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. FIG. 2. SECTION Box- 



These trees grow in two ways, " length ^S'^SSS' 

 growth " being distinct from " thickness (ENLARGED.) 

 growth." Trees thicken, as stated, by reason of new material 

 deposited upon the cross-section and lengthen by a sort of 

 telescopic extension 'at buds or extremities. These last, once 

 extended, later thicken, so as in turn to become trunks or 

 boughs. A nail driven at a certain distance from the ground 

 is never higher. 



Central pith cavities, as they exist in exogenous trees are 

 most pronounced in younger growth, saplings and small 

 branches. They are later absorbed or compressed and are 

 seldom directly evident in the sections of mature trunks. 



The young wood of exogenous trees is porous. It permits 

 the passage of sap and is known as sapwood (Alburnum). As 

 a layer is enclosed by others and retreats from the surface of 



