422 EFFECT OF LIGHT [Ch. XVII 



and light fayor transpiration ; and this means loss of water 

 and, consequently, of growth. Just how far these opposing 

 effects neutralize each other cannot be said, but our compara- 

 tive study gives assurance that the effect may well be pro- 

 duced by light acting alone. While the diurnal periodicity in 

 growth can clearly be ascribed to no other cause than the alter- 

 nation of day and night, it is an important fact that this perio- 

 dicity may be exhibited for several days after the plant has been 

 placed in a room kept constantly dark. There is apparently a. 

 persistence of an effect impressed by environment. 



Among animals the evidence of a retarding effect upon 

 growth is not so clear. Matjpas ('87, p. 1008), indeed, con- 

 cludes from actual experiment that various ciliate Infusoria 

 multiply with equal rapidity in the presence or the absence of 

 light. In the higher vertebrates, on the other hand, light, 

 acting through the retina, increases destructive metabolism, as 

 MoLESCHOTT ('55) first pointed out, so that many vertebrates 

 undergo a greater loss of weight in the light than in the dark. 

 Indeed, a diurnal periodicity in the weight of animals was 

 described as long ago as 1852, by Bidder and Schmidt, who 

 found that starving cats lost weight much faster in the day 

 than at night. These facts constitute a not unimportant par- 

 allel with the conditions in plant growth. 



In summarizing the foregoing facts on the retardation of 

 growth by light, we see that strong sunlight usually com- 

 pletely inhibits growth, so that the growing parts of organ- 

 isms, or entire organisms during the period of growth, are usu- 

 ally concealed. Even diffuse light retards growth, especially 

 in the following organisms : most seedlings, many of the 

 higher aerial fungi, germinating seeds in general, and spores 

 of fungi. Also, light hastens destructive metabolism in the 

 higher vertebrates so as to diminish weight rapidly. The 

 organisms thus brought together are without exception aerial. 

 This fact suggests that in plants, at least, the restraint of 

 growth by light may be due to the rapid loss of water which 

 accompanies illumination. The illumination of seedlings and 

 fungi for only a brief period — an hour or so — retards their 

 growth. These organisms likewise exhibit a diurnal perio- 

 dicity in growth corresponding to the alternation of day and 



