Root Morphology, Tap Root, Adventitious Root

The root may be defined as the cylindrical plant organ which is devoid of chlorophyll, bearing no buds or leaves, and tending to grow downwards away from light. It develops from radicle part of the embryo. The main functions of roots are anchorage and absorption of food from the soil. The root also perform certain special functions, e.g., store food, absorb moisture from air, provide shelter to nitrogen fixing bacteria, provide extra support to the plant, for which the roots undergo various types of modifications.

Generally, the roots are positively geotropic and negatively phototropic i.e., they grow downward into the soil and away from light. They are also Positively Thermotropic and exhibit Positive Hydrotropism, i.e. they bend in the direction of temperature that is most favourable for the growth, and have a tendency to grow in the direction of moisture supply.

When a seed germinates, the embryonal root (radicle) gradually elongates and forms the Primary Root. The primary root may give off branches, the Secondary Roots which in turn branch off to produce Tertiray and Quarternary Roots.

In dicotyledonous plants the primary root becomes the main root and is termed as a Tap Root. Monocotyledonous plants generally, lack tap root. In these plants the root formedfrom the radicle is short lived and from the base of the stem strong and vigorous roots develop. These roots are known as Adventitious Roots (roots developing from or the plant other than the radicle). In some members of Poaceae (grasses), additional embryonal roots known as Seminal Roots arise from the base of the radicle. These roots come out just after the radicle during germination.

A root, whether tap or adventitious, have the same different regions. Each root tip possesses a protective covering, the Root Cap. The root-cap protects the actively dividing cells and controls the direction. Root-cap is absent in aquatic plants. Below The root-cap, Zone of Cell is present this zone consists of meristelnatic cells (actively dividing cells). This region gradually passes into Region of Elongation. The cells of this region are columnar and are slightly thick walled. This region in turn merges into Region of Maturation. The cells of this region has root-hairs, which are extensions of epidermal cells. The cells of this region starts differentiating and permanent tissue of the root is produced, This region gradually passes into Mature Zone where the differentiation of the cells occur and tissues like Epidermis, Cortex and Stele can be recognized.

Root Systems

The roots and its branches together constitute the Root System.

Two main types of root systems are found in plants.

1. Tap Root System
2. Adventitious Root System

Tap Root System:

In this the root developing the radicle persists throughout the life of the plant. Such root is Tap Root or Primary Root. This root branch off to produce Secondary, Tertiary and Quaternary branches. The primary root along with subsequent branches constitutes Tap Root System.

The tap root system is of two types:

• Racemose tap root system:

The primary root continues to grow and penetrates the soil to considerable depth and is very prominent. The secondary and tertiary roots are smaller as compared to the primary root. The older secondary branches lie close to the soil surface while the younger secondary branches are present near the tip portion of the primary root. This type of root system is found in deep-feeding dicots, e.g., Oak. Such plants are known as Deep-Feeders.

• Cymose tap root system:

The primary root persists throughout the life of the plant but does not go deep into the soil and stops its growth after sometimes. Secondary roots developing from primary roots are quite strong but they also invade upper layers of the soil like primary root. This type of root system is found in surface-feeding plants. The plants having this type of root system are called Surface-Feeders.

Modifications of Tap Roots

In many plants the tap roots may get swollen due to storage of reserve food material in them. The stem proper in these modifications is very much reduced. A part of the hypocotyls may also be involved in storage of food. Due to storage of food the tap roots assume various shapes which determine a particular type of root.

Three main modifications are recognized:

(i). Conical Tap Root:

This root is broad at the base and gradually taper towards the apex, e.g Carrot, Radish.

(ii). Napiform Tap root:

The root is very much swollen above and abruptly tapers towards its lower end, as in Turnip. In Titrnip the swollen portion is mostly hypocotyl while the narrow portion is the root proper. In Beet, swollen portion is contributed by the hypocotyl and the root.

