SPRING AND SUMMER WOOD. 



137 



allowing a specific gravity of 0.40 for springwood and 0,90 for summcrwood, the values for the 

 entire disks as actually observed being given below : 



Summerwood per cent and specific (jraritij in various partK of a tree of Longhaf Pine. 



o Six rings next to pitb. liTwo rings. cOnering. 



The ohserved 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 in the section at the stump. This is fully iiccoanted 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. 



100 



Decades of Rings from periphery 



Fio. 14.— Variation of specific gravity with summerwooii per cent and age of section in Longleaf Pine, the .solid lines referring to a section 

 3 feet from the ground, tlie dotted lines to ouo 14 feet from the ground. (Specific gravity as actually observed nn pieces of 1 inch radial 

 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 beconiv^s conspicuous in the lighter color of the wood of such portions. (See 

 diagram, fig. 15.) 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 summcrwood 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. 1.").) In some of the rapidly grown Loblolly and Spruce Pine the summerwood forms but 

 a small part of the first ten to twenty years' growth, and in all cases the first few rings about the 

 pith have but little summerwood. In general, the summerwood per cent varies in the several 

 species as well as in the individual with the weight of the wood, which is least in the Spruce Pine, 

 greatest in Cuban and Longleaf Pine, and stands between these in Loblolly and Shortleaf. It 

 furnishes a very useful criterion to distinguish between these groups and especially to select 

 strong timber. 



