STEMS AND BRANCHES AND THEIR USES 213 



shrubs produce rubber in this way in large quantities. More 

 than half the total supply of rubber comes from the Amazon 

 Valley; the trees of that region whose juices yield " Para 

 rubber " belong to the spurge family. Gutta-percha, which 

 is used for the insulation of electrical wires, especially in un- 

 derground and submarine cables, as well as in the manufac- 

 ture of hose, belting for machinery, golf balls, and in a variety 

 of other ways, is obtained from the milky juices of some other 

 tropical trees. The similar juice of a tree that grows in 

 Mexico and Central America supplies chicle, the basis of most 

 chewing gums. 



232. Bark. — Each kind of tree has a bark of characteristic 

 appearance. Think, in this connection, of the very different 

 barks of the oaks, the poplars, the shagbark hickory, and the 

 paper birch. The appearance of the bark of any tree depends 

 in part upon the distance between the successive layers of cork 

 cambium, and in part upon the planes in which these layers 

 lie with reference to one another. If they are parallel or 

 nearly so, broad bark layers are produced of even thickness, 

 as in the case of the paper birch, whose successive layers of 

 cork cambium are very near together. If the layers are not 

 parallel but intersect one another, as they do in the oaks, the 

 bark cracks into narrower pieces of various shapes. The 

 thickness of bark depends largely upon the rate at which the 

 cork cambium cells divide and upon the number of layers of 

 cork cells that they form. 



In the cork oak of southwestern Europe, which supplies 

 the " cork " of commerce, a single layer of cork cambium 

 remains active for seveiral years. The first cork cells formed 

 each year are flatter and darker than the later-formed ones, 

 and so the whole thickness of cork tissue is marked off into 

 layers, each representing a year's growth. This division into 

 layers may be seen in large bottle corks. 



Sometimes the true bark (that is, the layers of cork and 

 other cells lying outside the cork cambium) is called " dry 



