Plant Qrowth'Suhstances 



tomato plants from 4 to 10 or more inches high occurred after 

 treatment of the undisturbed soil with any of the six acids 

 except phenyl-propionic. Pronounced responses were often 

 displayed within two hours of the application of the solution : 

 plants about 5 inches high required only about 3 mg. of the 

 substance in order to show the effect, but larger and older plants 

 required 10 to 20 mg. On the whole, the indole compounds 

 were more effective than the phenyl ; and toxic effects were 

 observed after application of more than 20 mg. of the indole 

 compounds [to the youngest plants?] or about 50 mg. of 

 the phenyl compounds. 



The responses of tobacco plants were generally similar to 

 those exhibited by tomatoes, except that no noticeable 

 response was evoked in tobacco in less than three to six hours. 



Root growth from the stems of tomatoes was essentially the 

 same after application of the active substances to the soil as 

 has already been described for local applications. However, 

 "Roots induced by phenyl-propionic acid (applied to soil) 

 appeared sooner and in larger numbers during the first eight 

 days than did roots induced by other compounds. Phenyl- 

 propionic acid was the only compound to induce many roots 

 to appear from parts of the stem that did not exhibit noticeable 

 swelHng or other type of proliferation. Naphthalene-acetic 

 acid was the most toxic to root systems in soil. ... In addition 

 to root growth from the stem of the tomato, roots also appeared 

 from the petiole [leaf-stalk] and along the entire midrib of the 

 middle leaves. This type of response was induced by indole- 

 propionic acid (27 mg. per pot). . . . Indole-butyric acid was 

 especially effective in causing roots to be initiated in stems of 

 tobacco plants 10 to 12 inches in height." Optimum initiation 

 of aerial roots in tobacco resulted from the addition of 27 mg. 

 of indole-butyric acid to the soil. 



REFERENCES A 



The lists appended to this chapter refer to but a few papers 

 published before 1935, as most of those already have only a 

 historical interest for chemists. Papers mainly of pure plant- 



3+ 



