TECHNIQUE OF AUXIN DETERMINATIONS 31 



on one side of the coleoptile without cutting the primary 

 leaf; the top of the coleoptile is then bent so that it breaks 

 at the incision and the topmost part is pulled off. Special 

 decapitation scissors (Figures 11, 12) which cut the coleop- 

 tile on two sides without touching the leaf are very con- 

 venient for this, because with the closed scissors the cut-off 



m 



\l 



A 



u \/ 



B 



D 



H 



Fig. 10. Diagrammatic summary of procedure in the Avena test, showing the 



stages photographed in Figure 9. 



portion can be removed in the same movement. The pri- 

 mary leaf, which now protrudes for 5 mm. or more, is then 

 pulled gently so that it breaks at the base and is partially 

 drawn out (F, Figure 10); this can be conveniently done 

 with cork-tipped forceps (Figure 11). If the cut surface is 

 very wet it should be dried with filter paper. The small 

 block of agar to be tested is then placed on one side of the 

 cut surface, resting against the leaf, so that it is held in 

 place by capillarity. The period elapsing between the 

 second decapitation and the apphcation of the block does 

 not usually exceed 20 minutes. 



To record the curvatures a piece of bromide paper is 

 placed behind the plants; they are then illuminated from 



