238 TRANSLOCATION IN PLANTS 



the appearance of having been ringed, but closer examina- 

 tion showed a layer of phloem was left intact and the 

 regeneration response confirmed this. Loeb refers to but 

 two layers which he calls cortex and wood. 



The composition of a stem also influences the type of 

 regeneration. A high carbohydrate content increases the 

 tendency to regenerate roots (Curtis, 1918) and one with 

 a low carbohydrate content to regenerate shoots (see also 

 Kraus and Kraybill, 1918; Reid, 1924; and others). That 

 extreme changes in composition alone will tend to induce 

 regeneration does not seem to follow, for McCallum (1905) 

 could induce no such regeneration in his attempts to cause 

 such changes in composition. The methods used by 

 McCallum, however, did not insure as extreme changes, 

 perhaps, as were brought about by decapitation, ringing, or 

 removal of roots. That adventitious root formation can 

 be induced by high carbohydrate content without removal 

 of active roots can be demonstrated by growing the 

 plant with deficient nitrogen; and the development of an 

 increased number of buds can be brought about by increas- 

 ing the nitrogen and water supply and decreasing the 

 carbohydrate. Whether the development of true adven- 

 titious buds can be so caused may be doubtful. 



Evidence pointing to composition as more important 

 than hormone or transmissive effects in causing regenera- 

 tion has been obtained from experiments on double-ringing. 

 I have found that defoliated parts between double rings 

 have a strong tendency toward shoot development. Buds 

 in these isolated regions developed shoots even more 

 readily than buds below the lower ring, at which point any 

 increased influence from the roots below or any increased 

 supply of substances from the roots might be expected to 

 be more effective. The carbohydrate supply in these 

 isolated defoliated regions was exceptionally low, lower 

 than that below the lower ring. These parts developed 

 shoots in spite of their difficulty in getting water in compe- 

 tition with tissue below or above with high osmotic con- 

 centrations. It was also noticeable that apical dominance 



