RUBBER (HEVEA) DISEASES 59 
If a wound exposes the true cambium one or more of several sets 
of tissues begin to repair the damage. Tyloses, or filling cells, are 
developed in the outer exposed vessels of the wood, which is the 
only reproductive ability the woody cells possess. 
The cambium, if uninjured by drying through exposure, by the 
transverse division of its protoplasmic cells soon forms an investing 
tissue, which under the influence of the sun becomes chlorophyl- 
laceous. The epidermis then develops, and the work of protecting 
the living tissues beneath is carried on by the cork cambium above 
described. If the wound is formed during the dry season the 
cambium may be destroyed with the exception of the parenchy- 
mous cells of the medullary rays. These cells of the wood, which 
retain their ability to divide, now take up the work of protecting 
the exposed tissues. Small areas of healing tissue appear over the 
surface of the wound, which later coalesce and cover the surface 
completely. Whether the new investing layer is formed by the 
cambium or by the cambium of the medullary rays, it is at first 
of a homogeneous structure. With the development of the periderm 
and the cutting off of an outer protective layer by the cork cambium 
separating the outer or cortical layer from the bast region, which in 
turn is separated from the wood by the meristematic cambial zone, 
the usual differentiation of tissues is restored. The cells of the 
new wood formed on the wound, however, are of an abnormal struc- 
ture, and from this fact the location of old wounds may be recog- 
nized years after all external signs have disappeared. This is espe- 
cially shown in trees which have died from excessive tapping and 
have split up in falling. 
If the cambium is completely destroyed before a protective layer 
can be developed over the wound or if the wood is exposed by nat- 
ural or artificial pruning of branches or tapping through to the 
wood, the only healing process that is possible is the formation of 
a callus from the edge of the wound. This is brought about by the 
rapid development of the cambium at the edge of the wound. The 
callus advances from all directions and finally coalesces in the center. 
The exposed wood of the wound which has become discolored is 
merely inclosed by the callus. If it is infected with a wood- destroy- 
ing fungus it may result in serious damage to the tree. Certain 
natural processes may tend to inhibit infection. The wood cells 
may become filled with tyloses, viz, ingrowths of the cells, which 
may effectually plug the lumina against the invasion of water. 
Gums may form over the surface of the wood and act in the same 
way as tyloses. The oxidation of cell substances resulting in dis- 
coloration of the wood is often deceptive and may not indicate decay 
by a wood-destroying fungus. Whatever natural processes may 
take place in Hevea they are not sufficient to protect open wounds 
from infection, and artificial treatment must be employed. This 
will be discussed later. 
KINDS OF WOUNDS 
NATURAL WOUNDS 
In a hard, compact soil the lateral roots of Hevea sometimes cross 
one another near their point of origin, become fused below, and form 
bowllike or triangular-shaped cavities. These cavities are eon- 
