62 THE MECHANICAL FUNCTIONS. 



the second, they are elongated into cylinders, or slowly ta- 

 pering 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 modifications of the same elemen- 

 tary 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 physio- 

 logy. 



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 neigh- 

 bouring cells, give the appearance of fibrous connexions 

 among these cells, which did not originally exist. Simple 

 membranous 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 coa- 

 lesced by their edges; thus presenting a more uniform sur- 

 face, excepting that a few interstices are left, where the pel- 

 lucid membrane, having no internal lining, presents the ap- 

 pearance of transverse fissures or oval perforations, (Fig. 10.) 

 Cells of this description are said to be reticulated or spot- 

 ted, and, together with those having more regularly formed 

 spiral threads, are very abundantly met with in plants be- 

 longino; to the tribe of Orchideae. 



It has been much disputed whether the cells of the vege- 

 table texture are closed on all sides, or whether they com- 

 municate with one another. Mirbel has given us delinea- 

 tions of what appeared to him, when he examined the coats 

 of the cells with a microscope, to be pores and fissures. But 

 subsequent observations have rendered it probable that these 

 appearances arise merely from darker portions of the mem- 

 branes, where opaque particles have been deposited in their 

 substance. Fluids gain access into these cells by transuding , 



