ROOT FORMATION 199 



thinking that it is this process which is influenced by the 

 factors discussed above; firstly, the length of the roots 

 which have been formed is not materially influenced by 

 the treatment, except by sugar; secondly, examination of 

 the pea stems has not revealed any appreciable number of 

 root primordia; thirdly, we know that the direct effect of 

 auxin on roots is to inhibit their growth in length. Further- 

 more, the total length of roots formed per cutting is more 

 or less constant, so that the more roots are formed the 

 shorter they are. This indicates that the outgrowth of the 

 roots is influenced not by the auxin treatment, but by an 

 internal factor, which may become distributed over a large 

 number of root primordia. This factor is probably the one 

 contained in the yeast extract which White (1933, 1934) 

 finds necessary for growth of excised root tips in synthetic 

 media (see IX D) . 



There is, however, another factor of quite different type 

 which apparently also takes part in the first stages of root 

 formation. If a pea cutting is divided into a number of 

 sections and each is placed in sugar solution and treated 

 with high concentrations of auxin (in paste form), then the 

 sum of the numbers of root primordia formed is about the 

 same as if the intact cutting were so treated. The majority 

 of these primordia are formed at the point of apphcation 

 of the auxin. Only a small proportion of them grow out as 

 roots, probably because of the high concentration of auxin, 

 which inhibits growth in length (c/. IX B). In one experi- 

 ment, there were 34 such primordia on the intact cutting; 

 on 2 cuttings which were di\4ded into 4 the sum of the 

 primordia produced at the 4 bases and apices was 27 and 33 ; 

 on 2 cuttings similarly divided into 8 the sum of the pri- 

 mordia was 38 and 43 {u). The total number of primordia 

 is thus nearly constant and must therefore be determined 

 by an internal factor other than the auxin, which factor only 

 becomes limiting when auxin is in excess. Further, the dis- 

 tribution of this factor inside the plant can be determined 

 from the distribution of the primordia on the dissected cut- 



