20 VEGETABLE ELEMENTS AND TISSUES. 



the cells at the same time mostly adhering to each 

 other, so as to be separated with difficulty. 



The forms thus produced are various and inter- 

 esting, and have all received names by which they 

 are distinguished. They are described in works on 

 botany in two ways according to the outline (which 

 is the most common, as this expresses the appear- 

 ance usually presented in sections and on the surfaces 

 of vegetable structures), or according to the entire 

 or solid form, which it is often a difficult matter to 

 determine. 



Cellular tissue. Cells aggregated thus form a 

 tissue, which is called cellular tissue or parenchyma 

 (Trapa, among, and ey^v/jLa, poured substance), be- 

 cause it fills up the interstices of the other tissues 

 of plants. 



In technical descriptions, the cell- structure is often 

 left out of consideration ; and bodies composed of 

 parenchymatous tissue are described as being reticu- 

 lated or netted, because the united sides of the cell- 

 walls appear as a network covering the surface. 



It must be understood that parenchymatous cells 

 are such only as have the three dimensions of solidity 

 (viz. the length, breadth, and depth) nearly equal. 



Intercellular passages. The observer will not have 

 examined many sections of cellular tissue,, without 

 noticing certain irregular black lines running be- 

 tween the cells, as in a piece of a Geranium- (Pelar- 

 gonium-) leaf (PL I. fig. 1). These lines arise from 

 the existence of passages between the cells, contain- 

 ing air; and they are called intercellular passages. 

 By gently warming a section containing them in 

 water over a spirit-lamp, or by moistening the section 

 with a drop of spirit, the passages will be filled up 

 with the liquid, so as to become transparent. When 

 the intervals between the cells are larger and broader, 

 they are called intercellular spaces. 



So far, cells have been considered simply in regard 



