VEGETABLE ORGANIZATION. 71 



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, hereafter to be described, sur- 

 rounded by assemblages of cells thus fortified, 

 and the whole cohering in bundles, so as to 

 present greater resistance to forces tending to 

 displace them, in the longitudinal direction 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 vesiculosus. But the 

 greater number of other plants have, in addition 

 to these cells, numerous ducts or vessels, con- 

 sisting of membranous tubes of considerable 

 length, interspersed throughout every part of the 

 system. These tubes exhibit different modi- 

 fications of structure, more especially with regard 

 to the form of the fibres, or other materials, 

 which adhere to the inner surface of their mem- 

 branes ; and these modifications correspond very 

 exactly with those of the vesicles already de- 

 scribed as constituting the simpler forms of 

 vegetable tissue. There can be little doubt, 

 indeed, that the vessels of plants take their 

 origin from vesicles, which become elongated by 

 the progress of developement in one particular 

 direction ; and it is easy to conceive that where 

 the extremities of these elongated cells meet, 



