74 . THE MECHANICAL FUNCTIONS. 



from the interior ; a mode of developement which 

 has been compared by De Candolle to the drawing 

 out of the sUding tubes of a telescope. The whole 

 stem, whatever height it may attain, never increases 

 its diameter after its outward layer has been con- 

 solidated. 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- 

 cular impression round the stem. The age of the 

 tree may accordingly be estimated by the number 

 of these circles, or knots, which appear along its 

 stem. The successive knots which are seen in the 

 stems of other endogenous plants, as may be ob- 

 served in growing corn, and also in various grasses, 

 have a similar origin. 



The structure of exogenous trees is more com- 

 plicated : for, when fully grown, they are composed 

 of two principal parts, the tvoocl and the bark. The 

 woody portion exhibits a further division into the 

 pith, which occupies the centre, and consists of 

 large vesicles, not cohering very closely, but form- 

 ing a light and spongy texture, readily permeable 

 to liquids and to air ; the harder ivood, which sur- 

 rounds the pith in concentric rings, or layers ; and 

 the softer wood, or aJburmmi, which is also disposed 

 in concentric layers on the outer side of the former. 

 Each of these concentric layers of wood and of al- 

 burnum may be farther distinguished into an inner 

 and an outer portion ; the former being of less den- 

 sity than the latter, and consisting of a lighter cel- 

 lular tissue : while the outer portion is composed of 

 the denser woody fibres resulting from the union 

 of numerous vessels with a cellular envelope. The 



