622 GROWTH CORRELATIONS 



auxin were applied to broad bean {Vicia faba) plants in place of the terminal 

 buds which had been removed, the blocks being replaced with fresh ones from 

 time to time in order to maintain the supply of auxin, inhibition of lateral 

 bud development occurred just as if the terminal bud were intact (Slcoog 

 and Thimann, 1934). The lateral buds on check plants, to which only plain 

 agar blocks were applied, developed rapidly. These investigators postulate 

 that an excess of auxin is produced by the terminal bud, and that some of 

 this auxin passes downwards into the lateral buds, exerting a direct retarding 

 effect on the development of the latter. This explanation implies that the 

 terminal buds will grow at auxin concentrations which are inhibitory to lateral 

 buds. 



A somewhat different interpretation of the mechanism of apical dominance 

 has been advanced by Snow (1937) and others. The auxin moving down- 

 ward from the terminal bud is supposed to favor growth of the stem through 

 which it passes, and inhibition of the axillary buds is supposed to result from 

 some secondary influence which arises as a result of the growth process. 



Still another interpretation of this phenomenon is offered by Went (1938). 

 He postulates that a hormone caulocaline (Chap. XXXH), necessary for 

 stem elongation, moves upwards in stems and accumulates near the regions 

 of auxin production. As long as the apical bud remains intact and produces 

 auxin, practically all caulocaline is considered to move to the tissues in its 

 vicinity and the lateral buds fail to develop. Inhibition of lateral bud develop- 

 ment will continue if auxin is supplied artificially at the cut surface as de- 

 scribed above. But if the apical bud is removed, or for some reason ceases 

 to produce auxin, it is believed that the caulocaline is diverted to the lateral 

 buds of lower auxin content and they begin to grow. 



A somewhat similar example of correlative growth can be observed in 

 many gymnosperms. The usual growth habit of most coniferous species, ex- 

 hibited most rigorously by the spruces and firs, is for the main stem to grow 

 vertically, while all of the lateral branches assume an obliquely upright or al- 

 most horizontal position. If, however, the apex of the terminal branch is 

 destroyed or seriously injured in any way one or more (often all) of the 

 lateral branches originating at the node next below the tip gradually turn 

 up as a result of greater growth on their lower than on their upper sides. 

 Eventually these branches assume an approximately vertical position, often 

 imparting a candelabrum-shaped top to the tree. Subsequent vertical growth 

 of the tree is accomplished by means of these reoriented branches. Main- 

 tenance of the more or less horizontal growth of the lateral branches in un- 

 injured trees is obviously a result of some kind of a control exerted by the 

 apical growing region. While no concrete evidence exists in favor of such 



