37 



the central pith to the bark, and which are 

 annually prolonged through each successive 

 pith and layer of wood, — in addition to 

 these, intermediate medullary rays are 

 developed from each new pith, which run 

 from that new pith to the bark, and are 

 annually prolonged. 



This growth sideways, between the 

 medullary rays, is doubtless the mode in 

 which alburnum is converted into heart 

 wood, and heart wood increases in density. 

 It must be from the upward sap ; and from 

 examination of branches which have been 

 rung, I have much reason to believe that 

 the alburnum below the rings is thus 

 converted into heart wood. This growth 

 may be said to partake of the principle 

 of the growth by intus-susception. One 

 cannot, however, suppose that this new 

 growth in the comparatively hard albur- 

 num is from the roots of the buds, accord^ 

 ing to Du Petit-Thouars's system. I have 

 remarked, that when -firs are cut across, 



c 3 



