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PLANTS 



characters, such as the color and texture of the bark and the 

 succession of branches, will also serve to determine the age of 

 any particular section of the branch. 



6i. If we cut across a twig of one year's growth, we find that 

 it consists of three parts, an outer bark which may be peeled 

 off, a central core of soft tissue— the pith — and between them 



L 



Fig. 4. — Photomicrograph of a cross section of oak wood showing one year's 

 growth, £, Early growth; L, late growth; m and w, large and small medullary 

 rays. (From Stevens.) 



a firmer cylinder, the wood. The outer surface of the bark is 

 smooth and rather tender, and covers a layer containing more 

 or less green substance. The inner layers, those which are next 

 the wood, are hard and consist largely of very tough fibers. 

 The pith is soft and spongy in texture and contains no fibers. 

 The wood is also fibrous, since it can be split lengthwise of the 



