CHAPTER XI 



THE PHOTOTROPIC EFFECT OF LIGHT 



Light induces movements of an extremely varied character. 

 Radial organs exhibit tropic movements in which the 

 position of equilibrium is definitely related to the direction 

 of the incident light. Such stems often bend towards the 

 light, while roots, generally speaking, are supposed to bend 

 away from it. It might be thought that this is due to a 

 specific difference of irritabihty between shoot and root, 

 the irritabihty of the former being of a positive, and of 

 the latter of a negative, character. There are, however, 

 numerous exceptions to this hasty generahsation. 



The intensity of the light has a modifying influence 

 on the character of the response. Thus, under unilateral 

 photic stimulation of increasing intensity and duration, a 

 radial organ may exhibit a positive, a dia-phototropic, and, 

 finally, a negative response. Strong sunlight brings about 

 a para-phototropic movement in which the apices of the 

 leaves or leaflets turn towards or away from the source of 

 illumination. The teleological argument advanced, that in 

 this position the plant is protected from desiccation by 

 transpiration, does not hold good universally ; for under 

 strong light the leaflets of Cassia alata assume a position 

 by which the plant risks excessive loss of water. 



x\n identical organ, as previously stated, may appear to 

 exhibit sometimes a positive and at other times a negative 

 response. Thus the leaflets of Mimosa pudica acted on by 

 light from above fold up towards the light, the phototropic 



