[Illustration: Fig. 464. Four cells from dried Onion-peel, each holding a crystal of different shape, one of them twinned. 465. Some cells from stalk of Rhubarb-plant, three containing chlorophyll; two (one torn across) with rhaphides. 466. Rhaphides in a cell, from Arisæma, with small cells surrounding. 467. Prismatic crystals from the bark of Hickory. 468. Glomerate crystal in a cell, from Beet-root. 469. A few 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.) § 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.]