DIURNAL MOVEMENTS IK PLANTS 531 



phenomenon of nyctitropism had hitherto proved so 

 baffling. It is indeed a difficult task to disentangle the 

 full explanation of each given case in the vast complexity. 

 It is, however, possible, by a process of judicious eli- 

 mination, to reduce the difficulties which at first appear 

 to be insurmountable. 



In the periodic movement of plants there are several 

 factors which are predominant, others being of minor 

 importance. The important factors are the effects of light 

 and darkness, of variation of temperature on differential 

 growth, and of thermal variation on geotropic curvature. 



For facility of treatment, I shall first take the three 

 ideal types : (1) where the variation of light is the important 

 factor, (2) where the movement is due to differential growth 

 under variation of temperature, and (3) where thermal 

 variation induces changes in geotropic curvature. I shall 

 then take up the movement of the leaf of Mimosa where 

 the combined effects of numerous factors give rise to a 

 highly complex diurnal curve. There remains now the 

 difficulty of discriminating the three types which approximate 

 to the ideal. 



DISCRIMINATING TESTS FOR CLASSIFICATION. 



Predominant effect of light and darkness. — Turning first 

 to the case where light exerts a predominant influence, 

 the obvious test of keeping the plant in continuous dark- 

 ness or continuous light is not practicable. One would 

 think that if the movement was due to periodic variation 

 of light, such movement would disappear under constant 

 light or darkness. But owing to the persistence of after- 

 effect, the periodic movement previously acquired is con- 

 tinued for a long time. 



There is, however, another possibility of discrimination. 

 The effect of variation of light will be most marked at 



