GENERAL CONCLUSIONS 237 



the weight of organism affected its acti\dty is not particu- 

 larly high (see Table XVI). The following table from Kogl 

 (1935) exemplifies clearly the interlocking action of the 



3 fractions: 



TABLE XVII 



Additions to a Medium Containing 42 Grams of Yield per Gram of 



Dextrose and 23 Grams of Salts Inocdlom in 10 Hours 



Control 1-1.5 



Bios I, 4 g. 1-1.5 



Bios III, 2 g. 1-1.5 



Bios I, 4 g. + Bios III, 2 g. 1-1.5 



0.167 7 Biotin 4 



1.67 7Biotin 7 



1.67 7 Biotin + 4 g. Bios I 10 



1.67 7 Biotin + 4 g. Bios I + 2 g. Bios III 14 



The strain of yeast here used evidently contains none 

 of the biotin, which thus acts as a limiting factor, but in 

 the presence of a sufficient amount of biotin the other con- 

 stituents become limiting and must be added to obtain 

 maximal growth. Some yeasts, like that of Williams et al. 

 (1933), need only one of the bios factors, presumably being 

 able to synthesize sufficient of the others. This parallels 

 closely the root formation of the different strains of Pisum 

 discussed in XI E, where some varieties respond to auxin 

 application and others not at all. In some of these, auxin 

 is already in excess, as is shown by the large number of roots 

 formed by untreated controls; in others some additional 

 factor is lacking, for roots are not produced by any treatment. 



Root formation in Pisum provides a particularly good 

 example of a system of limiting factors in higher plants, 

 there being at least 3 substances known whose action inter- 

 locks. These 3 factors, sugar, auxin, and biotin are all avail- 

 able in the pure state and hence their interlocking action is 

 easily studied (c/. XI E). A graphic representation of the 

 interaction of these factors, drawn in a manner comparable 

 to that of Blackman (1905), is shown in Figure 62. It will 

 be seen that each factor reaches a concentration at which 

 it is no longer limiting, and at this concentration root 

 formation may be increased by adding the next factor, the 



