38 



GROWTH HORMONES IN PLANTS 



having the last one come just before the agar blocks are to be 

 applied. 



Kornmann (1935) found that a bending response could be 

 obtained with extracts from corn meal or from tips of maize 

 coleoptiles applied in agar blocks to Avena coleoptiles. There 

 was no response if Avena-coleoptile tips or bases were placed 

 upon these blocks for a few hours before testing. Some sub- 

 stance diffusing from the Avena tips into the agar inactivated the 

 maize-agar preparation. When maize coleoptile tips were 



010 40 100 140 



Concentration in y per Liter 



200 



250 500 750 1000 



Concentration in y per Liter 



Fig. 18B. — Graphs showing growth curvatures in Avena and Cephalaria when 

 treated with different concentrations of heteroauxin (3-indole acetic acid). Left: 

 Cephalaria shows greater sensitivity to low concentrations of growth hormone. 

 Right: same, over greater range of concentrations {From Soding, 1936.) 



placed on corn-paste agar, better curvatures resulted than with 

 the corn-paste alone. Similarly, good curvatures resulted with 

 oat-flakes paste upon which Avena coleoptile tips had stood. No 

 curvature followed the application of a mixture of maize-tip 

 paste and oat-tip paste, each of which alone gave good results. 

 The suggestion is made that in certain of the mixed preparations, 

 destructive oxidation of the hormones resulted from the action 

 of enzymes. 



SUMMARY 



It has been demonstrated beyond question that growth sub- 

 stances occur in plants, although they are present in such small 

 quantities that no satisfactory means of microchemical detection 

 has yet been devised. The most commonly used biological 

 indicator is the Avena coleoptile. 



Seedlings of Avena which are to be used in testing for the 

 presence of a phytohormone are usually grown in a darkroom 



