DISEASES. 239 



opposite directions from each other, attaining firmness 

 of texture as they increase in age. These are to be 

 observed as general physiological facts, though there 

 may be particular instances which are exceptions to 

 them. The inner layers of woody matter, acquiring 

 firmness of texture as they recede from the more 

 recent and juicy layers, become at length the heart- 

 wood of the tree. In this view of the vegetable pro- 

 gress, it is of the utmost importance to inquire into the 

 causes which induce that change of texture which has 

 just been described, for if these can be traced with 

 something like accuracy, a double advantage will be 

 gained ; for not only will the vegetable structure itself 

 be better understood, but a remedy may be found for 

 diseases which, in more instances than the one under 

 inquiry, cause the destruction of the heart-wood. 



" But what are the vessels and organs of supply and 

 elaboration ? For the solution of this inquiry, I refer 

 to the luminous theory of Mr. Knight. The scientific 

 world has long been in possession of his theory. I 

 may, however, be doing the cause of science some 

 service by quoting passages of his own letters to me. 

 In the one received in answer to my last communication 

 upon the larch, he observes : — ' My opinion respecting 

 the motions of the sap in trees is, that it is absorbed 

 by the bark of the roots, carried up through the sapwood 

 then transferred to the leaf, in which it becomes what 

 is called the true sap or blood of the plant, correspond- 

 ingj with the arterial blood of animals ; that this fluid 

 descends by the bark, by which, or rather by its 