(iii). Fusiform Tap Root:

The tap root assumes a shape of a spindle, i.e., swollen in the middle and tapering towards the ends, e.g., English Radish. In English Radish the upper and middle portions are hypocotyl and lower part is root proper.

Special Types of Tap Roots

(i). Nodulated roots:

These roots are characteristics of the members of the family Leguminaceae. The root system resemble a typical tap-root system except for the presence of small globular swellings mostly restricted to secondary roots. These are known as Nodules or Tubercles. These provide shelter to nitrogen fixing bacteria.

(ii). Buttress roots:

These are laterally compressed and vertically elongated roots that provides extra support to stem, e.g., in Almond plants.

Adventitious Root System:

The adventitious roots are those roots which are not developed from the radicle but are formed from any other part of the plant. Adventitious roots arising from any part of the plants together constitute a system called Adventitious Root system.

Modifications of Adventitious Roots:

As compared to tap roots the adventitious roots undergo more diverse modifications. These modifications provide some advantage to the plants.

Different modifications met among the adventitious roots are as follows:-

(i) Fibrous: These are hair-like roots present in the form of clusters and these arise from the base of the stem (wheat), from nodes (grasses–Cynodon dactylon) or from leaves (Bryophyllurn).

(ii) Stilt Roots: These arise from the first few nodes of-the stem and run obliquely towards the soil surface. These help the plant to fix it more firmly in the soil, e.g., in Maize, Sugar-cane.

(iii) Prop or Pillar Roots: these roots arise from stem branches and grow vertically downward. These roots increase In girth with the increase in girth of branches. These roots provide support to the plant. In Banyan, these roots are quite thick and look like pillars.

(iv) Pneumatophores or Respiratory roots are found in Mangrove Plants, i.e., such plants which grow in marshy places. The waterlogged soil does not contain air. Special roots arise from the underground roots which have tiny pores called Pneumatophores through which the roots take oxygen for respiration.

(v) Epiphytic Roots: Such roots are found in some epiphytic orchids. These roots possess special spongy tissues at their apices. The tissue is termed as Velamen. It has the capability of absorbing and retaining the moisture so these roots help in absorbing Inoisture from the air.

(vi) Climbing Roots: Some plants have roots which twine around the support liketendrils, e.g., in sonle species of FiC11S and Hcdera helix.

(vii) Clinging Roots: These roots penetrate into the cracks and crevices of the support and hold the plant firnlly, e,g. in Betel plant and Epiphytic orchid species.

(viii) Assimilatory Roots: These roots arc green due to presence of chlorophyll and can perform photosynthestis.

(ix) Parasitic roots: These roots are produced by parasitic plants and thesepenetrate into the food channels of the host plant and draw food for the plant, e.g., in Cascuta.

(x) Reproductive roots: In many plants adventitious buds develop on roots. If these are separated and put into the soil, they establish new plants, e.g., in Dhalia, sweet Potato.

(xi) Floating roots: In some aquatic plant the roots ari.se from the nodes. These roots are spongy in nature and look like a cotton ball. These roots provide buoyancy and help the plant in floating.

(xii) Root Tubers: In som plants the underground adventitious roots become tuberous due to accumulation of food in thenl. These are called root tubers, e.g., in sweet Potato.

(xiii) Fasciculated roots: In some plants, e.g., in Dahlia and Asparagus, adventitious roots occur in a cluster and swollen. These are called fasciculated roots.

(xiv) Nodulose Roots: In some plants apices of the adventitious roots become swollen due to storage of food and appear like beads. Such roots are called nodulose roots, e.g., in Costus speciosus.

(xv) Annulated Roots: In this type, the roots give an appearance of, as if discs have been arranged in stacks, e.g.,in Ipecac.

(xvi) Contractile Roots: Certain plants with underground stems possess special roots that can contract and move the rhizome, bulb or corm more deeply into the soil, e.g., Gladiolus and Polygonatunt.