THE STEM OF VASCULAR PLANTS. 183 



grafted upon another, be cut longitudinally at a given 

 height, it is found that from the heart to the circum- 

 ference, the wood and the bark are both of them, below 

 the graft, of the same nature as the stock ; and that 

 above it they are of the nature of the graft. Three 

 opinions have been put forth to explain this funda- 

 mental fact : — either the albui'iium produces the bark, — 

 or the bark the alburnum, — or the alburnum and bark 

 each produce a layer of their own nature. The first 

 opinion has been supported only by Hales, and is 

 without very striking proofs. It is easily opposed by 

 the extreme difficulty that all plants present to live and 

 grow when deprived of their bark ; and by the collec- 

 tion of facts. 



The opinion that the alburnum is produced by the 

 bark, is subdivided into ^ two. Some, at the head of 

 whom is Malpighi, thought that the internal layer of 

 the liber changes into alburnum ; others, after the 

 example of Grew, believed that the liber produces the 

 alburnum, but does not change into it. Without abso- 

 lutely deciding between Malpighi and Grew, Duhamel 

 has remarked, that if a plate of silver be placed between 

 the woody and cortical bodies, it is fouLud, after some 

 time, covered by new woody layers ; whence he con- 

 cludes that their formation is due to the bark, and is 

 produced by means of the mucilaginous substance 

 which is found between them, and which he calls 

 Cambium. This experiment, which seems conclusive, 

 still leaves some points in doubt ; viz. 1st, The difficulty 

 of being assured that the plate of silver has been really 

 placed between the wood and the bark; and 2d, The 

 possibility that the cambium may be produced by the 

 wood, and that it may be sufficiently fluid, in its first 

 state, to flow over the plate of silver, and to cover it over 

 on the outside, although originally proceeding from the 



