LETTER IV. 



31 



They speak of trees as being either exogenous or en- 

 dogenous, according to the manner of their growth 

 and the disposition of their parts. In the first of 

 these, as the Greek word from which the name is 

 derived signifies, the woody matter yearly added to 

 the trunk (or the annual woody layer) grows and is 

 deposited outside that of the previous year ; in the 

 latter, as the Greek derivation of the name implies, 

 the woody matter of the one year is deposited 

 inside that of the previous year. The Fir is an 

 example of the first, the Palm of the second sort of 

 tree. 



3. Let us first of all see how the matter stands in 

 respect of the exogen ; and for illustration sake, let us 

 take a Fir-tree — the Larch, for example. The engrav- 

 ing on the opposite side, entitled " Plan of Fir-tree," 

 is an ideal representation of such a tree, — stript of its 

 bark, as high nearly as where the branches are seen 

 to come off, next sawn crosswise to the centre of 

 the trunk, and then cut vertically downwards, right 

 through the middle plane of the trunk and roots. It 

 exhibits the relations Avhich the several parts com- 

 posing the trunk and roots bear to one another. In 

 other words, it is a representation of the relations that 

 subsist between the organic remains — i. e,, the dead 

 stems and roots — of the annual fir-plants that have 

 grown successively one above another. To avoid com- 

 plexity in the details both here and in the "plan," 



