nt cells, which by
continued multiplication build up an embryo.]

399. =Cell-walls.= While this growth was going on, each grain of the
forming structure formed and clothed itself with a coat, thin and
transparent, of something different from protoplasm,--something which
hardly and only transiently, if at all, partakes of the life and action.
The protoplasm forms the living organism; the coat is a kind of
protective covering or shell. The protoplasm, like the flesh of animals
which it gives rise to, is composed of four chemical elements: Carbon,
Hydrogen, Oxygen, and Nitrogen. The coating is of the nature of wood
(is, indeed, that which makes wood), and has only the three elements,
Carbon, Hydrogen, and Oxygen, in its composition.

[Illustration: Fig. 437. Magnified view of some of a simple fresh water
Alga, the Tetraspora lubrica, each sphere of which may answer to an
individual plant.]

400. Although the forming structure of an embryo in the fertilized ovule
is very minute and difficult to see, there are many simple plants of
lowest grade, abounding in pools of water, which more readily show the
earlier stages or simplest states of plant-growth. One of these, which
is common in early spring, requires only moderate magnifying power to
bring to view what is shown in Fig. 437. In a slimy mass which holds all
loosely together, little spheres of green vegetable matter are seen,
assembled in fours, and these fours themselves in clusters of fours. A
transient inspection shows, what prolonged watching would confirm, that
each sphere divides first in one plane, then in the other, to make four,
soon acquiring the size of the original, and so on, producing successive
groups of fours. These pellets each form on their surface a transparent
wall, like that just described. The delicate wall is for some time
capable of expansive growth, but is from the first much firmer than the
protoplasm within; through it the latter imbibes surrounding moisture,
which becomes a watery sap, occupying vacuities in the protoplasmic mass
which enlarge or run together as the periphery increases and distends.
When full grown the protoplasm may become a mere lining to the wall, or
some of it central, as a nucleus, this usually connected with the
wall-lining by delicate threads of the same substance. So, when full
grown, the wall with its lining--a vesicle, containing liquid or some
solid matters and in age mostly air--naturally came to be named a Cell.