92 MISSOURI BOTANICAL GARDEN BULLETIN 
entire surface of the tree. It is the cambium also that con- 
veys all the digested sap through the tree. Next in import- 
ance is the sapwood which carries the crude sap up to the 
leaves, and during winter, together with the roots, acts as a 
storage house for the dormant food. The heartwood func- 
tions as the supporting tissue, although it may help convey 
some crude sap. The bark is the protective covering for the 
cambium layer and thus indirectly plays a very important 
role in the life of the tree. 
_ If a broken limb is cut off close to the trunk, the result- 
ing wound is oval, and plate 10, fig. 2, shows how the heal- 
ing will take place. As the exposed cambium offers less 
———JAPWOOD 
ek 2a, 
———-HEARTWOOD 
Fig. A. Cross-section of tree trunk. 
resistance to the flow of digested sap, it is accelerated, fore- 
ing rapid growth of cells which gradually cover the wounded 
surface by rolling over it. It is generally found that the 
cut heals most rapidly along the sides, less so at the top, 
and least of all at the base. This is explained by the fact 
that sap tends to flow in fairly straight lines, hence the 
greatest flow and growth are past the sides. For this rea- 
son, the narrow wound will heal more rapidly than one 
square or oval running perpendicular to the trunk. 
Proper treatment of wounds implies prevention of en- 
trance of fungi and insects and facilitation of healing. In 
order to prevent the entrance of enemies some protective 
dressing must be applied. The dressings are divided into 
two classes: those that disinfect or preserve the wood, kill- 
ing the spores of the fungi and eggs of insects, and those 
