STEMS AND BRANCHES AND THEIR USES 211 



The wood of angiosperms contains similar conducting cells, 

 and with very few exceptions it contains vessels in addition. 

 The wood of gymnosperms contains no vessels. A vessel 

 is formed by a row of long cells whose walls are thickened and 

 marked in the same ways as are those of the conducting cells 

 already described. The cross walls between the cells disap- 

 pear, and so the row of cells becomes a continuous tube. A 



FIG. 131. A lengthwise section (diagrammatic) of a woody stem; 

 a, epidermis ; b, cork ; c, cortex ; d, bast fibers ; e, sieve tube ; /, cambium ; 

 g, various types of wood cells; h, pith. After Kerner. 



vessel is usually broader as well as much longer than a conduct- 

 ing cell. Both conducting cells and vessels die when they are 

 fully formed, and so in any case it is through dead cells that 

 sap passes from the roots to the leaves and flowers. The 

 pores that are seen so plainly in cross sections of some woods 

 are the vessels that have been cut across. Angiosperm woods 

 differ much from one another in the number, size, and dis- 

 tribution of their vessels, and upon these points of difference 

 the density and the strength of a wood very largely depend. 

 For instance, while the vessels in a cross section of oak wood 



