In this article, we propose to discuss about the various regions and functions of the root.
The root is the descending portion of the plant axis. As opposed to the stem, it is positively geotropic, negatively phototropic and positively hydrotropic. The root surface is non-green and it is not divided into nodes and internodes.
In stems the branches arise from the nodes so that there is some regularity in their arrangement while this is not so in roots because of the absence of nodes. Nevertheless, the branching in roots also is acropetal.
Roots bear branches only of similar type while stems bear similar and dissimilar lateral members (branch, leaf and flower). Anatomically, root branches arise from an inner layer, i.e., they are of endogenous origin while shoot branches arise more superficially, i.e., they are exogenous.
It is seen during germination that the radicle is the first organ to grow in the seedling. This radicle gives rise to branches of primary, secondary or higher orders forming what is known as the primary or taproot system , the main root being known as the taproot which is a direct prolongation of the radicle while the branches are often designated by the general expression secondary roots.
In most dicots the root system ramifies more and more in the soil and anchors the plant very firmly. The ultimate root branches of large trees ramify through a very large and deep area, often occupying no less a space than what the tree occupies above. In most monocots, however, this primary root system does not develop so strongly and does not form the main root system. New roots grow from near the base of the radicle and they are called seminal roots.
These seminal roots grow and behave in the same manner as the primary root from the radicle. Later on, some more roots grow from the base of the plumule or from its lower nodes. These roots are like fibres and they form a fibrous root system which is usually the main root system of monocots. In many monocots, at least in the cereals, it is now known that the primary and seminal roots do not die quickly and usually persist.
But even in this case, it is the fibrous root system which does the real work. Since the rootlets are rather thin and do not branch enormously as in the case of dicots, fibrous roots cannot be as strong for anchorage as the primary root system. But, there are very few trees among the monocots.
A root system developing from the radicle (with its branches) is known as a normal root system while a root developing from anywhere excepting the radicle is known as adventitious.
Thus, the root system is usually normal in dicots and adventitious in monocots. Adventitious roots also arise to serve special functions. Such adventitious roots are seen to develop on various organs like underground stems, stem cuttings, etc. These are described later.
Zones or Regions:
Whether a root belongs to a normal root system or it is adventitious, it shows the same different regions. The zones or regions of a root are best studied from the tip of a rootlet. As the root remains under the soil and it is difficult to get the fine root endings of large plants, it is easier to study the regions from some germinating seed , e.g., mustard, gram or pea. It will be seen that the tip of the root is protected by a fine cap-like structure known as the root-cap or calyptra. This region may be called (1) the root-cap region.
This root-cap serves for the protection of the delicate root-tip while the root is growing down and down into the soil. Friction with- soil particles continually wears out the root-cap while it is being constantly renewed by new growth from its base.
While most plants show such a simple root-cap , there are some which show some variation. In Pandanus (screw-pine) the root-cap is multiple. The aquatic plants (Lemna and Pistia stratiotes ) usually do not have root-caps but their root apices are protected by what are known as root-pockets.
Root- pockets are like loose thimbles which keep the root apex covered but, these are not regenerated from the base (which is the case in true root-caps), being of different origin.
Just behind the root-cap is (2) the very short growing region. This is the part of the root which actually grows. This region -may be further subdivided into the zones of division and elongation according to the stage of growth of the cells.
Behind this growing region is (3) the root-hair or piliferous region. Here the root surface is covered by fine unicellular root-hairs which do the actual absorption of solutes.
Friction with soil is continually wearing out and destroying the root-hairs while the region of growth, as it extends downwards by growth, is developing new root-hairs forming new root-hair regions.
Aquatic plants do not have root-hairs and they do not need them. Absorption goes on along the root surface in these plants.
The first three regions take up little space on the tips of the final root branches. All the length of the root behind and above the root-hairs is occupied by (4) the permanent region.
Most of the root and all its older branches up to the base of the stem are occupied by the permanent region. All root-hairs have been killed and all growth has stopped in this permanent region. No absorption takes place through this region but what has been already absorbed is conducted to the stem above.
Normally, the root performs the functions of anchorage, absorption and conduction. Anchorage of the plant to the Soil is a mechanical function while absorption of solutes and their conduction upwards are physiological functions. Besides these normal functions, roots may sometimes carry on some special functions according to which even the structure of a root gets modified. These are the modifications of roots.
Rootless Plants and Plants That are All Roots:
While all normal Angiosperms are provided with roots, even among them there are some instances of plants with no roots. They are to be found among aquatic plants and among saprophytes. In aquatic plants like Utricularia some leaves are finely dissected and these submerged segments carry on the function of roots.
In Wolffia arrhiza which is one of the smallest known flowering plants, there is only a very small free floating green structure (leaf or phylloclade as in Lemna ). There is no root. Same is the case in Epipogium.
On the other extreme we find plants in which the main plant body is a root. In Podostemaceae the main plant body is a thalloid root which carries on assimilation as well as reproduction and grows freely in the shallow water on rock surface.
Podostemon is quite common in the Cherrapunji area of Meghalaya. The small flowering shoots grow on this root from time to time . In the parasites Arceuthobium, Rafflesia and Sapria the plant bodies are filamentous masses living within the tissue of the host stem or root without showing an, differentiation.
At the proper time buds develop from this filamentous mass which burst out of the host stem or root and give rise to minute (Arceuthobium) or gigantic (Rafflesia) flowers. A seed of the saprophyte Monotropa produces only a free-living root on germination.
This branches abundantly in the humus forming a perennial structure. Occasionally, aerial flowering shoots are formed on this root.