THE ACTION OF LIGHT 579 



Light causes changes of permeability in the cell membrane and, in plants, has 

 been shown to have the effect of retarding growth. 



Certain animals adapt the colour and pattern of their skin markings to suit 

 the background against which they are seen. In the frog, the receptors for the 

 reflex are situated in the toes and the eyes play no part. In the fish, the eyes are 

 th'e receptors. 



The effects of radium and of X-rays on living tissues are similar to those of 

 intense ultra-violet light. X-rays are generally regarded as being light waves 

 of extremely short wave length. 



LITERATURE 



General Theory of Photo-chemical Reactions. 



Weigert (1911). Nernst (1911, pp. 776-791). Sheppard (1914). 



Photo-electricity. 



Allen (1913). 



The Chlorophyll System. 

 Timiriazeff(1903). 



Chemistry. 



Willstatter and Stoll (1913). 



Photo-chemistry. 



Weigert (1911, pp. 98-107). 



Energetics. 



Brown and Escombe (1905). 



Ultra-violet Light. 



Victor Henri, etc. (1912). Hertel (1905). 



Radium. 



Rutherford (1913). 



Therapeutics of Radium. 

 Finzi (1914). 



Rontgen Rays. 



Kaye (1914). 



