SOLUTE DISTRIBUTION AND BEHAVIOR 251 



concerned in the growth of Avena coleoptiles, that this is 

 widespread in nature and may be concerned in the growth 

 of many cells other than those of the coleoptile test material 

 used. But it seems to me that the evidence is far from 

 conclusive that there is a specific hormone for growth of a 

 shoot apex, another for cambium, another for roots, and 

 others acting as inhibitors. By some it is claimed that 

 special hormones move chiefly through the xylem, while 

 many claim others move exclusively through the phloem. 

 It is true that lack of conclusive evidence for such hormones 

 does not prove their absence, but it seems there is danger 

 of depending too much upon special hormones to account 

 for phenomena that are not otherwise easily explained. 



SUMMARY 



40. It has long been recognized that ringing is likely to alter the compo- 

 sition and amount of growth of the tissues both above and below the ring. 

 If many leaves are present above, carbohydrates accumulate in this part 

 and there is an increased diameter growth of stems and increased growth 

 and sugar content of fruits. A decreased sugar content and cessation of 

 diameter growth in stems below rings has often been explained as due to a 

 diminished carbohydrate supply, as has also the cessation of root growth 

 as well as that of subterranean storage organs. Though diminished shoot 

 elongation above rings and increased shoot development below rings may be 

 due to altered distribution of carbohydrates and mineral elements, espe- 

 cially nitrogen, this behavior is usually explained on other grounds. 



41. Attempts to explain formation of roots above rings and the develop- 

 ment of dormant or adventitious buds below rings seem more commonly 

 to be based on an assumed change in the distribution of special hormones, or 

 inhibitors, or an interruption in the transmission of some influence. Evi- 

 dence from several sources indicates that an increased supply of carbo- 

 hydrate relative to that of nitrogen tends to reduce shoot elongation and 

 favors root formation and growth, whereas an increased supply of nitrogen, 

 especially an increase relative to carbohydrate, tends to favor shoot elonga- 

 tion or formation from dormant or adventitious buds. Effects of ringing 

 on these regeneration and growth phenomena may be explained on the same 

 basis, although it is also possible that the regeneration phenomena are con- 

 trolled by hormones or stimuli that are transported or transmitted through 

 the phloem. Conclusive evidence on these points seems lacking. 



42. No satisfactory explanation has been given to account for the receipt 

 of solutes, such as sugars and nitrogen, by dominant tissues when in compe- 

 tition with suppressed tissues, especially when the latter may have a lower 

 concentration of the particular material moving and may also be nearer 

 the supply. From the standpoint of diffusion gradients alone one would 

 expect the weaker fruit or bud, or other growing part, to receive the material 



