Natural auxins 



soaking period in MnS04-|-H20 (1 mg/l.) was adopted for coleoptile 

 sections and a 1-hour presoaking period in glass-distilled water for the inter- 

 node sections, the latter being maintained breaking the surface on stretched 

 cheese-cloth. 



In a given plant extract, the quantity of auxin is generally small. The 

 problem is how to make the best use of it to get the maximum growth 

 response. If we have, let us say, 0-01 y of lAA, is it better to dissolve it in 

 1 c.c. of assay solution, or in 10 c.c. ? It is known (cf. Schneider, 1938) that 



^ 



^ 



1^0 1 u me c.c. 

 M C 



0-5 -K^- 0-5 



1-0 — ■>--i-o 



Z-0 — ■— S-0 



180 



leo 



mo 



120 



r 



100 -^ 



80 



GO 



<fO 



% 



f 10 f-o 100 100 

 Total amount of lAA 



1000 



9000 10,000 



yxlO'^ 



Figure 13. Effect of the volume of solution on the growth of oat first internodes [M) and coleoptiles 

 (C). The total amounts of lAA indicated on the abscissa {logarithmic scale) were dissolved in either 0-5, 

 1-0, or 2-0 c.c. {when using first internodes) or 0-5, 1-0, or 5-0 c.c. {when using coleoptiles) of the usual 

 buffer solution plus 2 per cent sucrose and 0-1 per cent Tween 80. 



controls in distilled water tend to grow less in large than in small volumes, 

 presumably because of the dilution of a food factor. This effect can be ruled 

 out (1) by presoaking the sections in large volumes of solutions before the 

 actual assay and (2) by adding a food factor, such as sucrose, to the assay 

 solutions. Experiments in which these precautions were taken showed 

 repeatedly that the growth of the controls is the same in 0-5 c.c. or 5 c.c, and 

 that growth of both coleoptile and internode sections in lAA plus sucrose and 

 buffer does not change appreciably according to the volumes used {Figure 13), 

 provided, of course, that no evaporation takes place during the assay with 

 0-5 c.c. It should be mentioned that the coleoptile sections were incubated 

 in special glass dishes having flat bottoms in order to ensure a uniform depth 

 of the liquid with 0-5 c.c. 



Many workers have found that the addition of a sugar, especially sucrose, 



20 



