VEGETABLE FIBRE. 69 



The firmness of wood, and consequently its value as 

 timber, depends on the number and density of its lon- 

 gitudinal fibres. In the Guiacum and Mahogany they 

 are numerous and compact, in the Poplar they are few, 

 and the wood of the former is extremely hard, while 

 that of the Poplar is soft and spongy. But even in the 

 same plant these fibres are not uniform throughout, ac- 

 quiring their utmost degree of solidity only by age, 

 and being soft and tender in the more recent layers of 

 wood and bark. And the delicate fabric of these re- 

 cent layers, exhibits a difference, equally worthy of 

 observation. If for example, the bark is wounded, its 

 fibres will soon reunite, and no traces of the wound are 

 perceptible ; but when the wood is injured, its fibres 

 are never reunited and the injured part is never regen- 

 erated. We here see one of those admirable provis- 

 ions of Nature, which it is impossible to contemplate 

 without delight. The bark being exposed to frequent 

 injuries, is capable of restoring itself to health, but the 

 wood being comparatively secure, needs not and pos- 

 sesses not this preservative power. 



But even these fibres, so slender that the naked eye 

 can scarcely detect them, are found to be tubular, and 

 they constitute the channel through which the fluids of 

 the vegetable pass from one organ to another. They 

 form in the aggregate bundles of hollow cylinders of 

 which Mirbel has given a very satisfactory description, 

 accompanied with some instructive hints relative to the 

 peculiar functions of each variety. 



