578 PLANT RESPONSE 



Summary 



The action of the diffused stimulus of light on a radial 

 organ is, like that of other forms of stimulation, to induce a 

 longitudinal contraction. The action on mature does not 

 differ from that on growing organs. In a growing organ the 

 induced contraction has the effect of retarding growth. 



The response to light may thus consist not merely of a 

 retardation of growth, but sometimes also of an actual 

 shortening of the responding organ. 



When an organ is in a sub-tonic condition, absorption of 

 energy of light may give rise to a transient acceleration of 

 the rate of growth ; but when the plant has attained the 

 normal tonic condition, response is by the usual retardation of 

 growth. 



The after-effect of stimulus of light may consist of a 

 simple continuation of the characteristic responsive contrac- 

 tion or retardation of growth. This constitutes the positive 

 after-effect. There may also be a negative after-effect, con- 

 sisting of expansion or acceleration of the rate of growth. 



Under the long-continued action of light, fatigue is 

 induced. There is sometimes an exhibition of periodic fatigue 

 with oscillatory response. 



The response to light is not different from that evoked by 

 any other form of stimulation. The various responsive 

 movements which occur under the action of light are thus 

 explicable without the assumption of the possession by 

 different organs of different specific sensibilities to light. 



