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• times the walls of parenchyma cells in the cortex thicken

at the corners and form brace cells (Fig. 83) (collenchyma) for support; sometimes the whole wall is thickened, forming grit cells or stone cells (Fig. 84; examples in tough parts of pear, or in stone of fruits). Some parts serve for secretions (milk, rosin, etc.) and are called latex tubes.

Fig. 83.—Collenchyma in Wild Jewelweed or Touch-me-not (Impatiens).

Fig. 84.—Grit Cells.

The outer bark of old shoots consists of corky cells that protect from mechanical injury, and that contain a fatty substance (suberin) impermeable to water and of service to keep in moisture. There is sometimes a cork cambium (or phellogen) in the bark that serves to extend the bark and keep it from splitting, thus increasing its power to protect.

Transport of the "Sap."—We shall soon learn that the common word "sap" does not represent a single or simple substance. We may roughly distinguish two kinds of more or less fluid contents: (1) the root water, sometimes called mineral sap, that is taken in by the root, containing its freight of such inorganic substances as potassium, calcium, iron, and the rest; this root water rises, we have found, in the wood vessels,—that is, in the young or "sapwood" (p. 96); (2) the elaborated or organized materials passing back and forth, especially from the leaves, to build up tissues in all parts of the plant, some of it going down to the roots and root-hairs; this organic material is transported, as we have learned, in the sieve-tubes of the inner bast,—that is, in the "inner bark." Removing the bark from a trunk in