48 VARIETIES OF PARENCHYMA, 
stellate cells (figs. 67 and 93), or of cells with an irregular out- 
line produced by projecting rays, and in contact only by the 
extremities of such rays, so as to leave large irregular spaces 
between them (fig. 124, c). This occurs commonly in the tissue 
on the under surface of most leaves ; and frequently in the air- 
passages of plants, particularly in the stems and leaf-stalks of 
such as grow in water, or in marshy places, e.g. the Rush and 
Water-lily. 
ce. Regular or Polyhedral Parenchyma.—This is formed of 
polyhedral cells, the faces of which are frequently nearly equal 
(figs. 65 and 66), and so combined as to leave no interspaces. 
It is commonly found in the pith of plants. 
d. Elongated Parenchyma.—This is composed of cells elon- 
gated in a longitudinal direction so as to become fusiform (fig. 
70), cylindrical (fig. 69), or prismatic, and closely compacted. 
It occurs frequently in the stems of Monocotyledonous plants. 
e. Tabular Parenchyma is that which consists of tabular, 
closely adherent cells. It is found in the epidermis and other 
external parts of plants (figs. 68, 95, e, and 125-125). A variety 
of this kind of parenchyma is called muriform, because the cells 
of which it is composed resemble in their form and arrangement 
the courses of bricks in a wall (fig. 94); this variety occurs in 
the medullary rays of the stems of Dicotyledons. 
Such are the commoner varieties of parenchyma, all of which 
are connected in various ways by transitional forms ; but other 
special kinds also occur. Thus, in the tissue which is placed 
below the epidermis of plants, which has been termed the 
hypoderma, we sometimes find the parenchyma composed of 
cells which are especially thickened at their angles (fig. 95, 
cl, cl); and these thickened portions swell up considerably 
when such cells are placed in water. This kind of parenchyma 
is called collenchyma; it never becomes lignified. Another 
variety of parenchyma is termed sclerenchyma ; this consists of 
cells which have become much hardened by thickening layers 
and lignified, as in the stem of Palms (see page 95). When the 
parenchymatous cells become thickened so as to form pitted or 
fibrous cells, the tissues formed by their combination constitute 
respectively the Pitted Cellular Tissue and Fibro-cellular Tissue, 
of some authors. 
In some of the lower orders of plants there is a kind of 
tissue present which is quite as distinct from parenchyma as 
this is from prosenchyma and the tissues formed by the vessels 
of plants. To this the names of Tela contextu and Interlacing 
fibrilliform Tissue have been given. It occurs in the Fungi 
(figs. 3 and 4), and Lichens (fig. 71), and consists of very long 
thread-like cells, or strings of cells, simple or branched, with 
either thin, soft, readily destructible walls, as in Fungi; or dry 
and firm ones, as in Lichens ; the whole inextricably inter- 
woven or entangled with each other so as to form a loose fibril- 
