EXOGENOUS TREES. 



{Dicotyledons .^ 



Exogenous trees are those the trunks of which are built 

 up by rings 'or layers, each deposited consecutively upon the 

 outside of the others. A section exhibits first a central point 

 or canal known as a pith-cavity, next and consecutively the 

 annual layers, and finally the bark. The 

 woods of this series are familiar to all. The 

 oaks, pines, and practically all of the mer- 

 chantable lumbers are among them. The 

 forests are widely distributed, and the species 

 are so numerous as to present an almost 

 infinite range of possibilities. 



The young wood of exogenous trees is ^f'^f^^^IZl^'"'' 



■' => ° ELDER, SHOWING PlTH- 



porous. It permits the passage of sap and cavity at Centre. 

 is known as sapwood (Alburnum). As a layer is enclosed by 

 others and retreats from the surface of the tree, it becomes 

 denser, its canals are filled with gums or tannin, color changes, 

 and the result is heartwood (Duramen). This change goes 

 forward rapidly in some trees, such as locusts, so that their 

 sections appear to be almost wholly heartwood ; other species 

 require longer time, and sapwood then predominates. 



Heartwood gives stability to the tree, but is not needed in 

 its physiological processes. It is tougher, heavier, stronger, 

 and more valued in construction. Sapwood is vitally essen- 

 tial to the life of the tree, but is lighter, weaker, less durable, 

 and less valued in construction. Sapwood is pliable, and the 

 sapwoods of several trees are valued for this reason. 



Wood-making varies- as it takes place in the springtime 

 and in the summer. Consequent differences in the densities 



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