M i c ro s copical Essays. 



661 



S' great meafure connected with it. It is, therefore, found to 

 differ confiderably in fubftanee, quantity, and quality, in various 

 kinds. 



It is originally the outer membrane, covering the lobes of the 

 feed. Even there, as in the branch of a tree, it appears in form 

 of a kind of fpunge, or like a cruft of bread, compofed of flatted 

 bladders. ... 



It% fpunge-like nature may be further inferred from the con- 

 traction of it's pores when dry, and the eafe with which they 

 dilate when in water Grew has called it a moft curious and ex- 

 quifitely fine-wrought fpunge. In the courfe of it's growth, the 

 outer ranges of thefe bladders drying, it becomes what we call 

 the rind ; for the rind was once bark^ and has only fuffered a - 

 flight change in feparating from it.;. 



By the bark the tree is fed with a continual fupply of moiflure; • 

 protected from external injuries, and defended from the exceffes 

 of heat and cold ; for thefe purpofes it is varioufly difpofed in 

 different trees. In the hardy and flow growing, as the oak, it is 

 thin; in the quick growing, as willow, poplar, and the like, it 

 is thick. And what is more particularly to be attended to is, that 

 in fome it's inner verge is radiated. There are fome trees, and a 

 great many herbaceous plants, in which this part is continued ? - 

 inward, in form of rays, through t^ie blea into the wood, form- 

 ing as it were fo many wedges, that fplit as it were the fubflance 

 of both thofe parts,* a circumftance which accounts for the vege- 

 tation of fome particular trees, which are known to live when de- 

 prived 



* Hill's Conftru&ion of Timber, p. 118. Ibid. p. I2Q. 



