18 



WOOD AXD FOREST. 



as spruce and Ijat^s. IikIimm], some species never form a distinct lieart- 

 "wood. ])ircli. (BetuJa nJhii ) Ijeincj an example. 



In a living tree, sap-vood and heart-wood perform primarily 

 cjuite different functions. I'he sap-wood carries tlie water from the 

 roots to the leaves, stores away starch at least in winter, and in other 

 ways assists the life of the tree. The proportional amount of sap- 

 wood vai'ies greatly, often, as in long-leaf pine, constituting 40 per 

 cent, of the stem. 



As the sap-wood gi'ows (ildci-. its cells become choked so that the 



sa]j can no longer flow 

 I ^ thru tliem. It loses its 



V jirotoplasm anr] starch 



and becomes heart- 

 wood, in which all 

 cells are dead and 

 serve only the me- 

 chanical function of 

 holding up the great 

 weight of the tree and 

 in resisting wind pres- 

 sures. This is the rea- 

 son Mdiy a tree may 

 hct'ome decayed and 

 lioUow and yet be alive 

 and liear fruit. In a 

 tree that is actually 

 dead the sap-wood rots 

 liist. 



Chemical s it b - 

 stances infiltrate into 

 the cell walls of heart- 

 wood and hence it has a darker color than the sap-wood. Persimmon 

 turns black, walnut purplish brown, sumac yellow, oak light brown, 

 tulip and poplar yidlowisli, redwood and eedar brownish red. Manv 

 •\ioods, as mahogany and oak, darken under exposure, which shows 

 that the substances producing the color are oxidizable and unstable. 

 ^Vrjo.l d\es are obtained by boiling and distilling such woods as su- 

 mach, logwood, red sanders, and fustic. Many woods also acquire 

 distinct odors, as camplior, sandalwood, cedar, cypress, pine and 

 mahogany, indicating the presence of oil. 



Fig-. S. Section of Dou^'-las Fir, Sliowinrr Aniiual 



Rings and Knois al Cemer of Trunk. Anuricair 



Musiiim of Natui-al Utslory, X. }'. 



