FORMATION AND OCCURRENCE OF AUXINS 59 



The production of auxin in the tip of the Avena coleop- 

 tile may be considered in more detail. Went (1928) showed 

 that only the extreme tip, less than 0.7 mm. in length, pro- 

 duces auxin. In fact if the auxin is collected at a point 10 mm. 

 or more below the tip, the amount obtained may be very 

 much less, because auxin is used up in growth, or destroyed, 

 during its downward movement (c/. e.g. van Overbeek, 

 1936). Nagao (1936) found the same thing for Avena roots. 



15 AE/hr/tip 

 or mmyhr. 



'growth rale 

 "of primary leaf 



I X, auxin, .. 



I \ "'"-^uCllOft 



50 too hrs. 



Fig. 27. Auxin production (crosses) in AE per hour per tip, and growth 

 rate (circles), in mm. per hour, in Avena coleoptiles at different ages. Ab- 

 scissa, age in hours from soaking. Each point the mean of 30-50 plants. 



When cut off and placed on agar, the coleoptile tip continues 

 to produce auxin for many hours (van der Weij, 1931), pro- 

 viding it is kept at temperatures below 30°. The production 

 finally drops to zero, however, probably on account of 

 exhaustion of the auxin precursor {of. IV B). Very young 

 coleoptiles produce little auxin; the auxin production closely 

 follows the increase in growth rate and continues at a high 

 level even when the growth rate of the coleoptile decreases 

 again. This has been found both in Zea Mays (van Overbeek, 

 u) and in Avena (u); see Figure 27. These results contrast 

 with those of du Buy and Nuernbergk (1932) and du Buy 

 (1933), who state that coleoptile tips from short plants 

 produce no auxin at all, and that auxin production is after- 

 wards constant over a period of 4 days. Their findings. 



