FORESTS.

JOHN M. COULTER, Ph.D.

_Head Professor of Botany, University of Chicago._

Forests have always been admired, and in ancient times they were often

considered sacred, the special dwelling-places of gods and various

strange beings. We can easily understand how forests thus affected

men. There is a solemnity about them, a quiet grandeur, which is very

impressive, and the rustling of their branches and leaves has that

mysterious sound which caused the ancients to people them with spirits.

We still recognize the feeling of awe that comes in the presence of

forests, although we have long since ceased to explain it by peopling

them with spirits.

Once forests covered all parts of the earth where plants could grow

well, and no country had greater forests than North America. When

America was discovered, there was a huge, unbroken forest from the

Atlantic west to the prairies. Now much of this has been cut away, and

we see only small patches of it. Men must use the forest, and still

they must save it, and they are now trying to find out how they may do

both.

Forests are sometimes almost entirely made up of one kind of tree,

and then they are called "pure forests." Pine and beech forests are

examples of this kind. More common with us, however, are the "mixed

forests," made up of many kinds of trees, and nowhere in the world are

there such mixed forests as in our Middle States, where beech, oak,

hickory, maple, elm, poplar, gum, walnut, sycamore, and many others

all grow together.

Probably the densest forests in the world are those in the Amazon

region of South America. So dense are they that hardly a ray of light

ever sifts through the dense foliage, and even at noon there is only a

dim twilight beneath the trees. The tallest forests are the Eucalyptus

forests of Australia, where the trees rise with slender trunks to

the height of four or five hundred feet. But the largest trees in

the world, when we consider both height and diameter, are the giant

"redwoods" (Sequoias) of the Pacific coast. All concede, however, that

the most extensive, the most varied, and the most beautiful forests of

the world are those of the Atlantic and Middle States.

Perhaps it is well to understand how a tree lives, that we may know

better what a forest means. The great roots spread through the soil,

sometimes not far from the surface, at other times penetrating deeply.

The young root tips are very sensitive to the presence of moisture,

and turn towards it, no matter in what direction it may carry them.

In penetrating the soil the sensitive root tips are turned in every

direction by various influences of this kind, and as a result, when

the root system becomes old, it looks like an inextricable tangle. All

this tangle, however, but represents the many paths that the root tips

followed in their search for the things which the soil contains.

Roots are doing two things for the tree: They anchor it firmly in the

soil, and also absorb material that is to help in the manufacture of

food. It is the older roots that have long since stopped absorbing that

are the chief anchors. How firm this anchorage must be we can, perhaps,

imagine when we think of the strain produced by a great crown of leaves

swaying back and forth in the wind. It is only a cyclone that seems to

be able to overthrow a sound tree, and then it more commonly breaks its

trunk than uproots it.

The very important work of absorbing is given over to the very young

roots; in fact, chiefly to those of this year, for new rootlets must be

put out each year. These roots can only absorb water, so that if they

are to get anything from the soil it must be something that water will

dissolve. In this way the water is used as the carrier of soil-material

into the root. Just how this water carrying soil-material gets into the

root is not easy to explain, for the root has no holes to let it in,

and it must pass through living walls. That it does enter, however,

every one knows. It is evident, therefore, that the root is supplying

to the tree two kinds of raw material for food manufacture obtained

from the soil, namely, water and soil-material dissolved in it.

But the tree does not obtain all its raw material from the soil. A

very important material is taken from the air, the material commonly

called "carbonic acid gas," the same material that we breathe out so

abundantly from our lungs as one of our body wastes. This important

material is taken out of the air into the plant chiefly by means of the

leaves. Spread out as they are in the air, the leaves are in the most

favorable position for doing this work.

But where and how are these three kinds of raw material manufactured

into plant food? The leaves are specially constructed to be the

chief seat of this food manufacture. The carbon gas is received

directly into these manufactories from the air, but the water and

the soil-material are down in the roots, and it is necessary for

them to be carried to the leaves. As a consequence, a "current" of

water containing soil-material ascends from the roots, through the

stem, and is distributed through the branches to the leaves. This

movement is generally known as the "ascent of sap." The path of this

movement in the stem is through what is known as the "sap wood,"

and it is this very fact which gives to this region of the wood its

peculiar character. Just how the sap ascends through the stem and

reaches the leaves, no one knows. All of our explanations have proved

unsatisfactory, and only those who are not fully acquainted with the

facts claim to be able to explain it.

When the sap reaches the leaves, the water is no longer needed as a

carrier of soil-material. Some of it is needed in the manufacture of

food, but by far the greater part of it escapes from the leaves into

the air by a process which may be called "plant evaporation." The

amount of water thus brought from the soil and poured out into the

air by active plants is very great; and when we consider a forest at

work, we can hardly compute the vast amount of moisture which it is

constantly contributing to the air during the growing season.

