178 LIGHT AND PROTOPLASM [Cn. VII 



chlorophyll, nearly or quite essential to movement. Phototo- 

 nus is a convenient name for the condition induced by light. 



b. Effect of High Intensity of Liyht on Movement Light- 

 rigor, - - We have just seen that in some organisms the most 

 vigorous movements occur at an optimum intensity of light, 

 which produces phototonus. At a lower intensity there is no 

 movement. It appears, furthermore, that there is for many or- 

 ganisms a maximum intensity of light, below that which produces 

 death (ultramaximum), which causes a cessation of movement 

 that may be called light-rigor. This condition is distinguished 

 from that of death by the fact that diminished light brings 

 return of activity. ENGELMANN ('82, p. 109) observed this 

 condition in his Bacterium photometricum, and remarks that 

 it is common to all bacteria. Similar light-rigor has been 

 observed in green plants also. PRINGSHEIM ('81, p. 516) 

 found that when, in the presence of oxygen, strong sunlight 

 was let fall upon Nitella, the movements ceased after 1| 

 minutes. If the insolation was now interrupted, normal move- 

 ments were resumed. 



Summing up the effects of varied intensities of light, it 

 appears that for many organisms there is an optimum, which 

 produces a condition of phototonus, in which the organism 

 moves and responds regularly to stimuli. As the light inten- 

 sit}^ falls below, or rises above this optimum, the activity of 

 movement diminishes, ceasing at certain points in the condi- 

 tions of dark-rigor and light-rigor. Beyond each of these 

 points, again, is the point of death. 



c. Contraction produced by Change in Intensity of Illumina- 

 tion. - - We here consider a number of cases not closely related 

 except in this, that quick movements are produced after stimu- 

 lation by change in the intensity of the light. The cases are 

 found both among Protista and Metazoa. 



Among the sulphur-bacteria ENGELMANN ('88, p. 665 ; 88 a ) 

 has noticed that a sudden diminution in the intensity of the 

 light, produced by shading the mirror of the microscope, is 

 followed by a spring backwards, often to the distance of 10 

 to 20 times the organism's length. This reaction ENGEL- 

 MANN has called " Schreckbewegung." When the light is sud- 

 denly increased, a forward movement takes place, but this is 



