ROOT FORMATION 



197 



different concentrations of indole-3-acetic acid up to the 

 maximum, which produced 102 roots per 10 plants; at this 

 maximum concentration the biotin was subsequently ap- 

 phed to the base, when the number of roots formed in- 

 creased in proportion to the biotin concentration and reached 

 a second maximum of 179 roots per 10 plants. Similar 



ISO 



100 



50 



roots per ten plants 



-8 -7 -6 -5 -4 lo|.tnol. 



concentration of Indole acetic acid 



0.4 



2 10 50 250 1000 5000 S.E^^ 

 units biotin 



Fig. 55. Root formation on etiolated pea stems with their bases in 2% 

 sucrose solution. Left, indole-acetic acid alone applied to the apex (abscissa, 

 log. molar concentration); right, optimal auxin concentration applied at apex, 

 with biotin added to the sucrose solution at base (abscissa, Saccharomyces 

 units per cc). (Kogl, Thimann, and Went, 1935, u.) 



application of biotin at the tip gave no effect. Without auxin, 

 biotin, apphed either at tip or base, produces no roots, so 

 that its effect is exerted only in presence of excess of auxin. 

 This result is of special interest in \dew of the fact that up 

 till recently biotin has only been known to exert its effect on 

 growth of yeasts. However, Kogl and Haagen Smit (1936) 

 have found that biotin also increases the growth in length 

 of Pisum seedlings, from which the cotyledons have been 

 cut off. It should be noted that the lowest concentration 

 of biotin which definitely increases the number of roots is 

 between 2 and 10 Saccharomyces units ^ per cc, i.e. a con- 



1 1 mg. of biotin = 25 X 10^ Saccharomyces units. 



