od of trees which form no heartwood
changes but little, except when stained by forerunners of disease.

The different tints of colors, whether the brown of oak, the orange
brown of pine, the blackish tint of walnut, or the reddish cast of
cedar, are due to pigments, while the deeper shade of the summer-wood
bands in pine, cedar, oak, or walnut is due to the fact that the wood
being denser, more of the colored wood substance occurs on a given
space, _i.e._, there is more colored matter per square inch. Wood is
translucent, a thin disk of pine permitting light to pass through
quite freely. This translucency affects the luster and brightness of
lumber.

When lumber is attacked by fungi, it becomes more opaque, loses its
brightness, and in practice is designated "dead," in distinction to
"live" or bright timber. Exposure to air darkens all wood; direct
sunlight and occasional moistening hasten this change, and cause it to
penetrate deeper. Prolonged immersion has the same effect, pine wood
becoming a dark gray, while oak changes to a blackish brown.

Odor, like color, depends on chemical compounds, forming no part of
the wood substance itself. Exposure to weather reduces and often
changes the odor, but a piece of long-leaf pine, cedar, or camphor
wood exhales apparently as much odor as ever when a new surface is
exposed. Heartwood is more odoriferous than sapwood. Many kinds of
wood are distinguished by strong and peculiar odors. This is
especially the case with camphor, cedar, pine, oak, and mahogany, and
the list would comprise every kind of wood in use were our sense of
smell developed in keeping with its importance.

Decomposition is usually accompanied by pronounced odors. Decaying
poplar emits a disagreeable odor, while red oak often becomes
fragrant, its smell resembling that of heliotrope.

WEIGHT OF WOOD

A small cross-section of wood (as in Fig. 19) dropped into water
sinks, showing that the substance of which wood fibre or wood is built
up is heavier than water. By immersing the wood successively in
heavier liquids, until we find a liquid in which it does not sink, and
comparing the weight of the same with water, we find that wood
substance is about 1.6 times as heavy as water, and that this is as
true of poplar as of oak or pine.

[Illustration: Fig. 19. Cross-section of a Group of Wood
Fibres (Highly Magnified.)]

Separating a single cell (as shown in Fig. 20, _a_