ew
cells of Locust-bark, a crystal in each. 470. A detached cell, with
rhaphides being forced out, as happens when put in water.]

421. Besides these cell-contents which are in obvious and essential
relation to nutrition, there are others the use of which is
problematical. Of such the commonest are

422. =Crystals.= These when slender or needle-shaped are called
RHAPHIDES. They are of inorganic matter, usually of oxalate or phosphate
or sulphate of lime. Some, at least of the latter, may be direct
crystallizations of what is taken in dissolved in the water absorbed,
but others must be the result of some elaboration in the plant. Some
plants have hardly any; others abound in them, especially in the foliage
and bark. In Locust-bark almost every cell holds a crystal; so that in a
square inch not thicker than writing-paper there may be over a million
and a half of them. When needle-shaped (rhaphides), as in stalks of
Calla-Lily, Rhubarb, or Four-o'clock, they are usually packed in
sheaf-like bundles. (Fig. 465, 466.)

Sec. 3. ANATOMY OF ROOTS AND STEMS.

423. This is so nearly the same that an account of the internal
structure of stems may serve for the root also.

424. At the beginning, either in the embryo or in an incipient shoot
from a bud, the whole stem is of tender cellular tissue or parenchyma.
But wood (consisting of wood-cells and ducts or vessels) begins to be
formed in the earliest growth; and is from the first arranged in two
ways, making two general kinds of wood. The difference is obvious even
in herbs, but is more conspicuous in the enduring stems of shrubs and
trees.

425. On one or the other of these two types the stems of all
phanerogamous plants are constructed. In one, the wood is made up of
separate threads, scattered here and there throughout the whole diameter
of the stem. In the other, the wood is all collected to form a layer (in
a slice across the stem appearing as a ring) between a central cellular
part which has none in it, the _Pith_, and an outer cellular part, the
_Bark_.

[Illustration: Fig. 471. Diagram of structure of Palm or Yucca. 472.
Structure of a Corn stalk, in transverse and longitudinal section. 473.
Same of a small Palm stem. The dots on the cross sections represent cut
ends of the woody bundles or threads.]

426. An Asparagus-shoot and a Corn-stalk for herbs, and a rattan for a
woody kind, represent the first kind. To it belong all plants with
monocotyledonous embryo