RESPONSE TO STIMULUS OF LIGHT 



405 



subjected to unusual cold. Even in such a case, however, 

 though the first responses are positive, successive exposures 

 to light, by raising the tonic condition, are found to restore 

 the response to the normal negative. 



The same facts receive interesting illustration in the re- 

 sponse of growth. If the growing organ be in a normally 

 excitatory condition, the 

 stimulus of light, inducing 

 negative turgidity varia- 

 tion, causes retardation 

 of growth. If the tissue, 

 however, be in an ex- 

 tremely sub-tonic con- 

 dition, light stimulus, by 

 increasing the internal 

 energy, gives rise to the 

 positive effect, that is to 



FIG. 239. Record of Responsive Growth- 

 variation taken under condition of balance 

 in slightly Sub-tonic Flower-bud of 



say, the initiation, or en- 

 hancement, of the rate 

 of growth. If the grow- 



Crinum Lily under Diffuse Stimulation of 

 Light 



ing tissue, again, be only Continuous lines represent the effect during 



slightly sub-tonic, we 

 shall have a preliminary 

 positive, or enhancement, 

 followed by the negative 

 response, or retardation 

 of the rate of growth. 

 This is seen in the fol- 

 lowing record, which I 

 obtained from a slightly 

 sub-tonic flower-bud of Crinum lily, in the induced variations 

 of the normal rate of growth under the stimulus of light 

 (fig. 239). This record was made with the Balanced Cresco- 

 graph, where the normal rate of growth is recorded as a 

 horizontal line, enhancement or positive variations of the 

 rate being represented by up-curves, and retardation, or 

 negative variations, by down-curves. It will be noticed that 



application of light, the dotted line on 

 withdrawal of light. The plant was 

 originally in a sub-tonic condition, and 

 application of light at x , after short 

 latent period, induces preliminary ac- 

 celeration of growth. After this follows 

 the normal retardation. On withdrawal 

 of light, in the dotted portion of the 

 curve is seen the negative after-effect, 

 followed by return to the normal rate of 

 growth. A second and long-continued 

 application of light induces retardation, 

 followed by oscillatory response. 



