228 PROCESS OF DEVELOPEMENT. CHAP. IV. 



the former being apparently united together by 

 the latter. But it does not appear that an inter- 

 vening layer of cellular tissue is always to be found 

 distinct. There are some plants, however, which do 

 exhibit the layer in question distinctly enough ac- 

 cording to the description of Malpighi. Between 

 every two layers of the bark of the Fir-tree, there is a 

 thin layer of a substance evidently different in tex- 

 ture, which might have been the ground of Mal- 

 pighi's remark. And even in the body of the 

 trunk there is alternately a layer of wood that is 

 hard and white, and a layer that is brown and re- 

 sinous, from which the resinous drops exude when 

 the trunk is cut. 



Formation But how are we to account for the formation of 

 vergent the divergent layers, which Du Hamel erroneously 

 cording C to supposed to proceed from the pith ? If Du Hamel 

 UuHamel, na( j j^ happened to attend to the phenomena re- 

 lative to the point in question, which some of his 

 own experiments were the best calculated to exhibit, 

 he would readily have found the true solution of 

 the difficulty. This,, however, has been furnished 

 by Mr. Knight, who, in tracing the result of the 

 operation of budding, observed that the wood 

 formed under the bark of the inserted bud unites 

 indeed confusedly with the stock, though still pos- 

 sessing the character and properties of the wood 

 from which it was taken, and exhibiting divergent 

 layers of new formation which originate evidently 



