eadily known by
the appearance of the bark of the trunk, due to the many varieties of
surface caused by the allowance for growth. None of the characteristics
of trees afford a better opportunity for careful observation and study
than the outer bark.

The Birches have bark that peels off in thin horizontal layers--the
color, thinness, and toughness differing in the different species; the
Ashes have bark which opens in many irregular, netted cracks moderately
near each other; the bark of the Chestnut opens in large longitudinal
cracks quite distant from one another. The color of the bark and the
character of the scales are quite different in the White and the Black
Oaks.

In the woody portion radiating lines may be seen; these are the _silver
grain_; they are called by the botanist _medullary rays_.

The central portion of the wood of many large stems is darker in color
than the rest. This darker portion is dead wood, and is called
_heart-wood_; the outer portion, called _sap-wood_, is used in carrying
the sap during the growing season. The heart-wood of the Walnut-tree is
very dark brown; that of the Cherry, light red; and that of the Holly,
white and ivory-like. The heart-wood is the valuable part for lumber.

If examined under a magnifying glass, the _annual layers_ will be seen
to consist of minute tubes or cells. In most trees these tubes are much
larger in the portion that grew early in the season, while the wood
seems almost solid near the close of the annual layer; this is
especially true in the Ashes and the Chestnut; some trees, however, show
but little change in the size of the cells, the Beech being a good
example. In a cross-section, the age of such trees as the Chestnut can
readily be estimated, while in the Beech it is quite difficult to do
this. Boxwood, changing least in the character of its structure, is the
one always used for first-grade wood-engravings.

When wood is cut in the direction of the silver grain, or cut
"quartering" as it is called by the lumbermen, the surface shows this
cellular material spread out in strange blotches characteristic of the
different kinds of wood. Fig. 16 shows an Oak where the blotches of
medullary rays are large. In the Beech the blotches are smaller; in the
Elm quite small. Lumber cut carefully in this way is said to be
"quartered," and with most species its beauty is thereby much increased.

Any one who studies the matter carefully can become acquainted with al