PHYSIOLOGY OF TREES. 267 



alburnum of the first year; 2 is that of the 

 second; 3 of the third, and so on to the sixths 

 a is the first epidermis or layer of bark with its 

 rough exterior ; h the second ; and so on to 

 which is the last or liber of the sixth year. 



Figure B is a section of the same stem cut in 

 the month of June of the seventh year. The 

 layers of wood and bark are shewn in the same 

 order as in fig. A, with the addition of the cam- 

 bium of the seventh year partly developed. Its 

 outer boundary line is the liber of the seventh 

 year; and the dotted space within is the cam- 

 bium swelling into volume, and pressing before it 

 in an outward direction all the previously formed 

 layers of bark, thereby adding to the diameter 

 of the stem. By this internal increase the first 

 layers of bark are split into vertical fissures, as 

 in Oak, Elm, &c.; divergingly stretched, as in 

 Beech; or thrown off, as in the Plane, Vine, 

 and Arbutus andrachne. 



The cambium, thus increasing, is gradually 

 charged with its peculiar organisation of fibrous 

 matter, tubes, cells, &c, containing that vital 

 principle called vegetable life. It separates from 



