66 THE MECHANICAL FUNCTIONS. 



points, arranged in circular rows around the cylin- 

 drical surface of the vessel.* 



What are called the ivoody fibres originate, like 

 all other parts of plants, in cells. These are gene- 

 rally fusiform, that is, of the shape of a double 

 cone, very greatly elongated, and placed close and 

 parallel to one another, with the narrow extremities 

 of one set wedged in between those of another set 

 (Fig. 18). Their coats are more firm and elastic 

 than those of ordinary vessels, but do not appear 

 to contain any internal fibres, although they receive, 

 in the progress of their developement, large addi- 

 tions of solid matter. These fibres are generally 

 collected together into bundles or layers, and are 

 accompanied by cells and vessels of various descrip- 

 tions, and in different stages of transition. The 

 density of the woody fibres increases in proportion 

 as these incrustations are formed, till they have 

 become nearly impervious ; and have acquired a 

 degree of rigidity peculiarly fitting them for the 

 office of giving mechanical support to the fabric of 

 the plant. -)" Their assemblage thus constitutes a 

 kind of framework for the whole system, which 



* Many distinguished botanists, such as Rudolphi, Link, Trevi- 

 ranus, and Dutrochet, consider these spots as being produced not 

 by the deficiency of the internal coating, but by the addition of 

 granular bodies. See De Candolle's Organographie Vegetale, torn. 

 i. p. 5Q. 



t By drying different specimens of wood in a stove, Count Rum- 

 ford was led to the conclusion that the specific gravity of the solid 

 matter which constitutes timber is nearly the same in all trees. He 

 found that the woody part of oak, in full vegetation, constitutes 

 only two-fifths of the whole bulk : and that ordinary dry wood con- 

 tains above one-fourth of its weight of water. Thomson's Annals 

 of Philosophy, i. 388. 



