232 DISEASES OF TREES 



cambium, and complete coalescence results.* Thick bark may 

 retard this coalescence for many decades, as, for instance, in the 

 case of the pine (Fig. 138). 



When one considers that the pressure exerted by the bark 

 in consequence of the peripheral enlargement of the stem acts 

 for the most part horizontally, like the pressure of a barrel- 

 hoop upon the staves, it is evident that the formation of callus 

 must proceed much more vigorously in the case of a longi- 

 tudinal incision in the cortex than when the incision is a 

 transverse one. This sufficiently explains why callus is most 

 vigorously produced along the lateral margins of branch-wounds. 



Should an injury produce little or no reduction in the bark- 

 pressure, as in the case of bruises caused, for instance, by one 

 .tree knocking against another during felling, the formation of 

 callus is either absolutely prevented or proceeds with great 

 slowness. The dead cortex, which, without becoming detached 

 from the uninjured portion, retains its position on the bruised 

 and lifeless spot, does not admit of a reduction of pressure along 

 the edge of the wound, and consequently no formation of callus 

 takes place. 



Finally, it may be mentioned that the shape of the wound 

 may be recognized on the surface of the tree for many decades, 

 the boundary between the old and new cortex being usually 

 visible for a long time. 



It need hardly be mentioned that coalescence of the wood 

 exposed by a wound, with the wood of the callus that is 

 subsequently formed over it, is impossible, and especially so as 

 the external wood-layers of the wound have previously died, 

 dried up, and become decomposed to a greater or less depth. 



This leads us to the consideration of the changes that occur 

 in wood which is exposed by a wound. In the case of those 

 conifers which are furnished with resin-ducts, the surface of the 

 wound is more or less perfectly protected, owing to the outer 

 layers of wood becoming impregnated with resin. 



The resin-ducts, into which resin mixed with turpentine 

 is shed from the surrounding parenchymatous cells which 



* [I have proposed to call all such cases of covering over of wounded sur- 

 faces by the agency of a callus, "occlusion": the wound is said to be 

 " occluded.'' ED.] 



