ORGANIZATION OP THK STEM. 57 



bark, of wood, and of pith, but the cambium layer is not there, 

 and therefore it wants the elements necessary to the forma- 

 tion of a new layer of bark and a new layer of wood. 



In the second year, the gelatinous tissue of the cambium is 

 subjected to the following changes. We have seen that it occu- 

 pies an intermediate position between the bark and the wood. 

 This zone of cambium cells produces at every point beds of the 

 same nature as those with which it is in immediate contact, 

 and is developed into ligneous and cortical fibre, preserving its 

 cellular organization only in those portions which correspond 

 to the medullary rays. The inner portion of the cambium 

 layer forms the wood, the outer portion the bark, and the new 

 cells of both layers thus mould themselves entirely on the 

 older cells throughout all their points of contact. 



That the bark increases in diameter by the deposition of new 

 layers of bark internally, was first proved by Duhamel, a cele- 

 brated French physiologist, by the following simple experiment. 

 He passed a metallic thread between the liber and wood of a 

 young tree, and cutting down the tree several years after, he 

 found, on examination of the cross-section, that rings of bark 

 coinciding in number with the years elapsed since he placed 

 the wire next the wood, had grown between the wire and the 

 wood, so that the wire was separated from the wood by a con- 

 siderable thickness of bark. No experiment can be imagined 

 more decisive than this, of the growth of the bark in diameter 

 by internal deposition of matter. 



The wood. — This consists of two parts, the alburnum or sap 

 wood, and the duramen or heart wood. The alburnum or sap 

 wood, so called because the sap circulates through it, and also 

 in allusion to its white or pale color. The alburnum is the 

 zone or ring of wood last formed. It consists of elongated 



