THE ROOT—THE FORMS OF ROOTS
=The Root System.=—The offices of the root are _to hold the plant in
place_, and _to gather food_. Not all the food materials, however,
are gathered by the roots.
[Illustration: FIG. 32.—TAP-ROOT SYSTEM OF ALFALFA.]
[Illustration: FIG. 33.—TAP-ROOT OF THE DANDELION.]
The entire mass of roots of any plant is called its =root system=.
The root system may be annual, biennial or perennial, herbaceous or
woody, deep or shallow, large or small.
=Kinds of Roots.=—A strong leading central root, which runs directly
downwards, is a =tap-root=. The tap-root forms an axis from
which the side roots may branch. The side or spreading roots are
usually smaller. Plants that have such a root system are said to be
_tap-rooted_. Examples are red clover, alfalfa, beet, turnip, radish,
burdock, dandelion, hickory (Figs. 32, 33).
A =fibrous root system= is one that is composed of many nearly equal
slender branches. The greater number of plants have fibrous roots.
Examples are many common grasses, wheat, oats, corn. The buttercup in
Fig. 34 has a fibrous root system. Many trees have a strong tap-root
when very young, but after a while it ceases to extend strongly
and the side roots develop until finally the tap-root character
disappears.
[Illustration: FIG. 34.—A BUTTERCUP PLANT, with fibrous roots.]
=Shape and Extent of the Root System.=—The depth to which roots
extend depends on the _kind of plant_, and the _nature of the soil_.
Of most plants the roots extend far _in all directions_ and lie
comparatively _near the surface_. The roots usually radiate from a
common point just beneath the surface of the ground.
_The roots grow here and there in search of food_, often extending
much farther in all directions than the spread of the top of the
plant. Roots tend to spread farther in poor soil than in rich soil,
for the same size of plant. _The root has no such definite form as
the stem has._ Roots are usually very crooked, because they are
constantly turned aside by obstacles. Examine roots in stony soil.
_The extent of root surface is usually very large_, for the feeding
roots are fine and very numerous. An ordinary plant of Indian corn
may have a total length of root (measured as if the roots were placed
end to end) of several hundred feet.
The fine feeding roots are _most abundant in the richest part of the
soil_. They are attracted by the food materials. Roots often will
completely surround a bone or other morsel. When roots of trees are
exposed, observe that most of them are horizontal and lie near the
top of the ground. Some roots, as of willows, extend far _in search
of water_. They often run into wells and drains, and into the margins
of creeks and ponds. Grow plants in a long narrow box, in one end of
which the soil is kept very dry and in the other moist: observe where
the roots grow.
[Illustration: FIG. 35.—THE BRACING BASE OF A FIELD PINE.]
=Buttresses.=—With the increase in diameter, the upper roots often
protrude above the ground and become _bracing buttresses_. These
buttresses are usually largest in trees which always have been
exposed to strong winds (Fig. 35). Because of growth and thickening,
the roots elevate part of their diameter, and the washing away of the
soil makes them to appear as if having risen out of the ground.
=Aërial Roots.=—Although roots usually grow underground, _there
are some that naturally grow above ground_. These usually occur on
climbing plants, the roots becoming _supports_ or fulfilling the
office of tendrils. These aërial roots _usually turn away from the
light_, and therefore enter the crevices and dark places of the wall
or tree over which the plant climbs. The trumpet creeper (Fig. 36),
true or English ivy, and poison ivy climb by means of roots.
[Illustration: FIG. 36.—AËRIAL ROOTS OF TRUMPET CREEPER OR TECOMA. ]
[Illustration: FIG. 37.—AËRIAL ROOTS OF AN ORCHID.]
In some plants all the roots are aërial; that is, _the plant grows
above ground_, and the roots gather food from the air. Such plants
usually grow on trees. They are known as _epiphytes_ or _air-plants_.
The most familiar examples are some of the tropical orchids which are
grown in glass-houses (Fig. 37). Rootlike organs of dodder and other
parasites are discussed in a future chapter.
Some plants bear aërial roots, that may _propagate the plant_ or may
_act as braces_. They are often called =prop-roots=. The roots of
Indian corn are familiar (Fig. 38). Many ficus trees, as the banyan
of India, send out roots from their branches; when these roots reach
the ground they take hold and become great trunks, thus spreading
the top of the parent tree over large areas. The mangrove tree of
the tropics grows along seashores and sends down roots from the
overhanging branches (and from the fruits) into the shallow water,
and thereby gradually marches into the sea. The tangled mass behind
catches the drift, and soil is formed.
[Illustration: FIG. 38.—INDIAN CORN, showing the brace roots at _oo_.]
=Adventitious Roots.=—Sometimes roots grow from the stem or other
unusual places as the result of some accident to the plant, being
located without known method or law. They are called =adventitious=
(chance) =roots=. Cuttings of the stems of roses, figs, geraniums,
and other plants, when planted, send out adventitious roots and
form new plants. The ordinary roots, or soil roots, are of course
not classed as adventitious roots. The adventitious roots arise on
occasion, and not as a normal or regular course in the growth of the
plant.
=No two roots are alike=; that is, they vary among themselves as
stems and leaves do. _Each kind of plant has its own form or habit
of root_ (Fig. 39). Carefully wash away the soil from the roots of
any two related plants, as oats and wheat, and note the differences
in size, depth, direction, mode of branching, number of fibrils,
colour, and other features. The character of the root system often
governs the treatment that the farmer should give the soil in which
the plant or crop grows.
[Illustration: FIG. 39.—ROOTS OF BARLEY AT _A_ AND CORN AT _B_.
Carefully trace the differences.]
Roots differ not only in their form and habit, but also in colour
of tissue, character of bark or rind, and other features. It is
excellent practice to _try to identify different plants by means of
their roots_. Let each pupil bring to school two plants with the
roots very carefully dug up, as cotton, corn, potato, bean, wheat,
rye, timothy, pumpkin, clover, sweet pea, raspberry, strawberry, or
other common plants.
=Root Systems of Weeds.=—Some weeds are pestiferous because they seed
abundantly, and others because their underground parts run deep or
far and are persistent. Make out the root systems in the six worst
weeds in your locality.