The forms thus produced are various and interesting, 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 appearance 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 _paren´chyma_ (παρά, among, and ἕγχυμα, poured substance), because 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 reticulated 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 between the cells, as in a piece of a Geranium-(_Pelargo´nium_-) leaf (Pl. I. fig. 1). These lines arise from the existence of passages between the cells, containing 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 to their form, as vesicles, either rounded or altered in shape by mutual pressure. We have now to notice the matters contained within the cells, or the cell-contents. _Cell-contents._--In most cells, especially when young, a minute, rounded, colourless body may be seen, either in the middle or on one side, called the _nucleus_; this is very distinct in a cell of the pulp of an apple (Pl. I. fig. 2 _b_). And within this nucleus is often to be seen another smaller body, frequently appearing as a mere dot, called the _nucle´olus_.