206 Auxins in Agriculture 



This has led to the generalization that the ratio of carbohydrate to 

 nitrogen should be high for optimum rooting (Kraus and Kraybill, 

 1918; Pearse, 1943). The reports of many workers that nitrogen is 

 detrimental to the rooting of cuttings (e.g. Knight, 1926) undoubtedly 

 are explained by the requirement for an abundant supply of carbo- 

 hydrate in proportion to the nitrogen. It is interesting to notice that 

 in replacing the stimulatory effect of leaves, van Overbeek et al (1946) 

 obtained the best results with forty times as much sugar as ammonium 

 sulphate. 



The rooting response to auxin is a quantitative one. The quanti- 

 tative characteristic of the response can be seen in the data shown in 

 figure 89. The use of auxin concentrations higher than optimum re- 

 sults in a reduction of rooting, but the evidence available suggests that 

 this reduction may be owing more generally to the inhibition of 

 growth of the root primordia rather than to a reduction in number of 

 primordia formed. 



It is evident that auxins stimulate the formation of roots by an 

 interaction involving organic materials in the plants, particularly car- 

 bohydrates and nitrogenous materials. This interaction apparently 

 controls the basic step of morphological differentiation at the cellular 

 level. 



+ Rough Lemon 



o^Cherimoyo 



Trifolata 



200 400 600 800 MG/L 



CONG. OF AUXIN 



Fig. 89. The quantitative stimulation by auxin of rooting in cuttings of three 

 varieties of citrus. The auxin used was indoleacetic acid (Biale and Halma, 1937). 



