CHAPTER IX 



THE GROWTH OF ROOTS 



In the preceding chapters reference has been made only 

 to the relation of auxin to shoots. In roots the relations are 

 entirely different, although in many respects analogous. 



A. The Effect of the Tip in Root Growth 



Unlike shoots, in which decapitation almost invariably 

 reduces the growth rate, decapitation of roots does not 

 greatly influence the growth. In the 70's and 80's of last 

 century a flood of literature appeared deaUng with this 

 subject. Every investigator used a different object, and 

 opinions were about equally divided as to whether decapita- 

 tion retarded growth or was without effect; Wiesner (1884), 

 working with roots under water, even found an acceleration. 



Probably on account of the disagreement, the whole ques- 

 tion was shelved for almost half a century. It was Cholodny 

 who in 1924 and subsequent years took up the problem 

 again and showed that in fact, if precautions were taken to 

 ensure that the roots received sufficient water, there was an 

 acceleration after decapitation (Cholodny, 1926). He worked 

 first with Zea Mays and later confirmed the effect with 

 Lupinus (1926). The differences were in all cases small 

 (ca. 12 per cent), but real. Later Biinning (1928) showed 

 that the acceleration was limited to a few mm. of the grow- 

 ing zone only, and that if sections of tip longer than 1 mm. 

 were removed the acceleration did not take place. The 

 acceleration is preceded by a temporary decrease in growth 

 rate, which is probably due to a loss of turgidity of the 

 cells resulting from the wounding (Janse, 1929). Biinning 

 found this acceleration in Lupinus, Zea Mays, and Vicia Faba 

 roots, but not in Pisum roots. Gorter (1932) also found 

 that Pisum roots show no change in growth rate after 



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