38 



PHYTOHORMONES 



be taken to control the results with plain pastes, because 

 Schilling (1915) has shown that treatment of twigs or leaves 

 with vaseline, paraffin, cocoa-butter, etc., gives rise to 

 various kinds of outgrowths, and even root formation may 

 be induced in this way. 



Soding (1936) has found that seedlings of Cephalaria 

 are very sensitive to low concentrations of auxin. His tests 

 are carried out in diffuse light. If the concentration of 

 hormone is very low, the Cephalaria test will show good 

 curvatures where the (dayhght) Avena test shows none. 



cl\ 



02"^S 



Fig. 17. Cephalaria test of Soding. A, decapitated hypocotyls with agar 

 block applied; B, after 5 hours; curve C, relation between concentration of 

 indole-acetic acid in mg. per liter and curvature of Cephalaria; curve D, the 

 same for Avena. (From Soding, Jahrb. wiss. Bot. 82: 534-554, 1936.) 



The seedlings are decapitated, the stem being cut through 

 very obliquely, and the block of agar is placed upon the 

 lower half of the cut surface (see Figure 17). The curvature 

 is measured after 5 hours. As may be seen, the sensitivity 

 at low concentrations is much higher than with Avena but 

 the maximum curvature obtainable is about the same. 



C. 5. Evaluation of Results 

 The Avena coleoptiles, prepared in the way described, 

 are of great physiological uniformity, as is shown by plotting 

 the variability of the curvatures given by a large number 

 of identically treated plants. Most of the experimental 

 faults will tend to reduce rather than increase the curvature 

 so that in a faulty test the variability curve will be skew 

 towards the lower values (see Figure 18). If, in a test of 

 12 or more plants giving a fairly symmetrical distribution, 



