180 LIGHT AND PROTOPLASM [Ch. VII 



of assimilation is increased by increased intensity of light. The 

 chemical rays act to increase metabolic changes, and the output 

 of carbon dioxide. As these rays become more intense, the 

 metabolic changes go on with abnormal rapidity, until, finally, 

 death ensues; thus, intense light is fatal to many, perhaps to 

 all, organisms. Absence of light, however, is injurious only as 

 preventing assimila-tion in chlorophyllaceous organisms ; but 

 these supply the food for other organisms, so that continued 

 darkness in any environment must likewise be eventually fatal 

 to all life. All organisms, before succumbing to darkness or 

 to light, enter into a condition of rigor, from which they may 

 return to activity if favorable conditions are restored. Sudden 

 change of intensity often produces violent protoplasmic changes, 

 awakening quiescent organisms to activity, or causing, in the 

 higher organisms, violent contractions. 



All of these effects of light, whether produced by the thermic 

 or chemic rays, probably give rise to great chemical changes 

 by which disturbances of metabolism, and eventually death, 

 may be produced. Not all organisms find light immediately 

 necessary to their existence; but very powerful light, long 

 continued, proves fatal to most protoplasm. 



§ 4. Control of the Direction of Locomotion by 

 Light — Phototaxis and Photopathy * 



In this section we shall (1) distinguish between false and 

 true phototaxis ; (2) consider the observed cases of phototaxis 

 among Protista, the parts of higher organisms, and the Metazoa 

 as entire organisms ; and (3) discuss the general laws of photo- 

 taxis and photopathy. 



* In this section we shall deal with two sets of phenomena which very likely 

 are different, tout which, in our ignorance, we cannot always distinguish. The 

 first includes that active migration of organisms whose direction is determined 

 by that of the rays of light. This is phototaxis. The second includes the 

 wandering of organisms into a more or less intensely illuminated region, the 

 direction of locomotion being determined by a difference in intensity of illumina- 

 tion of the two poles of the organism. This is photopathy. According as the 

 migration is towards or from the source of light, we can distinguish positive ( + ) 

 and negative ( — ) phototaxis. According as the migration is towards or from 

 the more intensely illuminated area, we can distinguish positive ( + ) and negative 



