192 PHYTOHORMONES 



The same authors have made some calculations which compare the 

 minimum effective concentrations of ethylene and auxin {cf. X M). 

 Such calculations are without any basis in the present state of our 

 knowledge. In the first place it is impossible to compare an effect on 

 Avena cell elongation with one on apple-twig intumescences. In the 

 second place, we do not know whether the ethylene is really distributed 

 between the air and apple-twig tissue as its equilibrium solubility in 

 water would indicate, or whether it is completely absorbed. It is prob- 

 ably absorbed especially by those cells which react, which constitute 

 only a very small fraction of the total tissue treated. Thirdly, the 

 minimum effective concentration of auxin is not that which will produce 

 10° curvature as they assume, but that which will produce a just visible 

 curvature, which, using de-seeded plants and automatic recording, 

 is nearer to 0.1° than to 10°. (The value of one part of ethylene in 

 100,000,000 is recorded by Wallace for the smallest observable intumes- 

 cence.) In addition there is an error of 10 times in the arithmetic as 

 published, and a subsequent "erratum" distributed by Crocker in 

 1936 introduces a further factor of 1,000 times. Reliable data on the 

 activity and penetration of ethylene will be needed before any compara- 

 tive calculation of minimum effective concentrations is possible. 



It is of importance that all substances which act as auxins 

 are, so far as they have been tested, also active in promoting 

 root formation. Thimann (19356) showed that indene-3- 

 acetic and coumaryl-1-acetic acids, which, as discussed in 

 VIII (t, are active in promoting growth but appear to be 

 poorly transported, are also active in root formation. Their 

 activity is, however, largely local, and is best exerted when 

 applied at the base of the test cuttings. Phenyl-acetic acid, 

 found to be active in root formation by Zimmerman and 

 Wilcoxon (1935) was shown to act as an auxin by Haagen 

 Smit and Went (1935). a-Naphthalene-acetic acid is also 

 active for both functions (u). Indole-3-carboxylic acid, on 

 the other hand, is inactive for both functions, while indole-3- 

 propionic acid has very low activity both for growth pro- 

 motion and root formation (Thimann and Koepfli, 1935). 



D. Effect of Light on Root Formation 



The results concerning the effect of light on root formation 

 have been very conflicting. Vochting showed that white 

 light inhibited root formation of Salix cuttings. Some plants, 



