304 MOTOR RESPONSES 



Reversal in the direction of orientation from positive to negative is 

 therefore due to internal changes of such a nature that shock reactions 

 which v^'ere produced by decrease are produced by increase in the il- 

 lumination of the photosensitive substance. The shift from negative to 

 positive is due to the reverse. The nature of the response to light in 

 Volvox depends upon the state of adaptation and upon the intensity of 

 the illumination. If Volvox is fully adapted in a given intensity, it be- 

 comes positive if the intensity is increased or negative if it is decreased. 

 If the colony is not fully adapted, it becomes negative if the intensity is 

 increased or positive if it is decreased. 



The time required for colonies of Volvox to become negative or posi- 

 tive after the luminous intensity has been changed (the reaction time) 

 depends upon the degree of adaptation and the extent of the change. 

 If colonies which have been subjected first to strong light (1-2 hours) 

 and then to a variable period in darkness are exposed to strong light, 

 the time required to become positive (reaction time) increases with in- 

 crease in the length of the period in darkness (dark adaptation) from 

 0.04 minutes (with 2 minutes in darkness) to a maximum of 0.52 min- 

 utes (with 16 minutes in darkness) and then decreases to 0.18 minutes 

 (with 25 minutes in darkness). If the colonies are kept longer in strong 

 light and are then subjected to darkness, the reaction time decreases to 

 a minimum and then increases as the time in darkness increases. If they 

 are left in darkness until they are fully dark-adapted, and are then ex- 

 posed to light of different intensities, the reaction time (as the intensity 

 increases) decreases from 29 minutes in 5.24 meter-candles to a mini- 

 mum of 0.098 minutes in 7.5 meter-candles, and then increases to 0.358 

 minutes in 62,222 meter-candles. The energy required to make the colo- 

 nies positive varies directly with the light intensity, over the whole range 

 tested. Over most of the range this variation is nearly proportional to the 

 variation in intensity. No satisfactory explanation of this relation is avail- 

 able. 



If colonies are kept in a given intensity or in darkness, they become 

 adapted, i.e., they lose the ability to respond to light. Their responsive- 

 ness is regained if, after dark adaptation, the intensity is changed. The 

 processes associated with adaptation and those induced by change in il- 

 lumination are therefore antagonistic. The rate of these antagonistic 

 processes varies greatly, depending upon the magnitude of the change 



