SECT. II. COMPOSITE ORGANS. 22? 



in like manner divide into two portions, the outer 

 forming one or more layers of bark, and the inner 

 forming one or more layers of wood. And hence 

 the origin of the concentric layers of wood and of 

 bark of which the trunk was found to be composed 

 in treating of the anatomy or internal structure of the 

 plant ; where it was shown that the layers of wood 

 are by no means regularly developed, being often 

 thicker on the one side of the stem than on the 

 other, and often also more in number ; owing, as it 

 appeared, to a superabundance of sap flowing from 

 a large root, or to a large branch, or to sudden 

 changes of temperature. And even where the in- 

 dividual layers are of equal thickness throughout, 

 they are not always equal in thickness to one 

 another ; owing no doubt to the age and vigour 

 of the plant; for the larger the diameter of the 

 tree, the thinner the layer, even if the same quan- 

 tity of wood should be formed. But the layers of 

 wood are always thicker than the layers of bark ; 

 though the bark of sickly trees is thicker in pro- 

 portion to the wood than that of healthy trees. 

 But how are the different layers united together so 

 as to form but one body in the aggregate? Malpighi 

 thought the layers of liber, as formed by the lon- 

 gitudinal tubes, are united or cemented together by 

 means of a plate of cellular tissue interposed be- 

 tween them. In the formation of the layers it is 

 true that tubes and utricles are always conjoined, 



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