ORNAMENTAL AND SHADE TREES 



229 



For the purpose of discussing the effects of wounds 

 upon the life of a tree its stem may be considered to 

 consist of five components, each completely surround- 

 ing its axis. Naming them from the inside outward, 

 they are heartwood, sapwood, cambium, live bark and 

 dead bark. Sapwood, cambium and live bark are always 

 present, even in the smallest twigs. The heartwood and 

 dead bark develop as the stem gets older. 



Heartwood is formed from sapwood by the deposits of 

 waste compounds which accumulate in the older cells 

 and kill them. These waste products have preservative 



AN OPENING FOR DISEASE 



Trunk of chestnut tree showing effects of chestnut hark disease which 

 entered through the spur-mark slightly above and to the right of 

 the center of the picture. 



properties which make the heartwood relatively resistant 

 to fungi and other agents of decay. Nature has evolved 

 fungi adapted to living upon heartwood. They are com- 

 paratively slow-going so that considerable time elapses 

 before the growth of the rot overtakes the growth of new 

 wood outside. When it does, the tree either dies slowly 

 from inability to supply its leaves with moisture or is so 

 weakened that it breaks to pieces. The wounds through 

 which the heart-rot fungi get into the wood are usually 

 large enough to be easily detected. If a heart rot is dis- 

 covered in time it can be gouged out and the sound wood 

 protected from further infection by a preservative coat- 



ing or by a coating plus a filling which new growth may 

 cover over and seal up inside the tree. 



The live cells of the sapwood contain more moisture" 

 and less of the preservative compounds found in the 

 heartwood. The fungi which thrive upon them are more 

 rapid growers. New wood is attacked readily. A small 

 wound exposes the sapwood to infection, so that discov- 

 , ery often comes too late. The sapwood, as its name may 

 suggest, is the part of the stem that carries the sap or 

 water supply of the tree from the roots up to the leaves. 

 When it is attacked by a fungus, the water supply of the 

 branches which are dependent upon the sap currents 

 that formerly passed through the diseased tissue is cut 

 off. Then their leaves wither on the twigs and the 

 branches die. If a sap rot encircles the trunk the whole 

 tree dies. 



Photo by Alfred Macdonald 



DEVELOPMENT OF WOUND 



Wound surrounded by a fungus (Schizophyllum commune) caused 

 by spur injury. 



The cambium layer between the wood and the live 

 bark is by far the most sensitive part of the tree. Its 

 individual cells are as delicate as those of the tiny shoots 

 inside the buds before nature has prepared them for 

 exposure to the outside air. One has only to peel off a 

 bit of bark in the spring and see how quickly both the 

 outside of the wood and the inside of the bark change 



