LETTER IV. 



33 



of this root, and thence extending singly still farther 

 into the ground. 



5. Ideal as this plan is, the representation given 

 tallies very exactly with the appearances presented by 

 a young fir-tree cut lengthwise through the middle ; 

 and may at any time be verified, as regards the trunk, 

 by examining a railway sleeper — at least one that has 

 chanced to be suitably sawn. No continuous, uninter- 

 rupted, medullary cavity (as many fancy there is), 

 reaching from the top to the bottom of the tree, and 

 filled with pith, or the remains of pith, will there be 

 seen. What we actually find is a set of narrow spaces 

 or cavities, rising one above another, in number cor- 

 responding to that of the annual shoots (which in fact 

 they indicate), each circumscribed and shut off from 

 those directly above and below it by a thin layer of 

 woody tissue, and each containing a dry cellular sub- 

 stance, the remains of the succulent pith with which it 

 once was stored, and each varying in length from ten 

 inches or less to a foot and a half or more, according 

 as the year's growth has been. On either side of 

 these medullary cavities may be seen a series of woody 

 layers passing downwards in the direction of the 

 ground ; their number progressively increasing in that 

 direction, and each pair coming off from the lower end 

 of each one of those medullary cavities, — each one of 

 these, therefore, in succession downwards, having a 

 pair more investing it than the one directly above it. 



c 



