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The Structure And Requirements Of Trees




To be able fully to appreciate trees, their mode of life,

their enemies and their care, one must know something of

their structure and life requirements.



Structure of trees: Among the lower forms of plants there is very little

distinction between the various parts--no differentiation into root,

stem, or crown. Often the lower forms of animal and vegetable life

are so similar that one cannot discriminate between them. But as we

ascend in the scale, the various plant forms become more and more

complex until we reach the tree, which is the largest and highest

form of all plants. The tree is a living organism composed of cells

like any other living organism. It has many parts, every one of

which has a definite purpose. The three principal parts are: the

stem, the crown, and the root.



The stem: If we examine the cross-section of a tree, Fig. 86, we will

notice that it is made up of numerous rings arranged in sections of

different color and structure. The central part is known as the

_pith_. Around the pith comes a dark, close-grained series of rings

known as the _heartwood_, and outside the heartwood comes a lighter

layer, the _sapwood_. The _cambium layer_ surrounds the sapwood and

the _bark_ covers all. The cambium layer is the most important

tissue of the tree and, together with part of the sapwood,

transports the water and food of the tree. It is for this reason

that a tree may be hollow, without heart and sapwood, and still

produce foliage and fruit.






The crown: The crown varies in form in different species and is

developed by the growth of new shoots from buds. The bud grows out

to a certain length and forms the branch. Afterwards it thickens

only and does not increase in length. New branches will then form

from other buds on the same branch. This explains in part the

characteristic branching of trees, Fig. 87.






The leaves are the stomach and lungs of the tree. Their broad

blades are a device to catch the sunlight which is needed in the

process of digesting the food of the tree. The leaves are arranged

on the twigs in such a way as to catch the most sunlight. The leaves

take up the carbonic acid gas from the air, decompose it under the

influence of light and combine it with the minerals and water

brought up by the roots from the soil. The resulting chemical

combinations are the sugars and starches used by the cambium layer

in building up the body of the tree. A green pigment, _chlorophyll_,

in the leaf is the medium by which, with the aid of sunlight, the

sugars are manufactured.






The chlorophyll gives the leaf its green color, and this explains

why a tree pales when it is in a dying condition or when its life

processes are interfered with. The other colors of the leaf--the

reds, browns and yellows of the fall or spring--are due to other

pigments. These are angular crystals of different hues, which at

certain times of the year become more conspicuous than at others, a

phenomenon which explains the variation in the colors of the leaves

during the different seasons.



It is evident that a tree is greatly dependent upon its leaves for

the manufacture of food and one can, therefore, readily see why it

is important to prevent destruction of the leaves by insects or

through over-trimming.



The root: The root develops in much the same manner as the crown. Its

depth and spread will vary with the species but will also depend

somewhat upon the condition of the soil around it. A deep or a dry

soil will tend to develop a deep root, while a shallow or moist soil

will produce a shallow root, Fig. 88.



The numerous fine hairs which cover the roots serve the purpose of

taking up food and water from the soil, while the heavy roots help

to support the tree. The root-hairs are extremely tender, are easily

dried out when exposed to the sun and wind, and are apt to become

overheated when permitted to remain tightly packed for any length of

time. These considerations are of practical importance in the

planting of trees and in the application of fertilizers. It is these

fine rootlets far away from the trunk of the tree that have to be

fed, and all fertilizers must, therefore, be applied at points some

distance from the trunk and not close to it, where merely the large,

supporting roots are located. In the cultivation of trees the same

principle holds true.



Requirements of trees: Trees are dependent upon certain soil and

atmospheric conditions which influence their growth and development.



(1) Influence of moisture: The form of the tree and its growth and

structure depend greatly upon the supply of moisture. Botanists

have taken the moisture factor as the basis of classification and

have subdivided trees into those that grow in moist places

(_hydrophytes_), those that grow in medium soils (_mesophytes_), and

those that grow in dry places (_xerophytes_). Water is taken up by

the roots of the tree from the soil. The liquid absorbed by the

roots carries in solution the mineral salts--the food of the

tree--and no food can be taken up unless it is in solution. Much of

the water is used by the tree and an enormous amount is given off in

the process of evaporation.






