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); and 442, a few cells of same more magnified. The prolongations from the back of some of the cells are root hairs.] 403. The hairs of plants are cells formed on the surface; either elongated single cells (like the root-hairs of Fig. 441, 442), or a row of shorter cells. Cotton fibres are long and simple cells growing from the surface of the seed. 404. The size of the cells of which common plants are made up varies from about the thirtieth to the thousandth of an inch in diameter. An ordinary size of short or roundish cells is from 1/300 to 1/500 of an inch; so that there may generally be from 27 to 125 millions of cells in the compass of a cubic inch!