164 



during winter, will force through the sides of the cells 

 in which is lodged the gummy fluid, which, the mo- 

 ment this happens, loses its vitality and causes a 

 decay of the surrounding parts ; or heat, when the 

 sun beats fiercely on the branches, will produce the 

 same effect — the inevitable result of which will be 

 decay. The proximate cause of " the gum" may 

 therefore be either exposure of unripe wood to the 

 sun, or the action of frost upon it. Another cause, 

 distinct from all these, may be the following : — Sup- 

 pose that neither heat nor cold are sufficient to damage 

 the unripe wood, the new sap will do it ; for in unripe 

 wood the cells and vessels are filled unnaturally with 

 crude fluid before the new sap enters them ; and the 

 moment that new sap is introduced in addition, they 

 become so distended, that a portion of their contents 

 must escape. That portion flows into the intercellu- 

 lar cavities of the bark, thence finds its way to the 

 exterior, and, having lost its vitality, immediately in- 

 duces the decay of the surrounding parts. {Gard. 

 Chron. 1844, 355.) 



Although thorough drainage is a most effectual 

 check upon the occurrence of gumming, by preventing 

 the absorption of too much moisture under ordinary 

 circumstances, yet, despite the best of drainage, this 

 efflux of sap will occur if the soil of the border is 

 allowed to become too dry, and then to be exposed to 

 a fall of heavy rain. The vessels of the branches, 



