[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.)
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!
405. Some parts are built up as a compact structure; in others cells are arranged so as to build up regular air-channels, as in the stems of aquatic and other water-loving plants (Fig. [440]), or to leave irregular spaces, as in the lower part of most leaves, where the cells only here and there come into close contact (Fig. [443]).
Fig. 443. Magnified section through the thickness of a leaf of Florida Star-Anise.