hiliibition of Gibberelliii Action by Auxin 



659 



The Test 



On the fifth day the oats are ready for use. The length from the 

 coleoptile tip to the node is approximately 20 to 30 mm. Twenty-four 

 hrs. prior to use, the flat is jarred gently in order to pack the Vermicu- 

 lite and prevent the oats from growing crooked. Seedlings with a 

 coleoptile length between 20 and 30 mm. are selected in red light for 

 the test. The shoots are cut at the base and floated in a dish of water. 

 After all the shoots necessary for a test have been selected and placed 

 in the water, a 5-mm. section is cut from the base of each coleoptile 

 by means of two parallel razor blades motmted in a special holder. 

 The section includes the coleoptilar node, and the lower cut is made 

 just below it. The shoots are laid on a piece of glass under which is 

 placed a paper with a line on it (Figure 1). The node is placed on the 

 line and the lower razor blade is lined up with this line as a cutting 

 guide. New razor blades are always used, as dull blades may injure the 

 leaf, causing it to grow out crooked from the coleoptile. Immediately 

 after cutting, the sections are floated in a dish of distilled water and 

 held there until they are ready to be put into the final test solutions. 

 When a sufficient number of sections has been cut, they are drained 

 through cheesecloth and, using the cheesecloth as a sack, they are 

 dipped 10 to 15 times in a 0.2 per cent Clorox solution (100 p. p.m. 

 active chlorine). The Clorox is not rinsed off. Without this method of 



BUFFER 



6 A O.i P.PA 



Fig. 2. Basal 5 mm. coleoptile sections of Avena with leaves growing out of them. 

 Leaf growth is promoted l^y the addition of low concentrations of gibberellic acid 

 to the basic medium of phosphate buffer and sucrose. The sections floated in 2 ml. 

 of solution on a shaker in the physiological darkroom at 27° C. and were occasion- 

 ally exposed to red light for observation. 



