The root system is the part of the plant growing in the soil. This excludes the underground stems which are organs of perennation (survival from year to year by vegetative means). There are two kinds of root systems, namely: tap root and fibrous root systems.
Tap root system
This system is characteristic of the dicotyledonous plants.
There is just one main root known as the tap root growing deep down and giving rise to smaller branches, the lateral roots (figure 2.7).
The fibrous root system
This is characteristic of monocotyledonous plants. In this system, there is a mass of roots arising from the base of the stem. The mass consists of several main roots of approximately the same size. These give off numerous branch roots which in turn give off other branch roots of smaller sizes. The roots are slender in appearance.
Both the main and branch roots bear a root cap at their tips. The root cap protects the soft growing regions of the root tip. Here the cells are actively dividing to add new cells to the root. A little above the root tips are some unicellular root hairs used for absorption of water and mineral salts from the soil into the plant body. The root hairs are very numerous and tiny and should not be confused with the very small branch roots seen after an intact plant has been uprooted from the soil. One may need a microscope to observe the root hairs if they have not been damaged by uprooting. The root begins to thicken above the root hair region.
Root produced from parts other than the radicle of a seed are known as adventitious roots. For example, the prop roots of maize which grow from the lower nodes of the stem into the soil. The roots seen in stem tubers, rhizomes, bulbs and corms are all adventitious roots.
Characteristics of roots
- Roots grow inside the ground except for some modified aerial roots.
- Roots lack buds.
- Roots end in caps (stems end in buds).
- Roots do not bear green pigments or chlorophyll.
- Roots possess unicellular hairs known as root hairs. (Hairs found in some stems and leaves are multicellular).
- Nodes and internodes are absent in roots.
Functions of roots
The main functions of roots are:
- For absorption of water and dissolved mineral salts from the soil into the plant body through the root hairs.
- For anchoring the whole plant firmly to the soil.
- Certain roots become modified to perform some special functions such as food storage, vegetative reproduction, climbing, breathing in water-logged soil and for extra support to the plant.
Types of modified roots
These are roots which become swollen with food reserves. Hence they are called storage organs. Examples are seen in the tap roots of carrot, pig weed (Boerhaavia diffusa) and lateral roots of cassava (figure2.9).
2) Climbing roots
They are also known as clasping or adhering roots. These are aerial roots growing into cracks of walls, or barks of host plants. By clasping the host for support they are able to climb high.
In Ficus pumila, the aerial roots appear in clusters at the nodes.
In Scindapus sp ., the clasping roots grow downwards and on getting to the soil they develop lateral roots.
In black pepper, the small roots of the plant produce sticky substance that helps to gum it to the host tree or wall.
3) Breathing roots
These are special roots possessed by some plants in the swamps for breathing in the air. These breathing roots or pneumatophores, are lateral branches growing above the soil which is often water logged. Good examples are seen in raphia and the white mangrove (Figure 2.11)
4) Prop roots
These are clusters of roots growing from the nodes of a stem straight into the soil for extra support to the plant. Prop roots occur in maize.
5) Stilt roots
These are branches from the base of certain trees which grow in muddy places and they help in the support of the plant. In the umbrella tree, for example, (figure 2.13) the trunk narrows at the base from where the stilt roots develop to provide the necessary support. Stilt roots are also seen in the red mangrove.
6) Buttress roots
These are enlarged growths on the upper edges of normal lateral roots of some huge trees such as silk cotton tree. Buttress roots also act as support to the plant as well as providing good hiding places for some forest animals (figure 2.14).
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