28 



GROWTH HORMONES IN PLANTS 



sides of a curved coleoptile. The concentration of growth sub- 

 stance necessary to bring about a certain difference in the length 

 of the two sides is designated as one WAE (Wuchsstoff Avena 



0.10 



0,08 - 



0,06 - 



0,04 - 



0.02 - 



20 40 60 80 100 120 140 160 180 200 



Fig. 11. — Curve showing relationship between amount of bending in the 

 Avena coleoptile and the concentration of "rhizopin" applied unilaterally in 

 agar blocks. Ordinate: d value. Abscissa: relative concentration of the growth 

 substance. {After Nielsen, 1930.) 



Einheit — see 5, page 29). The method may be outlined briefly 

 as follows: 



1. The test plants are grown in soil cultures and decapitated as outlined 

 under "culture methods" (p. 18). 



2. The agar blocks 2 by 2 by 1 mm. containing the growth substance (see 



4, p. 25) are applied unilaterally to decapi- 

 tated coleoptiles and are allowed to remain 

 for 3 hours. 



3. The curvature produced by the unilateral 

 effect of growth substance is the result of a 

 difference in length between the convex and 

 concave sides; this difference, d, is the result 

 of unequal rates of growth. When measured 

 in millimeters, this difference becomes a clear 

 expression for the effect of growth substance 



l+d 



r t 



Fig. 12. — Diagram showing 

 derivation of d value. /, 

 length of coleoptile, inner 

 curved side; d, difference in "PO" the coleoptile (if the approximate 



growth rate of decapitated coleoptiles is 



known). The d value may be determined by 



the method employed by Purdy (1921) (see 



Fig. 12). If Hs the length of the curved part 



of the Avena coleoptile, d the difference in 



length, t the diameter of the coleoptile, and r the radius of the curvature, 



the following equations are obtained: 



length between inner and 

 outer curved side; I -\- d, 

 length of outer curved side; 

 t, diameter of the coleoptile; r, 

 radius of curvature. 



I 



r +t l+d' 



l_ 



d' 



, tl 

 a = — 

 r 



