Researches on Growth and Moveraent in Plants. 393 



proximal side by the direct action of intense and long-continued stimulation. 

 The following experiment was undertaken to decide the question. 



Fig. 15. — a, positive, dia-, and laegative phototropic reapoDse of Oryza under continued 

 nnilateral stimulus of intense light from arc-lamp ; h. record on moving plate of 

 modification of growth under continued stimulation of ultra-violet from mercury- 

 vapour lamp. Application of light at thick dot. Note arrest of growth, contraction 

 and subsequent renewal of growth. 



Eocpt. 14. Reversal of lurrmal Betanlation of Groivth under jprolcxnged, 

 Stimulatwii. — I took a seedling of Oryza and by means of inclined mirrors 

 applied light from a mercury-vapour lamp to act on all sides of the organ. 

 The record (fig. 15, h) shows that after the normal contraction and retardation 

 a reversal took place, resulting in expansion. The result is not unlike the 

 contraction of muscle passing into relaxation under continuous stimulation. 

 The first effect of stimulation was an arrest of growth and pronounced 

 contraction, but afterwards growth became renewed. It will thus be seen 

 that while under normal circumstances growth is retarded or arrested by 

 light, under prolonged illumination the organ may sometimes recover itself 

 from the state of arrested growth. 



16. Radio-thermotropism. 



Expt. 15. — I have explained (Section 13) that rise of temperature and 

 radiation of heat induce opposite physiological effects. In the thermonastic 

 reaction of the Crocus flower, rise of temperature induce-s an acceleration of 

 growth, but radiant heat causes a retardation ((/. Expt. 10). The tropic 

 effect of thermal rays is as strong, if not stronger, than that induced by the 

 effective blue rays in the spectrum. This will be seen in the following record. 



