268 Internal Modifications. 



cellular integument or the epidermis, and of an interior lining 

 of woody fibre, called the liber or inner bark; if more than 

 one year old, then it is composed of as many layers of cellular 

 integument and woody fibre as it is years old, the former being 

 invariably external, and the latter internal in each layer ; and 

 every layer being formed beneath the previous one, and there- 

 fore next the wood. In consequence of the new bark being con- 

 tinually generated within that of the previous year, it is necessary 

 that the latter, which is pushed outwards, should be extensible ; 

 and in many of the plants this extensibility takes place to a 

 considerable degree. As there is a double layer of cellular 

 integument and woody fibre formed every year, it follows that 

 the age of a tree ought to be indicated by the number of such 

 deposits contained in its bark. But the arrangement of the zones 

 is so very soon disturbed, and the distinction between them be- 

 comes so imperfect, that even when the outermost coating is still 

 entire, it is scarcely practicable to count the zones ; and as soon 

 as the outside begins to split or peel off, all traces of their full 

 number necessarily disappear. That the bark really increases 

 by constant deposits of new matter between it and the wood, is 

 demonstrated by introducing a piece of metal in the liber of a 

 tree, and watching it subsequently ; in process of time it will he 

 protruded to the outside, and finally fall away. Notwithstand- 

 ing the fibrous character of a certain portion of the bark, it is 

 generally so brittle as to be capable of breaking ill all directions 

 with a clean fracture, as soon as it becomes dry and ceases to 

 live ; but in many plants, when young, it is so tough as to be 

 applied to different economical purposes. For instance, the 

 Russia mats of commerce are prepared from the liber of two or 

 three species of Silea, that of many Malvaceae is manufactured 

 into cordage, and similar properties are found in that of many 

 other plants. When stems are old, the bark usually bears but a 

 small proportion to the thickness of the wood, yet in some plants 

 its dimensions are of a remarkable magnitude. For instance, 

 Pinus Douglasii specimens have been brought to Europe twelve 

 inches thick, and these are said to be not of the largest size. 



Beneath the bark and above the wood is interposed in the spring 

 .a mucous viscid layer, which when highly magnified is found 

 to consist of numerous minute transparent granules, and to ex- 

 hibit faint traces of a delicate cellular organization. This secre- 



