But the name was suggested by, and first used only for, cells in
combination or built up into a fabric, much as a wall is built of
bricks, that is, into a

401. =Cellular Structure or Tissue.= Suppose numerous cells like those
of Fig. 437 to be heaped up like a pile of cannon-balls, and as they
grew, to be compacted together while soft and yielding; they would
flatten where they touched, and each sphere, being touched by twelve
surrounding ones would become twelve-sided. Fig. 438 would represent one
of them. Suppose the contiguous faces to be united into one wall or
partition between adjacent cavities, and a _cellular structure_ would be
formed, like that shown in Fig. 439. Roots, stems, leaves, and the whole
of phanerogamous plants are a fabric of countless numbers of such cells.
No such exact regularity in size and shape is ever actually found; but a
nearly truthful magnified view of a small portion of a slice of the
flower-stalk of a Calla Lily (Fig. 440) shows a fairly corresponding
structure; except that, owing to the great air-spaces of the interior,
the fabric may be likened rather to a stack of chimneys than to a solid
fabric. In young and partly transparent parts one may discern the
cellular structure by looking down directly on the surface, as of a
forming root. (Fig. 82, 441, 442).

[Illustration: Fig. 438. Diagram of a vegetable cell, such as it would
be if when spherical it were equally pressed by similar surrounding
cells in a heap.]

[Illustration: Fig. 439. Ideal construction of cellular tissue so
formed, in section.]

[Illustration: Fig. 440. Magnified view of a portion of a transverse
slice of stem of Calla Lily. The great spaces are tubular air-channels
built up by the cells.]

402. The substance of which cell-walls are mainly composed is called
CELLULOSE. It is essentially the same in the stem of a delicate leaf or
petal and in the wood of an Oak, except that in the latter the walls are
much thickened and the calibre small. The protoplasm of each living
cell appears to be completely shut up and isolated in its shell of
cellulose; but microscopic investigation has brought to view, in many
cases, minute threads of protoplasm which here and there traverse the
cell-wall through minute pores, thus connecting the living portion of
one cell with that of adjacent cells. (See Fig. 447, &c.)

[Illustration: Fig. 441. Much magnified small portion of young root of a
seedling Maple (such as of Fig. 82); a