darkness. In the latter case it is 

 certain that responses coming under the head of phototropism are 

 excluded, and it seems entirely probable that such reactions are done 

 away with in the former case, as the experiments were conducted. 



Let us note, in summary, the results of observing behavior in a 

 column of water in darkness as compared with diffuse light. 



In Acartia tonsa surface specimens go up in the light and down in 

 darkness; animals from deep water are positively geotropic under 

 both conditions. Calanus and Eucalanus are always positive in dark- 

 ness as well as in diffuse light. In Labidocera some specimens are 

 negatively geotropic in darkness and positively geotropic in diffuse 

 light ; but this sort of behavior was not apparent in all sets of animals. 

 Metridia shows very strong positive geotropism in diffuse light and in 

 darkness. Sagitta is positively geotropic in brighter diffuse light, but 

 in more subdued light and in darkness it is negative 



So far as these experiments are concerned Calanus, EiLca.lanus, and 

 Metridia do not show alterations in geotropism as the light intensity 

 changes. A change is very clearly shown, however, in the behavior of 

 Sagitta and of some specimens of Acartia, though in the latter case 

 the habitat has some relation to the behavior. It is evident, therefore, 

 that change in geotropism because of change in light intensity does not 

 account for the vertical migration in most cases, on the basis of these 

 experiments. 



DOES THE GEOTROPISM CHANGE AS THE DIRECTION OF 

 LIGHT VARIES? 



Sagitta and surface specimens of Acartia move toward a source 

 of light above or below. All the others show a tendency to move away 

 from the light whatever its position, when direction of rays is the only 

 factor that is altered. This tendency to move from the light is more 

 marked in some forms than in others. That is, two out of six species 

 move toward the source of light while four move from it; or, the 

 vertical phototropism is positive in two cases, and negative in four 

 cases. It is not allowable, of course, to apply the results of experi- 

 ments with the light beneath the column to the migration as it occurs 



