PART I!.] INTRODUCTION. 235 



of pith, wood, and bark. At first the substance 

 within the bark is Httle more than pith, but as 

 the returning sap deposits every year a layer of 

 wood just within the bark, which presses against 

 the previous layers so as to contract them, they 

 press in turn against the pith, which becomes 

 smaller and smaller every year, till at last, in 

 old trees, it is scarcely perceptible. The layers 

 of wood are always perfectly distinct from each 

 other, and they are called concentric rings (b) ; 

 while faint lines, with which they are intersected, 

 and which proceed like rays from the remains 

 of the pith in the centre, are called medullary 

 rays. A layer of wood being deposited every 

 year, the age of the tree may be discovered by 

 counting the concentric rings ; also if the tree 

 has grown rapidly, the layers will be thicker 

 than if it has grown slowly ; and if it has had 

 one side more exposed to the sun than the other, 

 the remains of the pith will be on one side 

 instead of in the centre, and the layers will be 

 thinner on that side than on the other. The new- 

 est layer of wood, which is called the alburnum 

 or sap-wood, is of a paler colour and more porous 

 texture than the rest of the tree, and it is of less 

 value as timber. It is from the manner in which 

 the successive layers of wood are deposited that 

 Dicotyledonous trees are called exogens, which 

 signifies, to increase from the outside. 



