The Washington times., 1901-07-07


first year. Thus it is only by means of
these youngest twigs that a tree in
creases in height and in spread of
branches. After the first year their
length is fixed, younger twigs stretch
out from the buds, and the older ones
grow henceforth in thickness. A fresh
coat of new material covers them year
by year. There are two layers in this
coat, separated by a third one of ten
der forming tissues called the cambium,
in which the actual making of the new
substance goes on The inner side of
the cambium layer forms new wood,
the outer side new bark. Besides the
true cambium, which forms both wood
and bark, there is another cambium
which makes the corky outer bark, and
nothing else. This cork cambium may
encircle the whole tree, like the true
cambium, as in the red cedar, or it may
form little separate films in the bark,
but in either case it dies from time TO
time, and is reformed nearer the wood
THE STRUCTURE OF WOOD.
Wood is chiefly made up of very
small tubes or cells of various kinds,
which have special uses in the life Of
the tree. Some conduct water from the
roots TO the crown, some store away di-
gested food, and others merely
strengthen the structure of the wood
and hold it together. At times the
cells have thick walls and small open
ings, and at other times wide openings
and very thin walls. a fair idea of
how these cells look can be had by
comparing a cross section of wood
with a cross section of ordinary honey
comb.
In climates which have regularly one
season of growth and one of rest, like
that which exists in this section, the
cells of the layer of new wood formed
each year at the inner surface of the
cambium are arranged in a definite way
When the growth begins in the spring,
and the fresh twigs and leaves put out
there is great demand for water in the
crown TO supply these moist green new
parts Of the tree. Water rises in most
trees through the newer layers of the
wood. and especially through the last
ring. Consequently, at first the tree
openings. through which water can rise
rapidly to the ends of the branches.
Later on. when the demand for water
is not so great, and there is plenty of
digested food to supply building ma
terial. the cells formed are narrow and
thick walled. Thus the summer wood
in each years growth is heavier,
stronger, and darker in color than the
spring wood.
THE ANNUAL RINGS DESCRIBED.
1t is correct TO speak of these rings
of growth as annual rings" for as
long as the tree is growing healthily d
ring is formed each year. 1t is true
that two false rings may appear in one
year. but they are generally so much
thinner than the rings on each side
that it is not hard to detect them. Very
often they do not extend entirely
around the tree. as a true ring always
does if the tree is sound. Whenever
the growth of a tree is interrupted and
begins again during the same season,
such false rirg is formed. This hap
pens when the foliage is destroyed by
caterpillars and grows again in the
same season. or when a very severe
drought in early summer stops growth
for d time, after late frosts, etc.
An annual layer once formed does
not change IN size or place during the
healthy life Of the tree, except that it
is covered in time by other younger
layers. A nail driven into a tree six
feet from the ground will still be at
the same height after it is buried under
twenty or fifty or one hundred layers
Of annual growth. From living sap
wood, in which growth is going on, it
becomes heartwood, which is dead, be-
cause it has .otbing to do with growth
1t is simply .rong framework which
helps To support the living parts of the
tree. This is why hollow trees can
flourish and bear fruit. Sapwood rots
more easily than heartwood, because it
takes up water readily and contains
plant food. which decays very fast
Not all trees have heartwood, and in
many the difference in color between 1t
and sapwood IS very slight Since
water from the roots rises only in the
sapwood. it is easy to kill trees with
heartwood by girdling them, provided
all the sapwood is cut through But in
those which have no heartwood the
tubes of the older layers of wood can
still convey water to the grown, and
when such trees are glrdled it is often
several years before the tree dies.
THE NEED OF HEAT AND LIGHT.
The two main requirements for the
healthy growth of us are heat and
light. The latter seems to be the most
necessary. This is so we'll recognized
that foresters have divided trees into
tolerant and intolerant according to
the ability of various trees to thrive
under shade. Some trees cannot en-
dure shade, and die early from lack of
sufficient light Others can thrive well
under partial shade, but in each case
all trees thrive best in full daylight,
and a constant struggle goes on be-
tween the individual trees of a forest
to reach the light.
1t is curious fact that trees give out
heat. To be sure, it is scarcely appre-
ciable. but is nevertheless true. This is
shown after a heavy fall of snow.
When this occurs, it will be noticed
that for number of inches to foot
or more Immediately around the foot
Of the tree there viii be a ring of bare
ground entirely denuded of snow.
which has been melted by the heat