62 THE MECHANICAL FUNCTIONS. 



The cells of the ligneous portion of trees and 

 shrubs are farther encrusted with particles of a 

 more dense material, peculiar to vegetable organi- 

 zation, and termed Lignin. It contains a larger 

 proportion of carbon and hydrogen, and less oxy- 

 gen, than the primitive tissue of the simple cells 

 and vessels ; and is not, like the latter, coloured 

 blue by iodine. 



It is this substance which principally contributes 

 to the density and mechanical strength of what 

 are called the Woody Fibres, which consist of 

 collections of fusiform, or tapering vessels here- 

 after to be described, surrounded by assemblages 

 of cells thus fortified, and the whole cohering in 

 bundles, so as to present greater resistance to force, 

 tending to displace them, in the longitudinal direc- 

 tion than in any other. 



Most of the plants which are included in the 

 Linnean class of Cryptogamia have a structure 

 exclusively composed of cells, as has been already 

 shown in the Fucus vesiculosiis. But the greater 

 number of other plants have, in addition to these 

 cells, numerous ducts or vessels, consisting of mem- 

 branous tubes of considerable length, interspersed 

 throughout every part of the system. These tubes 

 exhibit different modifications of structure, more 

 especially with regard to the form of the fibres, or 

 other materials, which adhere to the inner surface 

 of their membranes ; and these modifications corre- 

 spond very exactly with those of the vesicles already 

 described as constituting the simpler forms of vege- 

 table tissue. There can be little doubt, indeed, 

 that the vessels of plants take their origin from 

 vesicles, which become elongated by the progress 



