262 Journal of Agricultural Research vol. xx,no. 4 



Another possible connection between leaf pruning and oxygen hunger 

 of root and stem is suggested by Prof. Livingston in a personal communi- 

 cation to one of the writers. A reduction of the transpiring surface by 

 pruning should result in less absorption by the roots. If it be supposed 

 that oxygen dissolved in water absorbed from the soil is important as a 

 source of oxygen supply for the root tissues, a decrease in the amount of 

 water absorbed might result in oxygen deficiency in the root tissues. 

 This suggestion might help to explain the earlier reports of the stimulated 

 growth of lenticels on stems of dicotyledons whose transpiration has been 

 experimentally reduced. It obviously complicates any attempt to 

 explain on an oxygen-hunger basis the effects of pruning on lenticel 

 growth described in the present paper. 



Of course it does not seem likely that any part of a plant accustomed 

 to the presence of free oxygen would be likely to make much growth in 

 the entire absence of oxygen. However, the condition existing in the 

 soil in which the hypertrophies occurred certainly did not involve the 

 entire absence of oxygen. Pfeffer concludes (14, p. 115), in spite of some 

 conflicting evidence, that experiments have shown that reduction of the 

 proportion of oxygen, at least in some cases, acts as an accelerating 

 'stimulus to growth. 



It is of course true that any strong local growth is probably dependent 

 on high local sap pressure. However, it is well known that such local 

 high pressures are not necessarily dependent on excessive turgidity of the 

 plant as a whole. Unusual chemical conditions, such as might conceiv- 

 ably result from local oxygen hunger, might easily cause them. The 

 writers do not consider that oxygen hunger is established as the main 

 cause of the lenticel hypertrophy found. They can not, however, agree 

 with De Vaux in attributing the effect of increased soil moisture on 

 lenticel growth entirely to increased water supply, excluding oxygen 

 hunger as a possible factor in stimulating lenticel growth. 



Experiments in which the oxygen, carbon-djoxid, and water supplies 

 in the soil are independently controlled, as by the technic of Livingston 

 and Free (12), and perhaps also with temperature control, will be needed 

 to make a beginning on determining the relative importance of these 

 various environmental factors in causing hypertrophy of root lenticels. 

 Since conifers are rather difficult to handle in experimental work, poplar 

 would perhaps be a better subject for preliminary experimentation. 

 It seems likely, as has been suggested for hypertrophied lenticels in 

 general by Tubeuf (21) and for intumescences by Hasselbring (9), that 

 these unusual lenticel enlargements on the roots of conifers depend on a 

 complex of conditions rather than on any one simple stimulus, and that 

 with different species the conditions which call forth lenticel hypertrophy 

 may be found to differ in relative importance. 