The three kinds of raw material thus brought together chiefly in the

leaves are there manufactured into plant food. On account of this work

the leaves have often been spoken of as the "stomachs" of the plant.

This is a very incorrect and misleading illustration, for the work

referred to is not digestion such as a stomach is concerned with, and,

in fact, it is a process entirely unknown in animals, and found only

in green plants. It is a wonderful process, which we do not at all

understand, but it consists in taking this dead raw material from soil

and air and manufacturing out of it living material. Not only does the

food of the plant, and hence its life, depend upon this process, but

all the life of the world, as we understand it, depends upon it. We

know at least two prominent conditions of this process, for it seems

evident that it cannot take place without light and the peculiar green

substance which gives the characteristic color to leaves. With the help

of light and this green coloring substance, known as "chlorophyll," the

living substance in the leaves is able to do this marvelous work.

The food thus manufactured is distributed throughout the tree, either

to be used wherever growth is going on, or to be stored up. While we

may say that there is an "ascending current" of sap through the sap

wood, it is an error to say that there is a "descending current." The

movement of prepared food has no definite channel, but it is drawn in

every direction wherever needed.

If we now consider the parts of a tree all together, we may be able

to get some notion of the meaning of their positions. The roots must

be related to the soil to secure anchorage and raw material for food

manufacture. The leaves must be related to the air and light to secure

more raw material and help in doing their important work of food

manufacture. The stem is simply to carry the leaves well up into the

air and sunshine, and has no meaning except as it is related to the

work of the leaves. In thus widely separating the roots and the leaves,

the stem must act as a channel of communication between them.

In the tree trunks with which we are familiar, everyone has observed

the concentric rings of wood that appear in a cross-section. These are

usually spoken of as "annual rings," with the idea that one ring is

made each year. In consequence of this it is the habit to estimate the

age of a tree by counting these rings. Not infrequently it happens,

however, that more than one ring may be made in a year, as a ring

represents a single season of growth, and there may be more than one

season of growth during a single year. It is much better to call them

"growth rings," and to recognize the fact that by counting them we may

be overestimating the age of a tree.

One of the most noticeable things about the principal trees of our

temperate climate is that they "shed" their leaves every year, being

covered with foliage during the growing season and bare during the

winter. This is known as the "deciduous" habit, and such trees are

called deciduous trees, in distinction from "evergreen" trees. This

is really a habit, brought about by the conditions in which trees of

temperate climates must live. The leaves of such trees are broad and

thin, fitted for very active work. When the winter comes, they would

be entirely unable to endure it. The tree might protect them by giving

them narrow forms and thick walls (as in pines), but it would be at the

expense of activity during the growing season. It is more economical

for the tree to make an entirely new set of leaves each year than to

protect the old ones.

Perhaps the most noticeable feature in connection with the fall of the

leaves is that so many of them take on a rich coloration. Our mixed

American forest is the most brilliantly colored autumnal forest in the

world, and there can be no landscapes richer in color than those which

include such a forest. While all this should appeal to our sense of

the beautiful, it should raise the question as to what it means in the

life of the trees. We are not at all sure that we know, for we cannot

as yet explain the conditions which cause the colors to be produced. We

observe that they occur towards the end of the activity of the leaf,

but that they are necessarily associated with cold, or drought, or

certain outside conditions, is not at all clear. The colors are various

shades of red and yellow, sometimes pure, sometimes mixed. It has been

recently suggested that the red color is to serve as a protection. It

is known that before the fall of the leaf the living substances are

gradually withdrawn into the permanent parts of the tree, and that when

these living parts cease to work they are peculiarly helpless. At this

unprotected period the red appears, and this color absorbs enough heat

from the light to raise the temperature, and so the needed protection

against chill is afforded. This seems reasonable, but the whole subject

of the meaning of plant colors is very obscure.

* * * * *

Gen. Robert E. Lee was a great lover of forest trees. He owned a large

and beautiful forest in northern Virginia at the time of the War of the

Rebellion. While the army of Virginia was encamped near Fredericksburg,

he was gazing at the great forest trees that beautified a homestead

near by, the property of his companion. This companion quotes him as

saying on this occasion: "There is nothing in vegetable nature so grand

as a tree. Grappling with its roots the granite foundations of the

everlasting hills, it reaches its sturdy and gnarled trunk on high,

spreads its branches to the heavens, casts its shadow on the sward; and

the birds build their nests and sing amid its umbrageous branches."