DEVELOPEMENT OF VEGETABLES. 73 



A Palm tree may be taken as an example of the 

 mode of growth in endogenous plants. The stem 

 of this tree is usually perfectly cylindrical, attains 

 a great height, and bears on its summit a tuft of 

 leaves. It is composed of an extremely dense ex- 

 ternal cylindric layer of wood ; but the texture of 

 the interior becomes gradually softer and more 

 porous as it comes nearer to the centre; though 

 with regard to its essential character 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 ordi- 

 n^iry exogenous trees. 



The first stage of its growth consists in the ap- 

 pearance of a circle of leaves, which shoot upwards 

 from the neck of the plant, and attain, during the 

 first year, a certain size. The following year, an- 

 other circle of leaves arises ; but they grow from 

 the interior of the former circle, which they force 

 outwards as their vegetation advances, and as lig- 

 neous 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 distension, and remain 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 

 within, the growth of the inner layers is immedi- 

 ately directed upwards ; so that they each rise in 

 succession by distinct stages, always proceeding 



