226 GROWTH OF PLANTS 



hormones were assumed to accumulate, the degree of curvature was 

 greatly reduced; when they were applied on the other side, the curvature 

 was greatly increased. 



Anesthesia. In Chapter 4 (p. 150) there is a discussion of ethylene-induced 

 anesthesia in plants. It need only be mentioned here that a condition can 

 be induced with growth substances which in many respects resembles 

 ethylene-induced anesthesia. The most effective growth substances 

 for anesthesia are a-naphthaleneacetic, /3-naphthoxyacetic, /3-naphthoxy- 

 propionic, 2,4-dichlorophenoxyacetic, 2,4,5-trichlorophenoxyacetic, and 

 o:-(2,4,5-trichlorophenoxy)-propionic acids. 



Activation of cinnamic acid with ultraviolet light. Trans-cinnamic acid 

 is not an active growth substance. If, however, it is exposed to ultraviolet 

 light, an active substance results. The light changes the trans to the cis 

 form, which is active. During the investigation of this subject it was found 

 that the vapors of the substance were active and induced responses similar 

 to ethylene. ^^ The vapor method has since become of considerable impor- 

 tance. 



Natural influences. Evidence continues to accumulate to show that the 

 growing plant produces natural hormones and that these vary with en- 

 vironment and orientation of the plant organs. Potted plants which were 

 placed in a horizontal position gave a pronounced geotropic response. 

 When the pots were again set upright, the stems recovered from the first 

 response but the leaves developed epinasty.^^ The final effect was as if 

 synthetic hormones had been applied. 



Unusual physiological responses were also induced on intact plants by 

 capping wdth black cloth.^^ Capping the tomato plant for 3 to 14 days is 

 assumed to bring about an over-production of natural hormones which 

 causes epinasty of leaves, swelling of stems, proliferations (including in- 

 tumescences), inhibition of growth, uiitiation of roots, and a disturbance 

 of correlation of organs. 



Comparative effectiveness of acids, esters, and salts. The part of the 

 molecule which brings about a physiological response is still unknown. It 

 may be the entire molecule or one of its component parts. It has been de- 

 termined, however, that the application of acids, esters, and salts brings 

 about similar results. For solutions it is convenient to use acid and salts, 

 while for vapor treatment it is best to use the esters, due to their high 

 vapor pressure. ^^ 



Literature Cited 



1. Boysen-Jensen, P., "tlber die Leitung des phototropischen Reizes in Avenakeim- 



pflanzen," Ber. Deutsch. Bot. Ges., 28 : 118-120 (1910). 



2. , "La transmission de I'irritation phototropique dans VAvena," Kgl. Danske 



Videnskab. Selskabs. Forhandl, 1911(1) : 3-24. 



3. , "tJber die Leitung des phototropischen Reizes in der Avenakoleoptile," 



Ber. Deutsch. Bot. Ges., 31 : 559-566 (1913). 



