TECHNIQUE OF AUXIN DETERMINATIONS 51 



vantage of the variability is largely offset by "calibrating" 

 the plants, whenever required, with an auxin solution of 

 known concentration. For this purpose a dilute indole- 

 acetic acid solution (0.1 7 per cc), steriUzed and kept sterile, 

 retains its activity very satisfactorily. 



D. Other Methods of Auxin Determination 

 1. Straight Growth 



It goes without saying that all determinations of growth- 

 promoting activity should be based on actual growth pro- 

 duced. ^ The convenience of curvature methods rests upon 

 two facts: (1), the residual growth, after decapitation, is 

 the same on both sides of the plant and thus is automat- 

 ically eliminated from the measurements — no controls are 

 necessary; and (2), only one measurement need be made; 

 there is no ''Zero reading," for the plants are chosen 

 to be straight. Besides this, the curvature magnifies the 

 amount of growth occurring and thus makes it easy to 

 measure.- 



Since the absolute changes in length caused by auxin apphcation 

 are rather small — of the order of a few mm. at most — their measurement 

 requires magnifications from 10 to 100 times. The most generally 

 used instrument for this purpose is the horizontal microscope (described 

 by Metzner, 1928, p. 133). The lengths of the plants, which must 

 be placed on rigid supports, are measured periodically in red light with 

 this instrument. The same end can be attained by photographing the 

 plants automatically with a lapse-time moving-picture camera, and 

 subsequently enlarging and measuring the i^ictures; this is especially 

 convenient for long period measurements (see, for instance, Nuernbergk 

 and du Buy, 1932). Both of these methods have been used in measure- 

 ments of the distribution of growth over different zones of the plant. 

 For the photography, small marks (tin foil, paraffin paper, sand parti- 

 cles, etc.) are affixed to the side of the plant; with the horizontal micro- 

 scope, dots or fine lines made with India ink are suflScient. Other 

 methods of growth measurement such as the interferometer (Meissner, 



'Correspondingly, the term "growth-substances" should be restricted to sub- 

 stances which are actually shown to increase growth, a precaution which has been 

 neglected by Hitchcock and Zimmerman (1935), and Zimmerman and Wilcoxon 

 (1935). 



