202 DISEASE IN PLANTS. 



indefinitely delayed, a thicker and thicker callus 

 projecting over from above. For similar reasons 

 no annual wood layers are formed below, but only 

 above the wound, and thus the branch or tree may 

 die. The latter contingency is the more likely 

 the further up the tree the ringing takes place, 

 owing to the risk of drying up which threatens the 

 exposed wood, and to the consequent interruption 

 of the transpiration current, and the likelihood that 

 lateral shoots below the wound may divert the 

 water to their own leaves. If the ringing occurs 

 low down on a stem, and the environment remains 

 damp, the upper thick callus may put out new 

 roots ; the part above the wound then behaves 

 like a cutting. If the ringing is done on a 

 young and vigorous branch of an old tree, the 

 lower lip may receive supplies from the leaves of 

 branches below the wound, or from shoots which 

 spring from adventitious buds close to it, and the 

 wound may heal over normally. Such healing 

 may be rendered more certain by keeping the 

 wounded surface moist e.g. by means of damp 

 moss, and so encouraging the formation of callus- 

 bridges from the medullary rays. 



If on ringing a tree or a branch the young wood 

 is removed as well as the cambium and cortical 

 layers, the death of the parts above the wound is 

 almost certain, owing to the stoppage of the tran- 

 spiration current : the exceptions to this rule de- 

 pend simply on the existence of other channels 

 of communication, such as internal phloems, very 

 thick sap-wood, and so forth. 



