320 THE EEASON WHY. 



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" The trees of the Lord are full of sap : and the cedars of Lebanon which he hath 

 planted." PSALM crv. 



respiration more actively than if subjected to direct heat ; they are 

 also protected from the injurious effects of dust ; and are moistened 

 by evaporation from the earth's surface. 



1269. Why have plants a formation of pith in their 

 centre ? 



The pith is the chief organ of nutriment, especially in the young- 

 plant. It is the structure which first conveys fluids to, and 

 receives them from, the newly-formed leaf. It communicates 

 with every branch, leaf, bud, and flower ; and also with the 

 bark, through the medullary rays, which radiate horizontally 

 from the centre of the plant. It is the centre of the movements of 

 the sap which occur in the horizontal vessels ; and it holds an 

 important influence over the life of the plant. 



1270. Why are trees covered with lark ? 



Because the bark serves to protect the woody structure, and also 

 to give a passage to the descending sap which flows abundantly in 

 the spring, and out of which the woody fibre is formed. It is also, 

 from its peculiar nature, well fitted to endure the changes of the 

 seasons for many years ; and from its non-conducting properties it 

 serves to maintain the equal temperature of the vital parts of the 

 tree. 



1271. What is cork? 



Cork is the bark of a description of oak-tree, which grows in 

 great abundance in Spain, Italy, and France. 



1272. Why does the cork-tree release its own lark ? 



Because it possesses a bark which is exceedingly useful to man ; 

 and it seems, therefore, to have been the design of providence that 

 the tree should cast it off, to be applied to the wants of the human 

 family ; for the cork-tree does not discharge its bark by the mere 

 cracking, or exfoliation, of its substance ; the tree retains the bark 

 for a number of years, until it has attained that consistency and 

 thickness which renders it useful, and then the tree forms within 

 the bark a series of tabular cells, which cut off the connection 

 of Ihe bark with the internal structure, after which it peels off 

 in l.irsre sheets. 



