A GROWTH-INHIBITING SUBSTANCE 241 



by A in figure 1. If the apical bud is arrested in its develop- 

 ment a neighboring lateral bud promptly develops and continues 

 the growth, as shown in the upper shoot of B in figure 1. 



The sequence of events in a regenerating shoot is so well known 

 that further discussion is unnecessary. Many other trees pro- 

 duce new shoots in the same consistent manner as the pear tree. 

 The cause for the regular basipetal development of shoots on a 

 branch has not been clearly understood. The object of this dis- 

 cussion is to emphasize the inadequacy of our former ideas on 

 the subject and to call attention to a more plausible explanation 

 of the facts. 



Among the statements current (but vague) there is an idea that 

 the apical bud, or buds, secures the major portion of the sap in 

 the branch and so develops at the expense of the sub-apical buds. 

 Another statement which carries the same idea in somewhat more 

 mystical language is to the effect that " shoot-f orming substances 

 rise in the plant while root-forming substances descend." If 

 this concept be the correct one, it would seem that the basal buds 

 should be the first to develop since they would first receive the 

 "shoot-forming substance." But this is precisely what does not 

 happen, as is shown by instances cited above. 



To assume that "polarity" determines the course of events in 

 the regenerating shoot is to render one's position somewhat more 

 unassailable because of our ignorance of the nature of "polarity," 

 but does little to elucidate the question. There is no basis in 

 fact for comparing a living shoot with a magnet. 



If we cast aside the older ideas and assume that a growth- 

 inhibiting substance is produced in the apical region of the shoot 

 which perpetuates a condition of dormancy in subapical buds, 

 we shall find that we have a theory which fits the facts. 



Loeb 2 has already shown that the presence of a growth-in- 

 hibiting substance would satisfactorily explain the course of 

 events in the regeneration of Bryophyllum calycinum. 



2 Loeb, J. The chemical basis of axial polarity in regeneration. Science, 

 n.s., 46:547, 1917. 



Loeb, J. The law controlling the quantity and the rate of regeneration. 

 Proc. Nat. Acad. Sci., 4: 117-121, 1918. 



