Natural auxins 



experiment designed to clear up this point gave the results shown in Figure 11. 

 It was evident that, as the initial length of the section increased, the absolute 

 amount of elongation increased. However, the growth of the controls also 

 increased, so that the difference Giaa" ^o "^^Y ^'^^ benefit from longer 

 initial lengths. This is the case for internodes {Figure 1 l{e)) in which the zone 

 which responds to the addition of lAA seems more restricted than in the case 

 of coleoptiles. If one looks for maximum absolute response to lAA over that of 

 controls, then a 10 mm coleoptile section or a 5 mm internode section is 

 better than shorter ones. But if one considers this response expressed as 

 percentage of the initial length, then a 4 mm section gives about the largest 



Coleoptile 



First internode 



15 20 



30 



35 V(7 15 



Length of organ 



30 



Figure 12. Effect of the physiological age of the oat seedling {represented by the length of the coleop- 

 tile or of the first internode at the time of sectioning) on the growth of the sections in buffer+ sucrose without 

 lAA {white circles, dotted lines) and with lAA {black circles, solid lines), and on the difference between 

 these growths {curves b and d). Scales at the left ordinate are in mm, on the right ordinate in per cent 

 of the initial 4 mm length. 



diflferential response {Figure 11, curves c and/). To be able to use small 

 volumes of solution and avoid the lack of straightness inherent to long 

 sections, an initial 4 mm length was adopted for both coleoptile and internode 

 sections. 



An important factor in the sensitivity of oat plants is their age. In general, 

 the younger they are, the more they grow, but they do so also without added 

 auxin. The optimal size at the time of sectioning is when the coleoptile and 

 the mesocotyl have reached about respectively 20 and 15 mm in length 

 {Figure 12). Longer internodes rapidly lose their sensitivity, whereas 

 coleoptiles up to 35 mm in length (in the variety Brighton) still respond 



18 



