THE FORMATION OF WOOD. 



19 



THE STRUCTURE OF WOOD. 



Wood is cliietiy made up of very small tubes or cells 

 of various kinds, wliicli have special uses in the life of 

 the tree. Some conduct water from the roots to the 

 crown, some store away digested food, and others merely 

 strengthen the structure of the wood and hold it to- 

 gether. The wood of cone-bearing or coniferous trees 



Fig. 19.— Slab sawed lengthwise from a Black Oak. Milford, Pa. The saw 

 passed about midway bet^Yeen the center of the tree and the hark. The line 

 between the heartwood and the sap is plainly shown. 



(like the Pines and Spruces) has but few kinds of 

 cells, while that of the broadleaf trees (such as Oaks and 

 Maples) is much less simple. (See figs. 14, 16, 20, and Pis. 

 VII-IX.) But in each case some of the cells have thick 

 walls and small openings, and others wide openings and 

 very thin walls. In climates which have regularly one 



