DOMINANCE AND PHYSIOLOGICAL ISOLATION 309 



istic of many conifers to the radial pattern of the primary axis. It is obvi- 

 ous that the growth forms of axiate plants must depend, to a great extent, 

 on the degree or persistence of dominance of the primary stem tip and of 

 other tips. If dominance is highly effective and persistent, the axis re- 

 mains unbranched or may bear short lateral branches, usually different 

 in pattern of secondary branching from the main axis. With less effective 

 or decreasing dominance the plant may become a highly branched spread- 

 ing form with several or many equivalent or nearly equivalent axes. Many 

 trees and other plants show the first form in earlier stages; later, some 

 degree of physiological isolation of branches evidently occurs, and some 

 or all of them become more or less equivalent. Growing leaves, as well 

 as stem tips, inhibit bud development. The root tip is apparently, also, 

 to some extent a dominant region inhibiting formation of a new root tip 

 within a certain distance or determining its development as a lateral root. 



Various hypotheses concerning the mechanism of this dominance of 

 stem tips and other active regions have been advanced. It has been held 

 by some that there is in plants something analogous to nervous control; 

 others have maintained that dominance is a matter of nutrition, the domi- 

 nant region taking so large a part of the available supply that other parts 

 are unable to obtain enough for their development; another view is that 

 the dominant region produces substance inhibiting development of buds 

 in other regions. Recently, however, discovery of the substances now 

 known as "auxins" and the rapid development of analytic investigation 

 concerning their production, distribution, and functions have thrown 

 light on some aspects of the problem of dominance in plants.' 



A biological method for comparative estimation of auxin amounts in 

 terms of their effects on cell elongation has been developed with the 

 coleoptile of the Avena (oat) seedhng, which also played an important 

 part in the discovery of auxin. The coleoptile is a sheath surrounding the 

 young shoot. After an early stage its growth is by cell elongation without 

 division, and the region of maximum elongation is some distance below 

 the tip. Auxin is produced by the tip, is transported basipetally, and is 

 concerned in the elongation. When coleoptile tips, stem tips, or other 

 parts are placed on agar blocks, auxin, if present, diffuses into the agar; 

 and if the block is placed on one side of a decapitated coleoptile, auxin is 



2 This field of investigation is developing so rapidly that any general survey is practically 

 out of date by the time of publication. The book P/iyk>kormones by Went and Thimann (1937) 

 is the chief authority for the few points mentioned here. The book includes an extensive 

 bibliography. See also Boysen-Jensen, 1936; Thimann and Bonner, 1938. 



