ROOT FORMATION 201 



If the number of roots at the base and the tip are added 

 the total number per cutting increases smoothly with auxin 

 concentration as shown in Figure 56. 



^Yhen auxin is appUed at the apex, the lowest concentra- 

 tion needed to produce localized roots in this way is about 

 100 times that which will produce roots at the base. The 

 concentrations needed to produce localized roots in the 

 experiments of Hitchcock (1935a) are evidently those corre- 

 sponding to these high values, since they too are of the order 

 of 100 times the concentration which will, in our experiments, 

 produce roots at the base. Experiments on intact plants in 

 soil, of course, preclude observation of basal root formation. 

 This ratio of 100 fits in very well with an observation of 

 another sort. It was stated above that auxins must be 

 applied at the apex to induce root formation at the base. 

 An exception, of course, is given by those substances, such 

 as indene-acetic and cumaryl-acetic acids, whose transport 

 from apex to base is limited. These substances give ex- 

 cellent rooting when applied at the base, while if applied 

 at the tip their activity is slight or zero (Thimann, 19356). 

 Thus, on pea cuttings, indene-acetic acid applied at the tip 

 gave, per 10 plants, 19 roots, while at the base the same 

 concentration gave 98 roots. Root formation when true 

 auxins are applied to the base can, however, also be ob- 

 tained if very high concentrations are used. In one experi- 

 ment basal root formation on Pisum cuttings was induced 

 by 0.02 7 per cc. of indole-acetic acid when applied at the 

 tip, but, when applied at the base, a concentration of 20 y 

 per cc. was necessary. From this and other experiments 

 the ratio of minimum effective concentrations applied at 

 base and at tip is between 100 and 1000. Gouwentak and 

 Hellinga (1935) report comparable data for Coleus cuttings. 

 A dab of paste containing 0.01 y indole-acetic acid caused 

 root formation at the basal cut surface only; with 20 y of 

 the same compound in the paste, roots were formed at both 

 apical and basal surfaces. When the 20 y was applied at the 

 base it did not, of course, produce roots at the apex. These 



