es, the annual growths are more or less distinctly marked, in the
form of concentric rings on the cross section, so that the age of the
tree may be known by counting them. Over twelve hundred layers have been
counted on the stumps of Sequoias in California, and it is probable that
some trees now living antedate the Christian era.

435. The reason why the annual growths are distinguishable is, that the
wood formed at the beginning of the season is more or less different in
the size or character of the cells from that of the close. In Oak,
Chestnut, etc., the first wood of the season abounds in dotted ducts,
the calibre of which is many times greater than that of the proper
wood-cells.

436. =Sap-wood, or Alburnum.= This is the newer wood, living or recently
alive, and taking part in the conveyance of sap. Sooner or later, each
layer, as it becomes more and more deeply covered by the newer ones and
farther from the region of growth, is converted into

437. =Heart-wood, or Duramen.= This is drier, harder, more solid, and
much more durable as timber, than sap-wood. It is generally of a
different color, and it exhibits in different species the hue peculiar
to each, such as reddish in Red-Cedar, brown in Black-Walnut, black in
Ebony, etc. The change of sap-wood into heart-wood results from the
thickening of the walls of the wood-cells by the deposition of hard
matter, lining the tubes and diminishing their calibre; and by the
deposition of a vegetable coloring-matter peculiar to each species. The
heart-wood, being no longer a living part, may decay, and often does so,
without the least injury to the tree, except by diminishing the strength
of the trunk, and so rendering it more liable to be overthrown.

438. =The Living Parts of a Tree=, of the exogenous kind, are only
these: first, the rootlets at one extremity; second, the buds and leaves
of the season at the other; and third, a zone consisting of the newest
wood and the newest bark, connecting the rootlets with the buds or
leaves, however widely separated these may be,--in the tallest trees
from two to four hundred feet apart. And these parts of the tree are all
renewed every year. No wonder, therefore, that trees may live so long,
since they annually reproduce everything that is essential to their life
and growth, and since only a very small part of their bulk is alive at
once. The tree survives, but nothing now living has been so long. In it,
as elsewhere, life is a t