68 THE MECHANICAL FUNCTIONS. 



into cylinders, or slowly tapering cones, thus 

 passing by insensible gradations into the tubular 

 form. Figures 6, 7, and 8, are representations 

 of some of these different states of transition from 

 the one to the other. These various modifica- 

 tions of the same elementaiy texture have been 

 distinguished into several classes of cells, and 

 dignified by separate technical denominations, 

 which I shall not stop to specify, as it does not 

 appear that they have as yet thrown any light 

 on vegetable physiology. 



Many of the cells are fortified by the addition 

 of elastic threads, generally disposed in a spiral 

 course, and adhering to the inner surfaces of the 

 membranous coats of the cells, which they keep 

 in an expanded state. (See Fig. 9.) When the 

 membranes are torn, the fibres, being detached, 

 unrol themselves, and being loosely scattered 

 among the neighbouring cells, give the appear- 

 ance of fibrous connexions among these cells, 

 which did not originally exist. Simple mem- 

 branous cells, containing no internal threads, 

 are often found intermixed with these fibrous 

 cells. In many of the cells, again, the original 

 spiral threads appear to have coalesced by their 

 edges ; thus presenting a more uniform surface, 

 excepting that a few interstices are left, where 

 the pellucid membrane, having no internal 

 lining, presents the appearance of transverse 

 fissures or oval perforations. (Fig. 10.) Cells 



