g2 General Care of Trees 
sets in, but this takes time, and meanwhile the exposed 
part is subject to inimical influences, drying out, or giving 
access to parasites. 
Of course, only so far as living tissues are touched or 
exposed is there any real injury, hence scraping or breaking 
off the dead outer bark does no direct harm and the cutting 
off of dead branches in the dead parts produces no further 
results. When live tissues have been injured, a certain area 
of the wounded and exposed live 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 rapidly; 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 finally 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 off 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 
