110 



FORESTRY INVESTIGATIONS U. S. DEPARTMENT OF AGRICULTURE. 



part in the volume of the butt than of tha top log, and thus fully explains the well-known difference 

 in the weight, strength, and value of the various parts of the tree. The following table serves to 

 illustrate this point. The numbers in each line refer to the average values for the same teu annual 

 layers through three sections of the tree at varying height. The figures in italh-x below refer to 

 specific gravity for the same layer. The values for specific gravity were calculated on the basis of 

 allowing a specific gravity of 0.40 for springwood and 0.90 for summerwood, the values for the 

 entire disks as actually observed being given below: 



Summerwood per cent and specific grarity in various parts of a tree of longleaf pine. 



a Six rings next to pith, b Two rings. c One ring. 



The observed values of specific gravity for the three sections are 0.700, 0.560, and 0.490, respectively. 



It will be noticed that the greatest difference between the calculated and the actual value of 

 specific gravity occurs iu the section at the stump. This is fully accounted for by the fact that 

 large amounts of resin, not considered in the values of summerwood per cent, always occur in 

 this portion, adding from 5 to 20 per cent to the weight of the wood. 



Fio. 18 Variation of specific gravity with summerwood per cent and ago of section in longleaf pine, the solid lines referring to a section 3 

 ftct from the ground, the dotted lines to one 14 feet from the ground. (Specific gravity as actually observed on pieces of I inch radin, 

 extent.) 



In stunted trees the summerwood forms nearly as great a per cent of the total volume for the 

 whole tree as in thrifty trees of the same age, but in the stunted growth, or extremely narrow 

 ringed portion of otherwise normal trees, the per cent of summerwood is markedly decreased, a 

 feature which becomes conspicuous in the lighter color of the wood of such portions. (See diagram, 

 fig. L'2, A.) Where, on the other hand, the rate of growth in an old tree is suddenly increased by 

 the accessibility of more light, for instance, the summerwood per cent also is disproportionately 

 increased, but this disproportion appears to be more transient, i. e., a decrease in the summerwood 

 per cent sets in sooner than for the rate of growth or the width of the rings. (See fig. 10.) In 

 some of the rapidly grown loblolly and spruce pine the summerwood forms but a small part of the 



