SPRING AND SUMMEK WOOD. 



137 



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 : 



Summeruwod per cent and epecifio gravity in various parts of a tree of Lonyleaf Pine. 



(I Six rings next to pith. !<Tworinga. cOnering. 



The observed values of specific gravity for the three sections are 0.700, 0.560, anil 0.400, respectively. 



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

 specific gravity occurs in the section at tlie stump. This is fully accounted for by the fact tliat 

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

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



Decades of Rings from peripheiy . 



Fia. 14.— Variation of specific gravity with summerwoocl per cent and age of section in Longleaf Pine, tlie solid lines referring to a sirctiou 

 3 feet from the ground, the dotted lines to one 14 feet from the ground. (Specitio gravity aa actually cbscrvod on pieces of 1 inch radial 

 extent.) 



In stunted trees the sunimerwood 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 tlie stunted growtli, or extremely narrow 

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

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

 diagram, llg. 15.) Where, on the other hand, the rate of growth in an ohl tree is suddenly 

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

 disproportionately iii(;reased, but this disproportion appears to be more transient, i. e., a decrease 

 in the sunimerwood per cent sets in sooner tlian for the rate of growth or the width of the rings. 

 (See fig. 15.) In some of the rapidly grown Loblolly and Spruce Pine the siuiunerwood 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 sunimerwood per cent varies in the several 

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

 greatest in Cuban and Longleaf Pine, and stands between these in Lobh)Ily and Shortleaf. It 

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

 strong timber. 



