402 F. W. GAMBLE AND FREDEEICK KEEBLE. 



ebbs from that sti'ip of sand and at which the green colonies 

 reappear, and looks again at the vessel containing the sample, 

 it, too, will be found to be covering itself with its green scum 

 of Con voluta. Convoluta serves for abrief space as a tide- 

 table. It exhibits aperiodic vertical movement whose rhythm 

 is that of the tide. The rhythm is not profoundly impressed 

 upon it; after a day the movements of the patch in the 

 vessel cease to synchronise with those in the open. Dark- 

 kept animals show no periodic "tidal" movements. From this 

 we conclude that the descending movement of Con voluta, 

 whereby it sinks in the sand just as the tide is reaching it, is 

 due to light. When the summation of light stimuli passes a 

 certain amount — if we may use this figure — the influence of 

 lateral light no longer produces a positive effect. The 

 animal's reaction to gravity becomes changed in sign. It 

 sinks. When the light experience — the after-effect of light 

 stimulation — is dissipated, during its sojourn in darkness, 

 the animal becomes once more negatively geotropic and rises 

 to the surface. 



To sum up : within certain limits of light-intensity Con- 

 voluta is positively phototropic ; at a higher intensity it is 

 temporarily negatively tropic. On certain backgrounds it is 

 aphototropic ; and at certain periods in its normal habitat it is 

 either aphototropic or more probably negatively phototropic. 



4. The Influence of Monochromatic Light on 

 Phototropism. 



The phototropisras of plants are chiefly due to the rays of 

 high refrangibility — blue and violet (Pfeffer, 1902, p. 117). 

 It was of interest, therefore, to determine whether the photo- 

 tropism of Convoluta is also due to these rays. Our experi- 

 ments were made in the following manner : — In each of five 

 very shallow (1 cm. high), circular, porcelain jars a central 

 heap of sand, a little water, and a batch of Convoluta were 

 placed. Each of four of these jars was covered by a 



