TECHNIQUE OF AUXIN DETERMINATIONS 35 





exactly filled by Yi cc. After cooling, the resulting disk of agar is 

 cut into rectangles with the same cutter as above. With either method 

 the rectangles are finally divided into 12 equal blocks by means of 

 other special cutters (see Figure 11). 



The sizes of blocks adopted vary in different laboratories 

 between 2 and 10 cmm. To avoid too much volume change 

 by drying out, etc., it is advisable to use blocks of volume 

 not less than 4 cmm., the volume changes being propor- 

 tionately less the larger the block. It has been shown 

 (Thimann and Bonner, 1932) that the curvature is pri- 

 marily proportional to the 

 concentration of auxin in the 

 block and that the rate at 

 which the auxin enters the 

 plant is proportional to its f 

 concentration in the block at 

 any moment; since part of 



the auxin passes out of the Fig. 15. Method of cutting thin 



1, 1 1 • ,1 ,, ,1 sheets of agar of uniform thickness. The 



block durmg the test, the agar, A, is embedded on 3 sides in paraf- 



change of concentration in fin, B, and sliced on a microtome with a 



the block so caused is small '^^"^^ ^^^^'^ ^^^^^- ^^™"^ ^^'^^' ^^^s.) 

 if the blocks are large. Thimann and Bonner (1932) have 

 measured the amount of auxin which passes into the plant 

 during 110 minutes' contact, and from the data have cal- 

 culated the curvatures which would be produced by different 

 sizes of blocks containing the same auxin concentration (see 

 Table III). The calculations were found to agree closely 

 with the curvatures observed. 



The discrepancy between Went's conclusion (1927), that 

 with 0.9 cmm. blocks the curvatures are proportional to the 

 absolute amount of auxin in the blocks, and the conclusions 

 of van der Weij (1931) and Thimann and Bonner (1932) 

 that the curvatures are proportional to the auxin concen- 

 tration in the block, is directly explained by Table III. In 

 Went's experiments 86 per cent of the auxin from the block 

 entered the plant; with larger blocks only 15-30 per cent 

 will pass into the plant. 



