STRUCTURE AND APPEARANCE. 13 



BARK AND PITH. 



The bark of a pine stem is thickest and roughest near the base, 

 decreases rapidly in thickness from 1£ inches at the stump to one-tenth 

 inch near the top of the tree, and forms in general about 10 to 15 per 

 cent of the entire trunk. 



The pith is quite thick, usually one-eighth to one-fifth inch in Norway 

 pine and in the southern species, though much less so in white pine, and 

 is very thin, one-fifteenth to one twenty-fifth inch in cypress, cedar, 

 and larch. 



In woods with a thick pith, this latter is finest at the stump, grows 

 rapidly thicker upward, and becomes thinner again in the crown and 

 limbs, the first 1 to 5 rings adjoining it behaving similarly. 



SAP AND HEART WOOD. 



A zone of wood next to the bark, 1 to 3 or more inches wide, and 

 containing 30 to 50 or more annual rings, is of lighter color; this is the 

 sapwood, the inner, darker part of the log being the heartwood. In the 

 former many cells are active and store up starch and otherwise assist in 

 the life processes of the tree, although only the last or outer layer of cells 

 the cambium, forms the growing part and the true life of the tree. In 

 the heartwood all cells are lifeless cases, and serve only the mechan- 

 ical function of keeping the tree from breaking under its own great 

 weight, or from being laid low by the winds. 



The darker color of the heartwood is due to infiltration of chemical 

 substances into the cell walls, but the cavities of the cells in pine are 

 not filled up, as is sometimes believed, nor do their walls grow thicker, 

 nor is their wall any more lignified than in the sapwood. Sapwood 

 varies in width and in the number of rings which it contains, even in 

 different parts of the same tree ; the same year's growth which is sap- 

 wood in one part of a disk may be heartwood in another. Sapwood is 

 widest in the main part ot the stem and varies often within considerable 

 limits, and without apparent regularity. Generally it becomes narrower 

 toward the top and in the limbs, its width varying with the diameter, and 

 being least, in a given disk, on the side which has the shortest radius. 

 Sapwood of old and stunted pines is composed of more rings than that 

 of young and thrifty specimens. Thus in a pine 250 years old, a layer 

 of wood or annual ring does not change from sapwood to heartwood 

 until seventy or eighty years after it is formed, while in a tree 100 years 

 old, or less, it remains sapwood only from thirty to sixty years. The 

 width of the sapwood varies considerably for different kinds of pines; 

 it is small for longleaf and white pine, and great for loblolly and Nor- 

 way pines. Occupying the peripheral part of the trunk the proportion 

 which it forms of the entire mass of the stem is always great. Thus 

 even in old trees of longleaf pine the sapwood forms about 40 per 

 cent of the merchantable log, while in the loblolly and in all young 

 trees the bulk of the wood is sapwood. 



