356 CONSERVATIVE ORGANS. [LKCT. VII. 



wood displayed in the transverse section of a stem. 

 As the alburnum changes into wood it loses its 

 irritability; but an unexceptionable theory of the 

 means by which this change is effected, is still a 

 desideratum in vegetable physiology. The opinion 

 that the lignification is the consequence of the 

 mere loss of fluid parts, and the approximation of 

 the solid, cannot be admitted as correct; because, 

 in this case, the drying of the alburnum even after a 

 tree is cut down, would give it the consistence and 

 characteristics of perfect wood, which is not the 

 fact. I am inclined to believe, that the ligneous 

 matter is deposited in a manner somewhat ana- 

 logous to the deposition of the phosphate of lime 

 in bones ; and that this deposition continues to be 

 effected long after the alburnum assumes the cha- 

 racter of wood. The heart wood, even of an old 

 tree which is sound, contains some moisture as 

 long as the tree continues to grow, which can only 

 be accounted for by supposing that it still lives *, 

 and, consequently, is in some degree under the 

 control of the organic power ; and, as a lateral 

 communication is preserved between the exterior 

 and the interior of the stem by means of the diver- 

 gent rays, there is no improbability in supposing 

 that a sufficient supply of proper juice finds its 



* Darwin (Phytologia, xviii. 2, 12.) asserts that the 

 heart or internal wood is not alive ; but this is not the case, for 

 the whole is alive whilst the tree remains sound. 



