442 BOARD OF AGRICULTURE. [Pub. Doc. 



in most trees during the spring months, when the buds swell 

 and the leaves grow. 



The sap, having been elaborated by the leaves, is retiu'ned 

 from them and passes toward the roots through the soft cells 

 of the inner bark, forming in its progress earthward the new 

 wood and bark. There is a tissue called the cambium lying 

 upon the outside of the wood, but beneath the bark proper. 

 From the inner surface of this tissue the new wood seems to 

 grow, while on its outer surface the bark is formed. As the 

 new wood is deposited outside the old wood and just beneath 

 the cambium, vigorous trees thus add each year a new annual 

 cylinder or ring of wood to trunk and limb. This we call 

 the annual wood ring. Om' observations show that, so far 

 as the trunk and larger limbs are concerned, this growth 

 occurs mainly in July and August. Some trees make a vig- 

 orous wood growth in both these months, some begin this 

 growth in June, while a few continue it well into September. 

 The growth of wood seems to be greatest nearest the leaves, 

 where it is manifested in elongation of the shoots which begin 

 growth earlier in the season than the trunk or larger limbs. 

 This wood growth is likely to lie more rapid with young 

 trees than with older ones. 



How THE Tree Wounds tieae. 



Look at the end of a stick of firewood, and you will see 

 the lines (generally irregularly circular, and always concen- 

 tric) which mark the annual wood rings. Note that it is 

 this annual growth that alone must be depended u})()ii to 

 cover with new^ wood and bark all wounds u})on the trunk or 

 larger limbs made by the amputation of branches. 



If a wound is made in the outer bark, and only a portion of 

 this bark is removed, it is replaced by the formation of new 

 cells from the inner bark ; but any wound which exposes or 

 penetrates the wood is healed by growth from the cambium, 

 for the wood itself has no power to -produce more wood. 

 Its cells soon become lifeless, and its surface when exposed 

 to the air is subject to the influences of decay. • 



If we cut off a limb, then, the wood cannot heal the 

 wound ; in fact, a wound so made can never really heal, in 



