STRUCTURE OF DICOTYLEDONOUS WOOD. 14.J 



energy of the stem resides, and it is through it that 

 the sap rises in the spring for the supply of the buds 

 and leaves ; an Exogenous tree can therefore lose the 

 whole of its inside without suffering much diminution 

 of growth, so long as the sapwood remains uninjured, 

 and this is the reason why trees that are hollow go on 

 growing century after century, just as if their inside 

 were still sound. On the outside of the wood is the 

 bark, which binds up and protects all the other parts, 

 and down which the returning current of sap descends 

 towards the roots. 



Next carry your eye attentively over the section, 

 from the bark to the pith, and you will remark that a 

 number of fine pale lines are drawn as it were from 

 one to the other, forming delicate but broken rays ; 

 these lines, which are composed of flattened cells, 

 and named medullary rays, are in reality the ends 

 of extremely thin plates connecting the pith and 

 the bark together ; they perform an important part 

 in the system of vegetation, for it is they which con- 

 vey the descending sap from the bark to the centre 

 of the stem ; it therefore is they which are the cause 

 of the production of heartwood, and all trees without 

 them must be destitute of it ; as is the case with 

 monocotyledonous plants, which are always softest 

 in the centre. 



Thus you see the sap, which rises from the roots is 

 carried upwards in the sapwood, down again in the 

 bark, and latterly into the hidden recesses of the 

 trunk, by the medullary rays, all three currents 



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