172 VEGETABLE ORGANOGRAPHY. 



long as it is not too much stretched, <it remains, as in 

 most fleshy plants, in a state of greenness, freshness, 

 and integrity ; but, sooner or later, the time comes when 

 the cellular envelope cannot suffer any more enlarge- 

 ment, and then it dies in consequence of the extension 

 which it suffers. It breaks longitudinally, and thus forms 

 the cracks of the bark ; these become still deeper when 

 the external cortical layers themselves split as their en- 

 velope. This presents, according to its texture and the 

 mode of growth of the tree, different phenomena : 

 sometimes, as in the Oak, or the Birch, after having 

 been some time smooth and even, it presents irregular 

 fissures, and is destroyed by the slow and irregular sepa- 

 ration of its fragments ; at other times, as in the Cork- 

 tree, it presents a texture at the same time dry and 

 flexible ; from whence it results that it can live several 

 years without falling off, and that it can, at a certain 

 period of its existence, be taken off in considerable 

 pieces. It naturally falls off in the Cork-tree every 

 eight or nine years ; and care is taken, a year or two 

 before this period, to take it off for commercial purposes. 

 For this purpose that time of the year is selected when 

 the bark adheres most strongly to the woody body; 

 because then, by means of proper instruments, the 

 whole of the cellular envelope can be removed without 

 any danger of raising the liber. 



An opposite extreme to the state of the Cork-tree is 

 the example of the Plane. Here the cellular envelope is 

 thin, and rapidly acquires a rigid and friable consistence ; 

 whence it results that as soon as the trunk has slightly 

 increased in diameter, the rupture and fall of the cellular 

 envelope of the bark is caused ; and this takes place every 

 year, towards the end of summer. When a zone of the 

 cellular envelope is detached from the tree, the external 

 part of the cortical layer being thus denuded (which is 



