TREES 391 



The sap-wood is still living and growing, whereas in the 

 heart- wood growth has ceased. Through the openings of 

 the cells becoming choked so that the sap can no longer flow 

 through them, the heart- wood is formed. It serves merely 

 as a framework to help support the tree. When the tree 

 is cut down, the sap-wood rots more quickly than the heart- 

 wood, because it takes up water more readily and because 

 it contains plant food which quickly decays. Some trees 

 have no heart-wood while in many others the difference in 

 color between the sap-wood and the heart-wood is very 

 slight. 



437. Sap. Sap is formed, mainly in the early spring, 

 from water rising from the roots through the sap-wood: In 

 the leaves this water is converted into true sap, which con- 

 tains sugar and soluble gums. The sap descends through the 

 bark and feeds the tissues in process of formation between 

 the bark and the sap-wood. The term "sap" sometimes is 

 used wrongly to mean the moisture in wood, and at other 

 times to mean the sap-wood itself. 



438. Structure and Growth. A tree grows from a seed 

 to a simple stem which puts forth branches and foliage. Its 

 food consists of carbon dioxide obtained from the air, water 

 and mineral matter from the ground. The leaves have 

 breathing passages through which they take in carbon di- 

 oxide which under the action of the sun and green coloring 

 matter (chlorophyl) breaks up into carbon and oxygen. The 

 carbon is retained and oxygen given off. 



The wood of a tree is composed of innumerable cells or 

 tubes each of which is long and hollow. Every one of these 

 cells has some special function in the life of the tree. Sorne 