These facts will explain some of the fundamental principles in the

care of trees. To a tree growing on a city street or on a lawn where

nature fails to supply the requisite amount of water, the latter

must be supplied artificially, especially during the hot summer

months, or else dead branches may result as seen in Fig. 89. Too

much thinning out of the crown causes excessive evaporation, and too

much cutting out in woodlands causes the soil to dry and the trees

to suffer for the want of moisture. This also explains why it is

essential, in wooded areas, to retain on the ground the fallen

leaves. In decomposing and mixing with the soil, the fallen leaves

not only supply the trees with food material, but also tend to

conserve moisture in the ground and to prevent the drying out of the

soil. Raking off the leaves from wooded areas, a practice common in

parks and on private estates--hurts the trees seriously. Some soils

may have plenty of moisture, but may also be so heavily saturated

with acids or salts that the tree cannot utilize the moisture, and

it suffers from drought just the same as if there had been no

moisture at all in the soil. Such soils are said to be

"physiologically dry" and need treatment.



In the development of disease, moisture is a contributing factor

and, therefore, in cavities or underneath bandages where there is

likely to be an accumulation of moisture, decay will do more damage

than in places that are dry and exposed to the sun.



(2) Influence of soil: Soil is made up of fine particles of sand and

rock and of vegetable matter called _humus_. A tree will require a

certain soil, and unsuitable soils can be very often modified to

suit the needs of the tree. A deep, moderately loose, sandy loam,

however, which is sufficiently aerated and well supplied with

water, will support almost any tree. Too much of any one constituent

will make a soil unfit for the production of trees. If too much clay

is present the soil becomes "stiff." If too much vegetable matter is

present, the soil becomes "sour." The physical character of the soil

is also important. By physical character is meant the porosity which

results from breaking up the soil. This is accomplished by ploughing

or cultivation. In nature, worms help to do this for the soil, but

on streets an occasional digging up of the soil about the base of

the tree is essential.



Humus or the organic matter in the soil is composed of litter,

leaves and animal ingredients that have decayed under the influence

of bacteria. The more vegetable matter in the humus, the darker the

soil; and therefore a good soil such as one finds on the upper

surface of a well-tilled farm has quite a dark color. When, however,

a soil contains an unusual quantity of humus, it is known as "muck,"

and when there is still more humus present we find _peat_. Neither

of these two soils is suitable for proper tree growth.








(3) Influence of light: Light is required by the leaves in the process

of assimilation. Cutting off some of the light from a tree affects

its form. This is why trees grown in the open have wide-spreading

crowns with branches starting near the ground as in Fig. 90, while

the same species growing in the forest produces tall, lanky trees,

free from branches to but a few feet from the top as in Fig. 91.

Some trees can endure more shade than others, but all will grow in

full light. This explains why trees like the beech, hemlock, sugar

maple, spruce, holly and dogwood can grow in the shade, while the

poplar, birch and willow require light. It also explains why, in

the forest, the lower branches die and fall off--a process known in

Forestry as "natural pruning," The influence of light on the form of

trees should be well understood by all those who plant trees and by

those designing landscape effects.








(4) Influence of heat: Trees require a certain amount of heat. They

receive it partly from the sun and partly from the soil. Evaporation

prevents the overheating of the crown. The main stem of the tree is

heated by water from the soil; therefore trees in the open begin

growth in the spring earlier than trees in the forest because the

soil in the open is warmer. Shrubs begin their growth earlier than

trees because of the nearness of their crowns to their root systems.

This also explains why a warm rain will start vegetation quickly.

Too much heat will naturally cause excessive drying of the roots or

excessive evaporation from the leaves and therefore more water is

needed by the tree in summer than in winter.



(5) Influence of season and frost: The life processes of a tree are

checked when the temperature sinks below a certain point. The tree

is thus, during the winter, in a period of rest and only a few

chemical changes take place which lead up to the starting of

vegetation. In eastern United States, growth starts in April and

ceases during the latter part of August or in early September. The

different parts of a tree may freeze solid during the winter without

injury, provided the tree is a native one. Exotic trees may suffer

greatly from extreme cold. This is one of the main reasons why it is

always advisable to plant native trees rather than those that are

imported and have not yet been acclimatized. Frosts during

mid-winter are not quite as injurious as early and late frosts and,

therefore, if one is going to protect plants from the winter's cold,

it is well to apply the covering early enough and to keep it on

late enough to overcome this difficulty.



