394 THIMANN 



ture has subsequently led not only to its synthesis in quantity, but to the 

 synthesis of many compounds of related chemical structure, which often 

 have turned out to possess valuable growth-promoting activity (see below). 



Even before these pure auxins became available, it was discovered that the 

 action of auxins in plants is by no means limited to the simple promotion of 

 growth, whether straight or one-sided. On the contrary, a variety of functions 

 are either controlled or modified by auxin, and it is this fact which has done 

 so much to open our understanding of how the different parts of a plant react 

 on one another to produce that integration which is characteristic of the 

 plant as a whole. 



In a dicotyledonous seedling the main center of auxin production appears 

 to be the terminal bud, especially the young leaves in it. From here the 

 auxin moves down the shoot by an "active" transport process which is neither 

 diffusion nor mass flow but depends on metabolism. The cells immediately 

 below the bud are caused to elongate. If the auxin is in a certain range of 

 concentration, it may cause some of them to divide. However, the action of 

 auxin in causing cell division seems to be exerted mainly on the cambium 

 and the pericycle or phloem parenchyma. Cambium is certainly stimulated to 

 divide by auxin in low (i.e., physiological) concentrations, and it is to this 

 that the downward-spreading wave of cambial activation in trees in the 

 spring is due. This was elegantly shown by Soding, in Germany, by applying 

 synthetic auxin to young poplar and willow twigs and then counting the 

 number of layers of wood cells laid down. When the auxin is in slight excess, 

 the wood cells formed under its stimulus are of the redwood, or "rot-holz," 

 type, with rounded cross section and thick walls; this is characteristic of the 

 underside of lateral branches or leaning tree trunks. Its formation in these 

 organs is thus connected with the geotropic accumulation of excess auxin 

 on the lower side, described above. Xylem can also be formed from parenchyma 

 cells under the influence of auxin; when a vascular bundle of a young stem 

 is broken or dissected out, new xylem is formed on the inside of the bundle 

 or between the bundles; for this, auxin is essential and its concentration 

 determines the number of layers of xylem elements produced. 



Continuing down the main axis of the shoot, the auxin reaches the lateral 

 buds, each in the axil of a leaf. Typically, these buds remain almost inactive 

 as long as the terminal bud is actively growing, a phenomenon known as 

 "apical dominance." Paradoxically enough, this inactivity of the buds is due 

 to the auxin which reaches them. For although the internode below the ter- 

 minal bud is caused to elongate by auxin, the rudimentary internode in the 

 axillary bud is inhibited from elongation by the same auxin. The reason for 

 this inhibiting action has been under discussion ever since the facts were 

 brought to light by Thimann and Skoog in 1934. Many theories have been 

 proposed, of which space does not allow a discussion here. The phenomenon 



