gt General Care of Trees 



sets in. but this takes time, and meanwhile the exposed 

 jjart is subject to inimical intluences, drying out, or giving 

 access to parasites. 



Of course, only so far as lixing tissues are touched or 

 exposed is there any real injury, hence scraping or breaking 

 oflf the dead outer bark does no direct harm and the cutting 

 off of dead branches in the dead parts jjroduces no further 

 results. V\ hen live tissues have been injured, a certain area 

 of the wounded and exposed li\-e tissues dries out and dies 

 before the healing process has begun, and it is only by the 

 growth of neighboring live tissue that a covering can be 

 gradually established. In other words, the cut surface or 

 wound consisting of dead tissues cannot heal over as a flesh 

 wound does, but the narrow ring of cambium cells at the 

 margin of the wound, being relieved from the pressure of 

 the bark, subdivides and grows rai)idly; and an excessive 

 growth of wood cells and bark cells takes place, forming 

 the so-called callus or wound wood, and this protrudes 

 from the old bark over the wound, like a thick mass boiling 

 over from the rim of a vessel. Year after year it increases 

 in mass, and tinaUy covers up the surface mechanically, 

 leaving only a scar where the margins meet; and in time 

 even this may vanish. The wound, then, is not really healed; 

 merely a mechanical cover or cap is established, not organ- 

 ically connected with the surface of the wound, and, if 

 properly cut, it comes oft' like the cover of a box. 



In conifers, especially in young trees, usually an exuda- 

 tion of resin first covers the wound, preventing loss of water 

 and entrance of fungi, but the callus itself forms more slowly, 

 and in older trees both processes of resin and callus forma- 

 tion may become feeble or fail altogether, so that careful 

 attention to the wounds is necessary. 



The growth of the callus, like all other growth, takes 



