[Illustration: 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 a girdle will not stop the upward rise of the root water so long as the wood remains alive; but it will stop the passage of the elaborated or food-stored materials to parts below and thus starve those parts; and if the girdle does not heal over by the deposit of new bark, the tree will in time _starve to death_. It will now be seen that the common practice of placing wires or hoops about trees to hold them in position or to prevent branches from falling is irrational, because such wires interpose barriers over which the fluids cannot pass; in time, as the trunk increases in diameter, the wire girdles the tree. It is much better to bolt the parts together by rods extending through the branches (Fig. 85). These bolts should fit very tight in their holes. Why? [Illustration: FIG. 85.—THE WRONG WAY TO BRACE A TREE. (See Fig. 118).] =Wood.=—The main stem or trunk, and sometimes the larger branches, are the sources of lumber and timber. Different kinds of wood have value for their special qualities. The business of raising wood, for all purposes, is known as _forestry_. The forest is to be considered as a crop, and the crop must be harvested, as much as corn or rice is harvested. Man is often able to grow a more productive forest than nature does. =Resistance to decay= gives value to wood used for shingles (_cypress_, heart of _yellow pine_) and for fence posts (_mulberry_, _cedar_, _post oak_, _bois d’arc_, _mesquite_).