5/2 PLANT RESPONSE 



Retarding effect of light on longitudinal growth.— We 



thus see that the fundamental contractile effect of stimulus is 

 precisely the same in the case of light as in that of any other 

 form of stimulation. We know also that the response of a 

 growing organ is the same as that of one which is not 

 growing. It has been shown further, that in a growing 

 organ the contraction due to direct action of stimulus had 

 the effect of retarding growth. Now, from the fact that the 

 effect of light is the same as that of other forms of stimu- 

 lation, it follows that the result of its direct action on a 

 growing organ should be to produce contraction, and 



Fig. 234. Longitudinal Contraction and Retardation of Growth under 

 Light in Hypocotyl of Sinapis nigra 



The first part of the curve shows the normal rate of growth. Arrow (|) 

 indicates moment of application of diffuse light, which is seen not only 

 to retard growth, but also to induce a marked contraction. The 

 second arrow indicates moment of withdrawal of light, and dotted 

 portion of the curve shows recovery. 



resultant retardation of growth. Though this conclusion, 

 however, was thus clearly established in theory and by other 

 experiments, using different forms of stimulus, I yet thought 

 it important to test the matter with regard to the specific 

 action of light, under conditions so simple as to bring out the 

 fundamental phenomenon unmistakably. 



In order to do this, I took a seedling ot Sinapis nigra in 

 which the hypocotyl was strictly radial, and made a record 

 of its longitudinal growth (fig. 234). Its normal rate of 

 growth, seen in the curve as movement upwards, was at the 

 rate of -015 mm. per minute. On now applying light to 

 the specimen on all sides at once, the growth is seen to 



