70 



Structure of Wood. 



of leaves from worms that appear at that interval, and that- have 

 thus left their record when every other proof of their presence . has, 

 perished. We have seen sections of trees in the museums of Schools 

 of Forestry, in which these proofs were recorded througli a century 

 or more of time, and the years could be definitely fixed by counting 

 inward from the year when the tree was felled. 



251. As a rule, the most favorable seasons for the growth of wood 

 are those that are warm, cloudy, and humid in the early part of 

 summer, followed by very warm and dry weather. The maturing 

 process in the newly-formed wood appears to be due to the excess 

 of evaporation from the leaves, as compared with the absorption 

 from the roots, resulting in a hardening of the tissues, and an im- 

 provement in the quality of the new wood. Where the season 

 continues wet, this hardening does not take place, and when fol- 

 lowed by a severely cold winter, the growth may be checked for 

 the season following, or the trees may be killed altogether, or at 

 least the branches of younger growth. 



In fact, so many causes aifect the amount of growth in difierent 

 years that there may be as much diiierence as is shown in the fol- 

 lowing engravings : 





'j> 





56. Differences in the Amount of Annual Growth of Wood. 



Upon irrigated land, in Colorado, the cotton wood has been known, 

 to grow to a diameter of fourteen inches in eight years. If grown 

 in rich, humid soil, but in a confined location, the growth may be 

 rapid, but the texture light and spongy, and the wood of poor 

 quality, although of ample volume. Even the teak, one of the 



