56 PHYTOHORMONES 



K the slope of the hne. This varies somewhat from 

 day to day. 

 In theory two values of a suffice to characterize the line, but 

 since the curve deviates from a straight line at its upper 

 and lower ends, a larger series of values is desirable. 



It is worth noting that in the Avena test there is no evi- 

 dence for any ''threshold" concentration Co, and the cal- 

 culation of activities is based upon this fact; the activity, 

 in "units" per milligram, is really a measurement of K. 



D. 3. Epinastic Responses 



Since Uyldert (1931) has shown that the epinasty of 

 stems is a phenomenon of asymmetrical growth due to 

 auxin (X M), it follows that a test could be based on the 

 application of auxin to petioles and measurement of the 

 resulting epinasty. This has been done by Hitchcock (1935, 

 1935a). His technique was to make serial dilutions of dif- 

 ferent substances in lanoline (Laibach's method) and apply 

 these to the upper side of the base of the petiole of a tobacco 

 plant. Eight different pastes were applied at the same time 

 to eight petioles of the same plant ! The change in the angle 

 between stem and petiole was used as measure of the activ- 

 ity. This method must be used with great caution, since 

 Hitchcock himself has shown (1935a) that a substance 

 applied to one petiole may cause curvature of the petiole 

 below. In addition, a number of simple acids such as pyruvic, 

 malonic, and benzoic cause the same response, so that the 

 test is not specific. (C/. also 'KM.) 



D. 4. Other Methods 



The fact that auxin is involved in a large number of 

 processes other than simple cell elongation makes possible 

 a number of means of assaying it. The inhibiting effect of 

 auxin on the growth of roots has been used as a test method 

 for auxin assay (IX B). The formation of roots on cuttings 

 (XI B), the inhibition of lateral bud development (Chapter 

 XII), and the production of swellings on stems and hypo- 

 cotyls (XIII C) have also been used. 



