144 GROWTH OP PLANTS. 



the inner bark is converted into pulp-wood, and this 

 every year is farther converted into hard wood. When 

 a tree,, accordingly, is cut across, these yearly layers 

 may be observed, and the age of the tree may be 

 thereby ascertained. 



Mr. T. A. Knight says, the bark is never changed 

 into pulp wood, nor into hard wood. 



M. Mirbel appears to have somewhat altered his 

 opinions at different times. He first says the inner 

 bark is changed into wood and augments the mass 

 of the ligneous body. Again, he says, the trunk is 

 formed of one and the same cellular tissue, of which 

 the rind forms the limit. But afterwards, in 1827, he 

 expressly contradicts the first of these statements, saying, 

 that the inner bark never becomes wood; but there is 

 annually formed between the inner bark and the wood 

 a new layer, which is a continuation of the wood and 

 of the inner bark. This regenerating layer is termed 

 pulp, which is not a fluid coming from one place or 

 another, but a very young tissue that continues the 

 older tissue. It is nourished and evolved by sap 

 highly elaborated. Its organisation appears to be uni- 

 form throughout, yet the part which is in contact with 

 the pulp- wood, changes insensibly into hard wood, and 

 that which is in contact with the inner bark, changes 

 in the same manner into the inner bark. The latter 

 change is perceptible to the eye of the observer. In a 

 young shoot, the pulp between the inner bark and the 

 pulp wood gradually thickens, while fine fibres begin 

 to appear therein, till at length it is crowded with 

 vessels and cells, slowly and gradually formed. 



