GROWTH SUBSTANCES FOR TRAUMATIC CURVATURES 215 



Chemical Treatment. — Positive traumatic curvatures can be 

 produced in roots and stems by applying silver nitrate. The 

 effects of certain dyes upon the role of growth substance and upon 

 the normal course of tropic curvatures (Boas, 1933; Schweighart, 

 1935; Blum and Scott, 1933) have been discussed in previous 

 chapters. Observations on the influence of certain fungicidal 

 preparations are of interest. Neukirchen (1930) treated grass 

 seeds with a series of commercial chemicals and investigated the 

 effect that they exerted upon the geotropic responses of the 

 coleoptiles. Dilute solutions of " Uspulun-Universal " and arseni- 

 ous acid produced an increase in geotropic response, Uspulim- 

 Universal and mercuric chloride were without effect, and 

 "germisan," formalin, copper sulphate, and eosin caused a retar- 

 dation of the response to gravity. It is notable that photot- 

 ropism was not affected by preliminary treatment of the seeds 

 with these same chemicals. Von Witsch (1934) found that very 

 dilute solutions (1/100,000 or 1/800,000 molecular in distilled 

 water) of the salts of heavy metals (copper, silver, uranium) 

 caused retardation of growth and negatively geotropic curvatures 

 in lateral roots of Phaseolus multiflorus. The addition of small 

 amounts of calcium or potassium distinctly reduced the growth- 

 retarding effects of the heavy metals without changing the 

 curving effect. Certain other experiments of Warner (1931) 

 indicate that the influence of metal ions upon geotropic response 

 may be due to the effect upon cell permeability. The bearing of 

 such observations upon the growth-substance explanation of 

 tropisms is not known. It may be said that while the traumatic 

 reaction of the root is generally negative, the wound response of 

 coleoptiles and stems generally takes the form of a positive curva- 

 ture. Meesters (1936) found that chemotropic curvature of the 

 root hairs of Agrostemma (growing in culture solution) did not 

 occur when various concentrations of 3-indole acetic acid, con- 

 tained in glass tubes, were introduced near the roots. 



Stimulus Transmission. — Studies on the transmission of stimuli 

 from the place of wounding to other regions where the response 

 occurs have thrown light upon the nature of the wound stimulus 

 as well as upon the mode of its conduction. 



Transmission in the Avena Coleoptile. — In the Avena coleoptile. 

 Stark (19216) showed that both traumatic and thigmatic stimuli 

 could be conducted over an incision. If the tip was removed and 



