oak.

The red and black gums are perhaps the simplest of the hardwoods in
structure. They are termed "diffuse porous" woods because of the
numerous scattered pores they contain. They have only vessels, wood
fibres, and a few parenchyma cells. The medullary rays, although
present, are scarcely visible in most instances. The vessels are in
many cases open, and might be expected to offer relatively little
resistance to drying.

Properties of Wood

Certain general properties of wood may be discussed briefly. We know
that wood substance has the property of taking in moisture from the
air until some balance is reached between the humidity of the air and
the moisture in the wood. This moisture which goes into the cell walls
hygroscopic moisture, and the property which the wood substance has of
taking on hygroscopic moisture is termed hygroscopicity. Usually wood
contains not only hygroscopic moisture but also more or less free
water in the cell cavities. Especially is this true of sapwood. The
free water usually dries out quite rapidly with little or no shrinkage
or other physical change.

In certain woods--for example, _Eucalyptus globulus_ and possibly some
oaks--shrinkage begins almost at once, thus introducing a factor at
the very start of the seasoning process which makes these woods very
refractory.

The cell walls of some species, including the two already mentioned,
such as Western red cedar and redwood, become soft and plastic when
hot and moist. If the fibres are hot enough and very wet, they are not
strong enough to withstand the resulting force of the atmospheric
pressure and the tensile force exerted by the departing free water,
and the result is that the cells actually collapse.

In general, however, the hygroscopic moisture necessary to saturate
the cell walls is termed the "fibre saturation point." This amount has
been found to be from 25 to 30 per cent of the dry wood weight. Unlike
_Eucalyptus globulus_ and certain oaks, the gums do not begin to
shrink until the moisture content has been reduced to about 30 per
cent of the dry wood weight. These woods are not subject to collapse,
although their fibres become very plastic while hot and moist.

Upon the peculiar properties of each wood depends the difficulty or
ease of the seasoning process.

Classes of Trees

The timber of the United States is furnished by three well-defined
classes of trees