72 University of California Publications in Zoology [Vol.19 



The Eelations Between Geotropism and Diurnal Migration 



When an animal in an experiment ascends it overcomes the influence 

 of gravity that is always acting in the same direction with the same 

 force ; when the specimen descends it moves in the direction of the 

 action of gravity. It is difficult to see how differences in distribution 

 can be attributed to reactions to an unvarying agent, and I use the term 

 geotropism to designate movement in a vertical direction or mainten- 

 ance of position in an upright container. Whether upward and down- 

 ward movements in a tube are reactions to gravity or not, it is certain 

 that their direction is changed as variable factors in the environment 

 are altered. In this sense it is permissible to speak of modification of 

 the geotropism ; the intention is to express the fact that direction of 

 vertical movement changes under certain conditions, or that the posi- 

 tion maintained in a column of water may vary as surrounding condi- 

 tions change. 



Variations in light intensity are constantly affecting organisms of 

 the plankton. Such alterations are due both to increase or decrease 

 in the amount of light entering the water and to movements that bring 

 the organisms into regions of greater or less intensity, depending upon 

 the direction of their locomotion. Since the source of the light that 

 may affect the animals in the sea is always above, it seems probable 

 that light will act as a directive stimulus. It has been suggested, 

 however, by Franz (1913, p. 264) that organisms in the sea may be 

 stimulated from all sides, even from beneath, by the diffusion of the 

 light, except when they are close to the surface or to the bottom. It 

 seems to me that this is a point worth considering. Is a copepod, for 

 example, when at a depth of one hundred feet, aware, so to speak, 

 that the light comes from one direction 1 It is possible that the light' 

 is so diffused that the external condition may be described as "more 

 light" or "less light" instead of as " light from above." We our- 

 selves have the experience, in a dense fog, of not being able to deter- 

 mine the position of the sun ; the intensity of the light is nearly the 

 same all about one, particularly toward the sky. 



If, however, an animal at a moderate depth in the sea is stimulated 

 by light from a given direction, it is necessary that a distinction be 

 made between phototropism, as such, and modifications of geotropism 

 that are related to the changes in light intensity apart from changes 

 in direction. Are we dealing with "vertical phototropism" or with 

 changes in the geotropism attributable to variations in the light in- 

 tensity ? 



