A thorough knowledge of the root system of plants is
essential if their growth, flowering, and fruiting responses are
to be understood. The structure and growth habits of roots have a
pronounced effect on the size and vigor of the plant, method of
propagation, adaptation to certain soil types, and response to
cultural practices and irrigation. The roots of certain vegetable
crops are important as food. Roots typically originate from the
lower portion of a plant or cutting. They possess a root cap, have
no nodes and never bear leaves or flowers directly. The principal
functions of roots are to absorb nutrients and moisture, to anchor
the plant in the soil, to furnish physical support for the stem,
and to serve as food storage organs. In some plants they may be
used as a means of propagation.
Types of Roots
A primary (radicle) root originates at the lower end of
the embryo of a seedling plant. A taproot is formed when the
primary root continues to elongate downward. This makes them
difficult to transplant and necessitates planting only in deep,
well-drained soil. The taproot of carrot, parsnip, and salsify
is the principal edible part of these crops.
A lateral, or secondary root is a side or
branch root which arises from another root. A fibrous root
system is one in which the primary root ceases to elongate,
leading to the development of numerous lateral roots. These then
branch repeatedly and form the feeding root system of the plant. A
fibrous root is one which remains small in diameter because of a
lack of significant cambial activity. One factor which causes
shrubs and dwarf trees to remain smaller than standard trees is
the lower activity rate of the cambium tissue which produces a
smaller root system.
If plants that normally develop a taproot are undercut
so that the taproot is severed early in the plants life, the
root will lose its taproot characteristic and develop a fibrous
root system. This is done commercially in nurseries so that trees,
which naturally have tap roots, will develop a compact, fibrous
root system. This allows a higher rate of transplanting success.
The quantity and distribution of plant roots is very
important because these two factors have a major influence on the
absorption of moisture and nutrients. The depth and spread of the
roots is dependent on the inherent growth characteristics of the
plant and the texture and structure of the soil. Roots will
penetrate much deeper in a loose, well-drained soil than in a
heavy, poorly-drained soil. A dense, compacted layer in the soil
will restrict or stop root growth.
During early development, a seedling plant nutrients
and moisture from the few inches of soil surrounding it.
Therefore, the early growth of most horticultural crops which are
seeded in rows benefits from band applications of fertilizer,
placed several inches to each side and slightly below the seeds.
As plants become well-established, the root system
develops laterally and usually extends far beyond the spread of
the branches. For most cultivated crops roots meet and overlap
between the rows. The greatest concentration of fibrous roots
occurs in the top foot of soil but significant numbers of laterals
may grow downward from these roots to provide an effective
absorption system a couple of feet deep.
Parts of a Root
Internally, there are three major parts of a root. The meristem
is at the tip and manufactures new cells. It is an area of cell
division and growth. Behind it is the zone of elongation,
in which cells increase in size through food and water absorption.
These cells by increasing in size, push the root through the soil.
The third major root part is the maturation zone, in which
cells undergo changes in order to become specific tissues such as
epidermis, cortex, or vascular tissue. The epidermis is the
outermost layer of cells surrounding the root. These cells are
responsible for the absorption of water and minerals dissolved in
water. Cortex cells are involved in the movement of water from the
epidermis and in food storage. A layer of suberized (a fatty
material in some cells), known as the Casparian strips, has
regulatory effect on the types of minerals absorbed and
transported by the roots to stems and leaves.
Vascular tissues conduct food and water and are
located in the center of the root. However, some monocots have the
vascular system of their roots distributed around the root center.
Externally there are two areas of importance. Root
hairs are found along the main root and perform much of the actual
work of water and nutrient absorption. The root cap is the
outermost tip of the root, and consists of cells that are sloughed
off as the root grows through the soil. The root cap covers and
protects the meristem and also senses gravity and directs in what
direction the root grows.
Roots as Food
The enlarged root is the edible portion of several vegetable
crops. The sweet potato is a swollen root, called a tuberous root,
which serves as a food storage area for the plant. Carrot,
parsnip, salsify, and radish are elongated taproots.