72 THE MECHANICAL FUNCTIONS. 



mode of growth, as being the simplest of these two kinds of 

 vegetable development. 



A Palm tree may be taken as an example of the mode of 

 growth in endogenous plants. The stem of this tree is usu- 

 ally perfectly cylindrical, attains a great height, and bears on 

 its summit a tuft of leaves. It is composed of an extremely 

 dense external cylindric layer of wood; but the texture of 

 the interior becomes gradually softer and more porous as it 

 comes nearer to the centre; though wdth regard to its essen- 

 tial character it appears to be uniform in every part, having 

 neither medullary rays, nor true outward bark, nor any cen- 

 tral pith; in all which respects it differs totally from the or- 

 dinary 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 following 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 

 lio-neous 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 farther dis- 

 tention, 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 immediately directed 

 upwards; so that they each rise in succession by distinct stages, 

 always proceeding from the interior; a mode of develop- 

 ment which has been compared by De Candolle to the draw- 

 ing out of the sliding tubes of a telescope. The whole stem, 

 whatever height it may attain, never increases its diameter 

 after its outward layer has been consolidated. A circle of 

 leaves annually sprouts from the margin of the new layer of 

 wood; these, when they fall off in autumn, leave on the stem 

 certain traces of their former existence, consisting of a cir- 



