18 SYLVA AMERICANA. 



cease, and in time it loses even its vitality ; not unfrequently 

 decaying in the centre of the trunks of trees ; which, often, still 

 flourish, and put out new shoots as if no such decay existed. 

 To carry on, therefore, the functions of the wood, a new circle 

 of it is annually forn;ed over the old ; and thus, also, the diameter 

 of the trunk and branches present, by the number of these 

 annual zones, a pretty correct register of their age, each zone 

 marking one year in the life of the part. The hardness of these 

 zones of wood increases with the age of the tree, being most 

 dense in the centre, and less and less hard as they approach the 

 circumference. 



Various opinions have been entertained respecting the origin 

 of the alburnum. Mr. Knight, however, by various experiments, 

 has satisfactorily proved that it is formed from the secretion 

 deposited by the vessels of the liber, but that it is not, as had 

 been supposed by Du Hamel, Dr. Hope and Mirbel, a trans- 

 mutation of the liber itself. 



Mr. Knight is of opinion, that the bark deposits the alburnous 

 matter ; but that the leaves are the organs in which this matter is 

 elaborated from the sap ; or, that the alburnum is generated from 

 the cambium of Grew, which is part of the proper juice of the 

 plant, formed by the exposure of the sap to the light and air in 

 the leaf, and returned from it by the vessels that pass down from 

 the leaf into the interior bark, by which it is deposited, and we 

 may add, elaborated by the action of the vital principle inherent 

 in this part of the plant. To determine this point, he removed 

 narrow circles of bark from roots of apple trees, " leaving a leaf 

 between the places where the bark was taken off; and on 

 examining them frequently during the autumn," he found that 

 the diameter of the shoot between the insertion of the leaf stalk 

 and the lower incision, was as much increased as in any other 

 part of the tree ; but when no leaf was left " on similar portions 

 of insulated bark, on other branches of the same age, no apparent 

 increase in the size of the wood was discoverable." 



These experiments explain the reason why trees and shrubs 

 having their leaves destroyed by caterpillars form scarcely any 

 new wood in that season ; and, indeed, every one who has 



