24 



GROWTH HORMONES IN PLANTS 



1. DETECTION OF GROWTH SUBSTANCE IN FLUIDS. — If a fluid is to 



be tested for growth substance, the reaction must be weakly acid; 

 one neutrahzes where necessary with sodium bicarbonate and 

 adds a little citric or acetic acid (0.2 cc. per liter). If the solution 

 is very pure, it may be necessary to add some potassium chloride 

 (169 mg. per liter) (Kogl and Haagen Smit, 1931, Mitt. I). The 

 substratum is prepared in the following way : A computed amount 

 of agar is carefully washed with tap water for 24 hours; after- 

 ward the agar plus the absorbed fluid is weighed again, and 

 enough water is added to produce a 3 per cent agar. The solution 



Fig. 9. — Curvatures resulting from application of agar blocks containing saliva 

 to one side of decapitated coleoptiles. (After Seubert, 1925.) 



which is being investigated for growth substance is then mixed 

 with an equal amount of the substratum. After solidification, 

 small blocks of equal size are cut out and placed unilaterally upon 

 decapitated Avena coleoptiles (for size of blocks, see description 

 under quantitative determination). Instead of mixing the solu- 

 tion to be tested with melted agar, agar blocks can be placed in 

 the solution for 1^^ hours. Through control experiments it can 

 be shown that the agar substratum has no effect upon the Avena 

 coleoptile. This method was originally used by Stark (19216) 

 and later by Nielsen (1924), Seubert (1925), and others (Fig. 9). 



It is possible also to detect the presence of growth substance in 

 a fluid by mixing it in various proportions with lanolin (Laibach, 

 19336). The lanolin growth-substance paste may be applied 

 unilaterally to intact coleoptiles; if bending occurs, it may be 

 concluded that growth substance is present (Fig. 37 A). 



