TREES AXD FORESTRY 21 



from root to leaves. The living green substance (chlorophyll) of the leaves 

 under the influence of sunlight breaks apart the hydrogen and oxygen, 

 elements in this water gained from the roots, and at the same time sepa- 

 rates the carbon and oxygen, element- in carbon dioxide gained from the 

 air, and recombines the three, hydrogen, oxygen and carbon, to make 

 various starch-like ] roducts. This chemical work of green plants, defying 

 man's efforts to imitate, creates food for animal life on the globe and is, in 

 fact, the only source of that food. Later, the starch is changed to complex 

 substances, largely because of the addition of nitrogen, and passes through 

 the inner bark (bast) of the trunk, upward to give food to the growing buds, 

 downward to supply the living cambium throughout its length with mate- 

 rial for a new wood layer, and to give nourishment to the living tips of the 

 roots (Fig. 1(3). 



These plant activities, mechanical and chemical, are not fully under- 

 stood. Osmosis or the passage of liquids through an organic membrane 

 explains the entrance of water carrying soluble substances into the roots; 

 and since in osmosis the movement is more rapid in the direction of the 

 stronger solution, which in this case is within the roots, the continual pas- 

 sage of water into the roots must create considerable pressure upward. This 

 root pressure accounts in some degree for the rise of water in the trunk. 

 Root pressure in birch trees will lift water in the tree trunk to a height of 

 84.7 feet. A second force acting to carry the current upward through the 

 sapwood is probably capillarity (the force which causes rise of a liquid 

 through any porous substance). A third force is the strong attraction 

 naturally existing between particles of wood and particles of water. A 

 fourth is certainly the continual and often rapid evaporation of water from 

 the tree's enormous leaf surface which lies spread out under the heat of the 

 sun (Fig. 17). 



Other structures of the trunk, complicating an understanding of the 

 appearance of lumber, are short, thin plates made of a substance like the 

 pith, arranged radially and vertically through the wood but irregularly 

 with reference to one another (Fig. 18). These plates, called pith rays, 

 show as radial lines in a cross-section of a log (see upper section, Figs. 

 19 and 20). They produce what is known as the silver grain of lumber, 

 inconspicuous short lines on the surface of tangential boards because the 

 plates are cut through their length and thickness, but often conspicuous in 

 quartered lumber where, cut through length and width, the plates are 

 sometimes exposed for two or more inches on the board's surface (see lower 

 section, Figs. 19 and 20). The pith rays in different species of trees differ 

 to such an extent thay like the variations of spring and summer wood, they 

 serve to identify different kinds of wood. In oaks the rays are long and 



