CIRCULATION OF THE SAP. 21 



all over the leaves, and is exposed to the full action of the 



sun's rays and to the air, in these thin and broad organs 



It here undergoes a great change. A large 



portion evaporates through the pores of the 



leaf, (which are so small that 30,000 are found 



on a square inch of the pear or apple leaf;) 



the remainder absorbs carbonic acid from the 



air, which is decomposed, the oxygen being 



given off again, and the carbon retained for Pores in the under 



the manufacture of the solid substance of the leafrgTeady mag- 



w^ood. The light of the sun is necessary to ^"^^^• 



effect this decomposition, — for in the dark it will not take 



place ; hence the reason that the branches of trees grow 



feebly or cease growing when their leaves are densely 



shaded. 



This newly formed juice, greatly reduced in bulk, but 

 enriched with materials for the new w^ood, now begins to 

 descend. It flows downward, not again through the sap- 

 wood^ but through the tubes in the i7iner hark., and in its 

 passage deposites on the outer surface of the wood, next to 

 the bark, a soft, mucilaginous substance, called cambium, 

 which gradually hardens into new wood. This commences 

 in spring as soon as the leaves open, and continues till the 

 cessation of growth in autumn, by which time a new layer 

 of wood has been deposited.* The distinct successive por- 

 tions thus laid on by each season's 2:rowth, indicate with 

 accuracy the age of the tree, when cut across and counted. 

 This may be proved by the simple and interesting experi- 

 ment of slitting down the bark of a hardy young tree, care- 

 fully loosening the bark back from the wood, and slipping 

 in around the freshly stripped stem a sheet of tin-foil, and 

 again replacing the bark without injur}^ The cambium is 

 then deposited outside the tin-foil ; the new wood forms 

 around it ; and at any subsequent year, by cutting down the 

 tree, the number of concentric layers outside the foil will be 

 found to indicate w^ith precision the number of years since 

 the operation. The ages of large forest trees are thus in- 

 delibly recorded, and the author has counted the rings of a 



* All ordinary trees in temperate climates, known as exogenous^ (which means 

 growing outside) are formed in this way; but palms and other endogenous trees, oi 

 tnside growers, have their new wood deposited at the centre, which swells out the 

 exterior. 



