ORGANIC STRUCTURE. 



Fig. 33. 



107 



11 



Third year. 



These examples of the annual accretion of the 

 stem for three years will suffice to show the manner 

 of all subsequent growth ; for, in fact, an oak of a 

 century old (if not decayed at the heart) will be 

 found composed of one hundred layers of wood and 

 one hundred layers of bark, whether they are dis- 

 tinguishable or not. 



The zones of wood are more visible in some kinds 

 of trees than others. Slow growers have their layers 

 so closely posited on each other, that they are very 

 indistinct ; and in the case of the oak and some 

 others, they appear twinned or double, owing no doubt 

 to the spring and midsummer growths usually made 

 by these trees. The same observation may be made 

 of the bark ; the layers of which are so extremely 

 thin that they are not easily identified. 



It often occurs that on viewing a cross section of 

 a felled tree, the pith is not exactly in the centre, 

 where it would be did the tree swell equally all 

 round. This irregularity proceeds from various 

 causes ; sometimes from the roots being better 



