THE ASCENT OF SAP 57 



growth in length, which takes place only when the joints 

 are very young, occurs in several joints and not at the tip 

 alone, as in roots. The leaves and branches spring from 

 the nodes. The internal structure (Fig. 12) corresponds 

 with this external arrangement, there being growing, con- 

 ducting, and mechanically strengthening tissues in the 

 stem which persist from year to year. These are united 

 into a cylinder surrounding the central pith and encircled 

 by the protective tissues, epidermis, cork, etc., collectively 

 termed the outer bark or cortex. The greater part of 



Fig. 12. — Cross-section of a Tulip Tree Branch, four years old, 

 showing the wood to consist of both conducting tissues (ducts, etc.) 

 and mechanically supporting tissues (fibers, etc.) surrounded by 

 the bark. 



the strengthening and vascular tissues are wood, which 

 is composed of fibers and of tubes through which water 

 and dissolved raw materials move upwards from roots 

 to leaves. 



The Ascent of Sap. — The name sap is given to the 

 water and raw materials carried up the stem through 

 the wood. The mechanics of the movement are not yet 

 completely understood, but it is evident that if water 

 is lost by evaporation from the leaves, and is combined 



