Investigation of natural auxins and growth inhibitors 



horizontal shaker. To be able to section the coleoptiles and internodes 

 accurately, we used Thimann's 'guillotine' {Figure 9), an improvement over 

 the original Van der Weij's 'coleoptile microtome' (1932). The primary leaf 

 was always left included in the coleoptile. This procedure simplifies the test 

 without impairing its sensitivity or reliability. 



The first question to be decided was: 'Where should we cut the sections?' 

 A series of experiments in which 4 mm sections were cut, first including the 

 tip or the node, then starting 1 mm, etc., below them, gave the results shown 

 in Figure 10. It should be noted that what is important here is not so much 



Coleoptile 



First internode 



Figure 11. Effect of the initial 

 length oj oat sections on their 

 growth in buffer + sucrose without 

 added 1.4.4 (white circles, dotted 

 lines) and with I.AA {white circles, 

 solid lines) : graphs (a) and {d) . The 

 other curves represent this effect on 

 the difference Gix^ — G^ expressed 

 in mm {curves {b) and {e)) or in per 

 cent of the original lengths {curves 

 {c) and if)). 



2 3 1 S 



10 2 3 15 

 Initial lengtt) of section 



10 



mm 



the maximum growth in lAA as the maximum differential growth between 

 the lAA-treated sections and the controls. This difference, which we can 

 represent by Gj^^ — Gq (growth in lAA minus growth without lAA), is 

 maximum under our conditions when the 4 mm section is cut 3 mm below the 

 tip or 3 mm below the node of the oat seedling. Since the growth of internode 

 sections taken 3 mm below the node is very low without auxin (which makes 

 it difficult to detect any inhibitor on chromatograms), we decided always to 

 cut the internode sections only 2 mm below the node. 



The second question was: 'What length should we cut the sections?' 

 Some workers use 3 mm sections, others prefer them 10 mm long. An 



17 



