DETECTION AND QUANTITATIVE DETERMINATION 21 



<\ 



Fig. 8. — Technique of testing for growth hormone with agar blocks applied to 

 the decapitated coleoptiles of Avena seedlings. A, diffusion of growth hormone 

 into agar; a, agar plate: h, plant material (e.g., coleoptile tips) to be tested for the 

 presence of growth hormone is placed in contact with agar for 2 hours; c, the agar 

 plate containing growth hormone is cut into small blocks. {After Went, 1928a.) 

 B, decapitation of Avena coleoptile and unilateral application of agar block. 

 The tip of the coleoptile is removed and the foliage leaf is pulled out part way 

 and cut off; a small portion is left extending from the apex of the coleoptile 

 stump. An agar block containing the hormone to be tested is placed over a 

 vascular bundle on one side of the cut apex (see Fig. 13). The ensuing curvature 

 is proportional, within limits, to the concentration of growth hormone in the 

 agar block. C, end view of decapitated coleoptiles in contact with agar blocks. 

 The contact area is the same in both a and b though the volume of one block 

 is 8 times that of the other. {After Thimann and Bonner, 1932.) 