The mechanical injuries from frost are also important. Snow and

sleet will weigh down branches but rarely break them, while frost

will cause them to become brittle and to break easily. Those who

climb and prune trees should be especially cautious on frosty days.



(6) Influence of air: On the under side of leaves and on other

surfaces of a tree little pores known as _stomata_ may be found. In

the bark of birch and cherry trees these openings are very

conspicuous and are there known as _lenticels_. These pores are

necessary for the breathing of the tree (respiration), whereby

carbonic acid gas is taken in from the air and oxygen given out. The

process of assimilation depends upon this breathing process and it

is therefore evident that when the stomata are clogged as may occur

where a tree is subjected to smoke or dust, the life processes of

the tree will be interfered with. The same injurious effect results

when the stomata of the roots are interfered with. Such interference

may occur in cases where a heavy layer of soil is piled around the

base of a tree, where the soil about the base of a tree is allowed

to become compact, where a tree is planted too deep, or where the

roots are submerged under water for any length of time. In any case

the air cannot get to the roots and the tree suffers. Nature takes

special cognizance of this important requirement in the case of

cypress trees, which habitually grow under water. Here the trees are

provided with special woody protuberances known as "cypress knees,"

which emerge above water and take the necessary air. See Fig. 18.



Conclusions: From the foregoing it will be seen that trees have certain

needs that nature or man must supply. These requirements differ

with the different species, and in all work of planting and care as

well as in the natural distribution of trees it is both interesting

and necessary to observe these individual wants, to select species

in accordance with local conditions and to care for trees in

conformity with their natural needs.



Tree Studies


How To Identify Trees
Group I The Pines
The White Pine (pinus Strobus)
The Pitch Pine (pinus Rigida)
The Scotch Pine (pinus Sylvestris)
Group Ii The Spruce And Hemlock
The Norway Spruce (picea Excelsa)
Hemlock (tsuga Canadensis)
Group Iii The Red Cedar And Arbor-vitae
Red Cedar (juniperus Virginiana)
Arbor-vitae; Northern White Cedar (thuja Occidentalis)
Group Iv The Larch And Cypress
The European Larch (larix Europaea)
Bald Cypress (taxodium Distichum)
Group V The Horsechestnut, Ash And Maple
The Horsechestnut
The White Ash (fraxinus Americana)
Sugar Maple (acer Saccharum)
Silver Maple (acer Saccharinum)
Red Maple (acer Rubrum)
Norway Maple (acer Platanoides)
Box Elder (acer Negundo)
Group Vi Trees Told By Their Form: Elm, Poplar, Gingko And Willow
American Elm (ulmus Americana)
Lombardy Or Italian Poplar (populus Nigra, Var Italica)
Gingko Or Maidenhair Tree (gingko Biloba)
Weeping Willow (salix Babylonica)
Group Vii Trees Told By Their Bark Or Trunk: Sycamore, Birch, Beech,
Blue Beech, Ironwood, And Hackberry
The Sycamore Or Plane Tree (platanus Occidentalis)
Gray Or White Birch (betula Populifolia)
American Beech (fagus Americana)
Blue Beech Or Hornbeam (carpinus Caroliniana)
Hackberry (celtis Occidentalis)
Group Viii The Oaks And Chestnut
White Oak (quercus Alba)
Black Oak (quercus Velutina)
Red Oak (quercus Rubra)
Pin Oak (quercus Palustris)
Chestnut (castanea Dentata)
Group Ix The Hickories, Walnut And Butternut
Shagbark Hickory (hicoria Ovata)
Mockernut Hickory (hicoria Alba)
Black Walnut (juglans Nigra)
Group X Tulip Tree, Sweet Gum, Linden, Magnolia, Locust, Catalpa,
Dogwood, Mulberry And Osage Orange
Tulip Tree (liriodendron Tulipifera)
Sweet Gum (liquidambar Styraciflua)
American Linden (tilia Americana)
The Magnolias