338 CONSERVATIVE ORGANS. [LECT. VII. 



produces merely a simple condensation of the solid 

 matter, a hurried crystallization of the salts, and 

 a hasty consolidation of the other secretions. In- 

 deed, when wood acquires its firmness by the na- 

 tural means connected with its growth, it is a well- 

 known fact that the hardest is always of the slow- 

 est growth; as exemplified in the comparative 

 hardness of the wood of the Oak, which is of very 

 slow growth, compared with that of the Willow or 

 Horse Chesnut, which are trees of rapid growth ; 

 and even in that of the wood of the same tree 

 when growing in a dry and in a moist situation. 



Much difference of opinion has existed among 

 Phytologists regarding the origin of each suc- 

 cessive layer of wood. Linneus conceived that 

 it is formed from the pith, and added internally ; 

 but the absurdity of this opinion must be imme- 

 diately obvious to any one who examines the trans- 

 verse section of any stem or branch more than 

 two years old. Dr. Hales supposed that it is 

 formed from the zone of the prior year, by the 

 horizontal dilatation of the vessels, and "the 

 " shooting of the longitudinal fibres lengthways, 

 " under the bark, as young fibrous shoots of roots 

 " do in the solid earth * ;" an hypothesis which 

 has had almost as few followers as that of Linneus. 

 Malpighi taught that the liber is annually trans- 



* Vegetable Staticks, p. 340. 



