RESPONSIVE ACTION OF PLANT-TISSUES TO LIGHT 567 



some unknown specific sensibility. Thus, Sachs differentiates 

 some of the principal effects as follows : 



* In the case of that stimulation of light which produces 

 waking and sleeping, the stimulus lies in the variations of the 

 intensity of light ; it is not the light as a constant force which 

 effects these movements, but the varying intensity. A further 

 great difference between the heliotropic curvatures and those 

 which bring about the sleep movements, lies in the fact that 

 the organ can make heliotropic curvatures in all directions. 

 The movement of waking and sleeping, on the contrary, only 

 takes place in one plane, which divides the leaf and motile 

 organ symmetrically, and it is thus unimportant here in what 

 direction the rays of light fall upon the motile organ, but only 

 important that light is present at all, or increases or decreases 

 in intensity. The above will suffice for the distinction of the 

 movement of waking and sleeping from the heliotropic 

 curvatures.' * 



Finally, he summarises the differences of motile effects as 

 follows : 



'We may thus say shortly, the movements of waking and 

 sleeping are called forth by paratonic light stimulus, whereas 

 the spontaneous movements of the same leaves are in- 

 dependent of any light stimuli, but probably dependent on 

 phototonus. Heliotropic curvatures, on the contrary, have 

 nothing to do with phototonus.' 



It will be found, however, that all these effects, sharply 

 differentiated as they are, may be seen in one and the same 

 organ. We may take for example the terminal leaflet of 

 Desmodium gyrans. This exhibits under favourable condi- 

 tions autonomous movements, whose period is short. It 

 exhibits also daily periodic movements, with the very long 

 period of twenty-four hours. It further, as I shall show, 

 exhibits positive heliotropic curvature when exposed to one- 

 sided illumination. Of these effects, it is supposed that the 

 autonomous movement is independent of the paratonic action 

 of light, but probably dependent on phototonus. The daily 



1 Sachs, Physiology of Plants, English translation, p. 628. 



