USE OF THE OAT COLEOFTILE AS A TEST PLANT 575 



(usually from a genetically uniform variety) are grown in a dark room at a 

 temperature of 25° C. and a relative humidity of 90 per cent. All manipula- 

 tions are performed under phototropically inactive orange or red light (Chap. 

 XXXVII). The coleoptiles are used when about 2.5 to 4 cm. in length. 

 The extreme tip of the coleoptile is first cut off, and after three hours the 

 topmost 4 mm. of the stump is removed. For reasons which can not be con- 

 sidered in a brief discussion, the coleoptiles are more sensitive when this 

 method of double decapitation is employed than when the tip is cut off in one 

 operation. 



The primary leaf, which is enclosed by the coleoptile, is then pulled loose 

 so it will not interfere with the determination by its continued growth. If 



A 



A M A# 



i M M ^■-- '"' 



' '1 > ■ 



1 : : I : 



I [ [ t . 



A B C D E F 



Fig. 127. Diagram of method of determining auxin content of agar block quanti- 

 tatively. (.1) intact coleoptile enclosing primary leaf, (B) tip of coleoptile removed, 

 (C) primary leaf pulled loose so its elongation will not dislocate agar block, (D) tip 

 of primary leaf cut off, (£) agar block affixed unilaterally to coleoptile tip, (F) curva- 

 ture resulting from movement of auxin into side of coleoptile below agar block. Re- 

 drawn from Went (1935). 



guttation water exudes at the cut surface it is carefully blotted of^. An agar 

 block (2 X 2 X I mm. is a commonly used size) containing the substance 

 to be tested is then affixed unilaterally to the cut tip. After a standard length 

 of time (usually 90 min.) the resulting degree of curvature of the coleoptile 

 is determined. This is usually done by obtaining their "shadowgraphs" on 

 bromide paper and measuring the curvature of the shadowgraphs with a 

 suitable protractor. The greater the degree of curvature, within limits, the 

 greater the hormone concentration in the agar block. 



Various methods of applying the material to be tested for auxins to de- 

 capitated coleoptiles have been employed. Sometimes small plant organs or 

 pieces of plant organs are affixed directly to the cut surface of the coleoptile. 

 IVIore commonly plant tissues are placed in contact with moist 3 per cent 



