SECT. V. THE LIGNEOUS LAYERS. 320 



SUBSECTION I. 



Concentric Layers. The concentric layers (P/. Discover- 

 VIII. Fig. 13.), which constitute by far the greater horizontal 

 part of the mass of the wood, are sufficiently con- se 

 spicuous for the purpose of exemplification on the 

 surface of a horizontal section of most trunks or 

 branches, as on that of the Oak and Elm. particu- 

 larly after being some time exposed to the weather. 

 But in some trunks, as in that of the Laburnum, 

 they arc marked even by a diversity of colour. 

 Like the concentric layers of the bark thev have been Indicate 



*yp 'IT QJ 



supposed to correspond in number to the years ofthepfaat. 

 the plant's growth. But the supposition seems to be 

 here equally unconfirmed by accurate observation, 

 though their number is no doubt in proportion to 

 the age of the plant ; as the trunk is always found 

 to contain more than the branches. But it has not 

 been by any means proved that there is only one 

 layer formed in the course of a year. On the con- 

 trary Du Hamel has shown that a tree of twenty 

 years old has not always twenty distinct layers ; and 

 that a tree of ten years old has sometimes more 

 then twelve. 



But though they are generally described as being Are often 

 concentric, they are not always strictly so. For 

 they are often found to extend more on the one 

 side of the axis of the stem or branch, than on the 

 other. Some authors say the excess is on the north 

 side, but others say it is on the south side. The 



