152 



THE CELL AND PROTOPLASM 



in a specific chemical reaction, as we 

 reasoned in the preceding section, it must 

 be possible to find a common denominator 

 for all of them, or in other words, to 

 discover a specific molecular arrangement 

 which is involved in this growth reaction. 

 After the discovery of indole acetic acid as 

 a growth-promoting substance, Kogl (1935) 

 suggested that either this substance per- 

 forms the same function in connection 

 with growth as a pass key in forcing a 

 lock, or it opens a different lock altogether, 

 a sort of back door to growth. This second 

 possibility is now ruled out, and we can 

 try to find out which bits in the key to 

 growth are necessary to open the lock; 

 that is, to take part in the growth reaction. 

 The grooves which are not essential for the 

 actual process of opening, but which only 

 determine whether the key will be able to 

 penetrate the lock, could be compared with 

 the secondary properties of growth sub- 

 stances, which are essential only for their 

 penetration into the cell. 



As a result of an extensive comparison 

 of the different substances able to produce 

 growth, involving the preparation of a 

 number of critical compounds (Koepfli, 

 Thimann and Went 1938), we can now say 

 that a substance must contain a ring with 

 a double bond; next to the double bond 

 it must have a side-chain with a carboxyl 

 group; this carboxyl group must be at 

 least one carbon atom removed from the 

 ring; and must in addition have a very 

 definite space-relation towards the double 

 bond. It is immaterial whether the con- 

 figuration is realized with an auxin, indole, 

 naphthalene, anthracene, or benzole nu- 

 cleus; within this series activity depends 

 only upon reactivity of the double bond. 

 Thus, out of the almost infinite complexity 

 of widely differing activities of very differ- 

 ent compounds in producing growth, there 

 appears an exceedingly simple picture of 

 the effect of substances in the growth 

 reaction. 



After clearing up the activity of so many 

 seemingly unrelated compounds, we will 

 consider how it is possible that auxin takes 

 part in so many processes inside the plant. 



To reach a better understanding, we must 

 first distinguish between two different 

 states in which the auxin is present. In 

 the first place, a certain amount of auxin 

 is moving freely in the plant and can be 

 obtained by letting it diffuse out of the 

 tissue into agar. This free-moving, or 

 diffusible, auxin is responsible for the cor- 

 relation phenomena which are affected by 

 auxin, such as bud inhibition, geotropism, 

 etc. In the second place, auxin is present 

 in the plant in a form which does not 

 diffuse out, but can be obtained by extract- 

 ing the killed cells with organic solvents. 

 It is this form of auxin (bound auxin) 

 which is responsible for growth. There 

 are many differences in properties and 

 behavior between these two states of auxin 

 in the cell (Stewart and Went 1940). The 

 diffusible auxin is not inactivated by light, 

 its effect is independent of pH, the double 

 bond is not essential for activity, and it 

 affects the transport of other growth fac- 

 tors, although it does not seem to combine 

 with them. The story is quite different for 

 the bound auxin. This is inactivated by 

 light, its effect depends upon the pH of 

 the cell, the double bond is essential for 

 its activity, and it is this form which re- 

 acts with other growth factors to produce 

 growth. 



If we list now the various processes in 

 which auxin takes part, the differentiation 

 between the effects of diffusible and bound 

 auxin will prove to be significant. 



The first known function of auxin was its 

 effect on cell elongation and all processes 

 depending upon cell elongation, such as 

 tropistic responses. A close analysis of the 

 process of cell elongation has shown that 

 the effect of auxin on it is dual : a prepara- 

 tory reaction precedes the growth reaction 

 proper (Went 1939b). This preparatory 

 reaction, just as the phototropic induction, 

 can be identified with the effect of the dif- 

 fusible auxin, and depends on the move- 

 ment of other growth factors along an 

 auxin gradient. 



The second auxin effect to be discovered 

 was its role in bud inhibition (Thimann 

 and Skoog 1933). This effect is wholly due 



