nd 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).

[Illustration: Fig. 443. Magnified section through the thickness of a
leaf of Florida Star-Anise.]

406. All such soft cellular tissue, like this of leaves, that of pith,
and of the green bark, is called PARENCHYMA, while fibrous and woody
parts are composed of PROSENCHYMA, that is, of peculiarly transformed

407. =Strengthening Cells.= Common cellular tissue, which makes up the
whole structure of all very young plants, and the whole of Mosses and
other vegetables of the lowest grade, even when full grown, is too
tender or too brittle to give needful strength and toughness for plants
which are to rise to any considerable height and support themselves. In
these needful strength is imparted, and the conveyance of sap through
the plant is facilitated, by the change, as they are formed, of some
cells into thicker-walled and tougher tubes, and by the running together
of some of these, or the prolongation of others, into hollow fibres or
tubes of various size. Two sorts of such transformed cells go together,
and essentially form the

408. =Wood.= This is found in all common herbs, as well as in shrubs and
trees, but the former have much less of it in proportion to the softer
cellular tissue. It is formed very early in the growth of the root,
stem, and leaves,--traces of it appearing in large embryos even while
yet in the seed. Those cells that lengthen, and at the same time thicken
their walls form the proper WOODY FIBRE or WOOD-CELLS; those of