ANAT0M* OF STEMS. 265 



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 

 formed 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 cen- 

 tre, and less and less hard as they approach the cir- 

 cumference. 



Various opinions have been entertained respecting 

 the origin of the wood or alburnum. Mr. Knight, how- 

 ever, by various experiments, has satisfactorily proved 

 that the alburnum is formed from the secretion deposit- 

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

 been supposed by Du Hamel, Dr. Hope, and Mirbel, 

 a transmutation 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 shoots 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 

 23 



