574 



GROWTH HORMONES 



A 



A 



A 



A 



A 



O-O. 



if the excised apex of the organ is carefully relocated on the cut surface of 

 the stump. 



Use of the Oat Coleoptile as a Test Plant in the Quantitative Deter- 

 mination of Auxins. — Auxins are now known to be of widespread distribu- 

 tion in plants. They occur in such small quantities, however, that detection 

 of their presence in an organic material by chemical methods is usually diffi- 

 cult and often impossible. Re- 

 course is had, therefore, to sensi- 

 tive biological tests in order to 

 demonstrate the presence of these 

 substances. The oat coleoptile test 

 is the most commonly used method 

 of determining the relative quan- 

 tities of auxins present in plant tis- 

 sues or other materials. 



If an agar block containing 

 auxins from one source or another 

 is affixed one-sidedly on the decapi- 

 tated stump of an oat coleoptile, 

 elongation is found to be more 

 rapid on the side of the coleoptile 

 below the portion of the tip on 

 which the block is perched, result- 

 ing in curvature of the coleoptile (Fig. 127). Translocation of the hormone 

 is almost strictly longitudinal, the elongating cells on the side of the coleoptile 

 covered by the block receiving much more than cells on the opposite side, with 

 a corresponding differential effect on growth. When the block is centered on 

 the decapitated tip, as in the experiment described in the preceding section, all 

 sides of the coleoptile receive approximately equal quantities of auxin, and 

 growth proceeds in a vertical direction. 



Furthermore, it has been found that the curvature resulting from the 

 eccentric attachment of agar blocks to the decapitated stumps of oat coleoptiles 

 is proportional, within the range of about O to 20 degrees, to the concentration 

 of the auxin in the agar block. This proportionality between hormone con- 

 centration and curvature makes possible the use of oat coleoptiles as living 

 test objects in the quantitative estimation of the auxin content of plant tissues, 

 etc. (Went, 1928). 



Quantitative measurements of auxins by the oat coleoptile technique must 

 be carried out under carefully standardized conditions. Only a generalized 

 description of the usually employed methods will be given. The oat seedlings 



Fig. 126. Effect of agar blocks contain- 

 ing auxin on elongation of oat coleoptile. 

 (A) check, (B) block containing auxin placed 

 on decapitated tip, (C) block of pure agar 

 placed on decapitated tip. The effect of the 

 treatment is shown by relative increase in 

 length of coleoptiles at right. 



