30 HEART-WoOOD ETC. 
29. The thicker the stem is, the thicker the bark 
generally, and the rougher and more cracked it becomes 
outside, as in the Elm; in which the bark remains 
without being cast, but cracks gradually ; while in the 
Plane, it falls off in plates every year or two. Cork is 
the bark of a kind of oak; it is so soft that it does not 
crack, and consequently it remains on the tree, and 
consists of a number of layers, as may be seen on 
examining a bottle cork. F. 6. is a piece of a cork stem 
cut across, representing at p. the pith; w. the wood, 
which is in seven layers as seen marked off by fine 
dots, with the pith rays passing through them to the 
bark 0. which is also composed of seven layers, the 
dark innermost layer, being the newest formed, while 
the outermost layers are cracked. 
30. The stem of a Tree of some years growth, 
presents the following appearances: in the centre, very 
little if any pith will be seen; the wood is of a dark 
colour, and harder in the middle, while that which is 
nearer the bark, is white and soft (f. 5.); the reason is 
this, that the wood in the middle of the stem, is older 
than that which is outside it; and being fully 
formed and fit for use, is called the heart-wood, or 
duramen; while the lighter coloured portion is termed 
the sap wood or alburnum, and which in time will 
become of a darker colour, and harder texture. 
81. The Bark varies in thickness according to the 
nature and age of the Tree; it is usually divided into 
two principal layers, one which is outermost and much 
cracked, and an inner layer called the liber, which lies 
upon the new wood; this inner bark in the Lime Tree, 
