DEVELOPEMENT OF VEGETABLES. 85 



compared by De Candolle to the drawing out 

 of the sUding tubes of a telescope. The whole 

 stem, whatever height it may attain, never in- 

 creases 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 exist- 

 ence, consisting of a circular 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 suc- 

 cessive knots which are seen in the stems of 

 other endogenous plants, as may be observed in 

 growing corn, and also in various grasses, may 

 be traced to a similar origin. 



The structure of exogenous trees is more com- 

 plicated : for, when fully grown, they are com- 

 posed of two principal parts, the wood and the 

 bark. The woody portion exhibits a further 

 division into the pit/i, which occupies the 

 centre, and consists of large vesicles, not co- 

 hering very closely, but forming a light and 

 spongy texture, readily permeable to liquids 

 and to air ; the harder wood, which surrounds 

 the pith in concentric rings, or layers ; and the 

 softer wood, or alburnum, which is also disposed 

 in concentric layers on the outer side of the 

 former. Each of these concentric layers of 

 wood and of alburnum may be further distin^ 



