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?
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).
Hardness and strength are qualities of great value in building. Live oak is used in ships. Red oak, rock maple, and yellow pine are used for floors. The best flooring is sawn with the straight edges of the annual rings upward; tangential sawn flooring may splinter. Chestnut is common in some parts of the country, being used for ceiling and inexpensive finishing and furniture. Locust and bois d’arc (osage orange) are used for hubs of wheels; bois d’arc makes a remarkably durable pavement for streets. Ebony is a tropical wood used for flutes, black piano keys, and fancy articles. Ash is straight and elastic; it is used for handles for light implements. Hickory is very strong as well as elastic, and is superior to ash for handles, spokes, and other uses where strength is wanted. Hickory is never sawn into lumber, but is split or turned. The “second growth,” which sprouts from stumps, is most useful, as it splits readily. Fast-growing hickory in rich land is most valuable. The supply of useful hickory is being rapidly exhausted.
Softness is often important. White pine and sweet gum because of their softness and lightness are useful in box-making. “Georgia” or southern pine is harder and stronger than white pine; it is much used for floors, ceilings, and some kinds of cabinet work. White pine is used for window-sash, doors, and moulding, and cheaper grades are used for flooring. Hemlock is the prevailing lumber in the east for the framework and clapboarding of buildings. Redwood and Douglas spruce are common building materials on the Pacific coast. Cypress is soft and resists decay and is superior to white pine for sash, doors, and posts on the outside of houses. Cedar is readily carved and has a unique use in the making of chests for clothes, as its odour repels moths and other insects. Willow is useful for baskets and light furniture. Basswood or linden is used for light ceiling and sometimes for cheap floors. Whitewood (incorrectly called poplar) is employed for wagon bodies and often for house finishing. It often resembles curly maple.