84 THE MECHANICAL FUNCTIONS. 



softer and more porous as it comes nearer to the 

 centre ; though with regard to its essential cha- 

 racter it appears to be uniform in every part, 

 having neither medullary rays, nor true outward 

 bark, nor any central pith ; in all which respects 

 it differs totally from the ordinary exogenous 

 trees. 



The first stage of its growth consists in the 

 appearance of a circle of leaves, which shoot 

 upwards from the neck of the plant, and attain, 

 during the first year, a certain size. The follow- 

 ing year, another circle of leaves arises ; but they 

 grow from the interior of the former circle, which 

 they force outwards as their vegetation advances, 

 and as ligneous matter is deposited within them. 

 Thus each succeeding year brings with it a fresh 

 crop of leaves, intermixed with ligneous matter, 

 which leaves, exerting an outward pressure, 

 stretch out the preceding layers that enclose 

 them ; until the latter, acquiring greater density, 

 no longer admit of further distention, and re- 

 main permanently fixed. This happens first to 

 the outermost layer, which is the oldest : then 

 each succeeding layer becomes consolidated in 

 its turn. As soon as the outer layer has become 

 too hard to yield to the pressure from w ithin, the 

 growth of the inner layers is immediately directed 

 upwards ; so that they each rise in succession by 

 distinct stages, always proceeding from the in- 

 terior ; a mode of developement which has been 



