VII.] INJECTION. 87 



structure is intimately related to, and adapted for the 

 conduction of fluids (containing dissolved salts, etc., and 

 therefore comparable physically with the fluids to be in- 

 jected) from the base to the apex, and vice versd. As the 

 trunk, or branch, ages, however, its inner portions undergo 

 the changes which convert them into heart-wood. Now 

 these changes are principally of two kinds. In the first 

 place the elements of the heart-wood are more and 

 more shut off, by peculiar structural changes, from par- 

 taking in the function of transport of water, therefore 

 rendering it more and more difficult for air or liquids to 

 traverse their walls ; and, in the second place, these 

 walls and the cavities of the elements become blocked 

 up with such materials as resins, tannin, colouring 

 matters and the like, and thus the heart is rendered 

 more and more impervious to such fluids as we refer to. 



Exact experiments prove that it requires very little 

 pressure to inject the sap-wood of a Conifer or Dicoty- 

 ledon with any coloured solution capable of wetting the 

 walls, if the pressure drives the fluid up or down the 

 stem ; whereas very much higher pressures are needed 

 to even partially inject the heart-wood in the same way. 

 Again, while it is still comparatively easy to press such 

 liquids through the sap-wood in a horizontal direction* 

 at right angles to a medullary ray, it is almost impossible 

 to drive them in one parallel to the ray, even in the 

 sap-wood. 



This being the case, it only remains to add that the 

 timber of different species of trees differs considerably as 

 to the depth to which it can be injected with anti- 

 septics, and as to the pressures necessary to force the 

 fluids in ; and that, while it remains true that no large 



* In all these statements concerning direction I assume the tree trunk to 

 be in its normal vertical position. 



