LETTER V. 



43 



figure 5. Hence the central point of the trunk is by 

 no means always the medullary cavity, which is con- 

 sequently more Fig. 5.*^- 

 or \es^ eccentric. 

 How is this? 

 These zones are 

 sections of the 



roots of the , / y - „ . 



plants above, 

 and their thick- ' ' ' 



ness indicates , ; . _J 



the quantity of ' ^ 



roots sent down, i . , ' 



which will of ^j-' ' 



course be in pro- ^^ ' 



portion to the \^ f 

 number of the ^ _ \ / ^ 



plants, and the "^-- --^-.=^555^^^ 



vigour of their growth. In these respects there are 

 great differences in different years and on different sides 

 of the tree, corresponding to variations in the tempera- 

 ture and character of different seasons, and likewise to 

 variations in respect of exposure to light, to prevaiHng 

 winds, and other causes. That is the explanation.f 



* Fig. 5. Concentric woody circles of the Box-tree, — from the 

 drawing made by Nature on the block from which it is printed. 



t " In a crowded plantation there is a marked difference between 

 the trees on the outside and those in the centre ; the former, having 



