312 PATTERNS AND PROBLEMS OF DEVELOPMENT 



by arrangement of leaf primordia (phyllotaxis) is species-characteristic 

 under natural conditions — alternate, opposite, whorled, etc. — but often 

 alterable experimentally.^ This pattern suggests, first, that a dominance 

 of the extreme apical region of the main stem tip is effective over a very 

 short distance in preventing new buds from originating; and that beyond 

 that distance, at most a few millimeters, some degree of physiological 

 isolation occurs, permitting initiation of lateral bud development and 

 often continued development of a leaf; and second, that each actively 

 developing bud or its leaf also dominates a certain area about itself and 

 so prevents other buds from arising within this area, that is, each center 

 of activity gives rise to a gradient system. That of the main tip has be- 

 come axiate; that of the lateral bud is at first more or less radial, or 

 symmetrical with respect to the axiate pattern of the stem tip, and be- 

 comes axiate by differential growth, like other buds. Nothing is known 

 concerning the factors effective in determining and maintaining these 

 orderly patterns within the stem tip. That they depend on auxin seems 

 at present rather improbable. Doubtless, electric-potential gradients are 

 associated with each growing primordium, but whether they are con- 

 cerned in determining the spatial and chronological order remains to be 

 determined. 



The activity of the root system of higher plants inhibits more or less 

 completely development of roots elsewhere, although physiological isola- 

 tion may be brought about in many plants by subjecting other regions to 

 conditions favorable to root formation. Experiment indicates that this 

 general dominance of a root system depends largely on transport of water 

 and salts from the roots. Depletion of these in other parts of the plant 

 favors root formation, chiefly at the more basal levels of the stem, unless 

 experimentally or otherwise inhibited there, presumably because deple- 

 tion is most rapid there. This also is a secondary dominance depending 

 on a pattern already differentiated. There is, however, some evidence of a 

 local dominance with limited range at the more apical levels of a root axis. 

 In the presence of an actively growing root tip a new root primordium 

 appears only at a certain distance from the tip, and removal or inactivation 

 of the tip destroys the dominance. Here, as in the stem tip, there appears 

 to be some degree of physiological isolation with increasing distance along 

 the root axis from the tip. 



Evidences of dominance and physiological isolation also appear in the 



5 See, e.g., Mary and R. Snow, 1931, 1934; R- Snow, 1929; and earlier work on phyllo- 

 taxis cited by these authors. 



