A root is the part of the plant present under the soil. It develops from the radicle. The main function of the root is to absorb water and minerals that a plant needs to survive, from the soil. The roots fix the plant to the soil and support the entire part of the plant above the ground. Roots may also serve as organs for vegetative propagation or storage organ.
Roots that store food for future use are known as storage roots or fleshy roots. This is a modification of the taproot system. Other modifications of roots include nodulated roots in lentils (harbor nitrogen-fixing bacteria in nodules), pneumatophores in mangroves (roots come out of waterlogged marshy areas to help in gaseous exchange) or buttress roots (support heavy trunk).
The taproot system is a mass of roots that develops from the radicle of the embryo. It consists of the main taproot (primary, thickest, longest) which branch into secondary roots. Each secondary root subsequently branches into many tertiary roots and rootlets.
The taproot with or without hypocotyl becomes enlarged and swollen due to the storage of food. Hypocotyl (embryonic region present between cotyledons and radicle) may also join the taproot in storing food. The stem is reduced (less prominent and underdeveloped) and discoid In shape in the beginning and bears radical leaves. Secondary roots bone over it later remain thin and do not swell up.
Types of storage roots
The storage roots are of following types like
- Conical roots
- Tuberous roots
- Napiform roots
- Fusiform roots
Here the fleshy storage root modifies to become conical in shape. The root is thickest towards the base, i.e. towards the stem and things down (tapers) towards the apex (away from the stem). The secondary roots are thread-like in shape and numerous in the count. They develop over the entire length of the conical root. This conical shape ensures the least resistance to the root as it burrows through the soil. An example of a conical fleshy root is Carrot ( Daucus carota).
They are also referred to as tubercular roots. This type of taproot has a secondary or tertiary branch that is swollen in an irregular shape. The structure has multiple swellings and bumps. One tap root system may possess more than one tubercles at multiple branches from the main taproot. A few examples of this type of modification include Mirabilis jalapa ( 4 o’clock plant), Trichosanthes (Pointed gourd), Echinocystis lobata, etc. In Echinocystis lobata the tuberous root is lobed and weighs as much as 22 kg.
This type of fleshy root has a top like a body. The body is nearly globular in shape and tapers (thins out) suddenly towards the apical region. Prime examples of napiform modification of taproots are seen in Brassica rapa (turnip), Beta vulgaris (Beet), etc.
In Beta vulgaris, the swollen part containing the food is formed by the combination of hypocotyl and taproot. The taproot can be distinguished from the hypocotyl region because the taproot has thread-like secondary roots over it, while the hypocotyl region does not have any secondary roots.
In Brassica rapa, the major part of the storage root is formed from the hypocotyl part of the embryo. The taproot is present towards the tapering apical region and is modified to store food through its contribution is minor. As the taproot is only present towards the apical region, the thread-like secondary roots are formed only at the apex. The swollen part of the root is composed of hypocotyl and does not have any secondary roots.
The fusiform root is spindle-shaped. It is narrow towards both the base and the apex. The root is thickest in the middle part. The hypocotyl also contributes to the formation of the modified root. A well-known example of fusiform root modification is Radish (Raphanus sativus). In European radish, the hypocotyl part constitutes more than half of the modified root while in Indian radish, it makes up less than half of the storage root.
In European Radish, the taproot forms only the terminal tapering towards the apex of the root. The middle and the basal fleshy parts of the root are formed by the hypocotyl.
The fusiform root of Indian Radish consists of a swollen hypocotyl near the base. The rest of the root(apical and middle) is formed by the swollen taproot. The taproot possesses thin and thread-like secondary roots.
Differences between fusiform and napiform roots
|Sl.NO||Fusiform root||Napiform root|
|1||The outline is spindle-like||The outline is top-like|
|2||The middle path is the thickest||The basal part is the thickest|
|3||The apex narrows gradually||The apex thins out suddenly|
|4||The hypocotyl forms half or less than half of the storage root||The hypocotyl forms half or more than half of the storage root|
|5||The taproot is a part of the storage root||The taproot may or may not be a|
part of the storage root. It is
unthickened in Turnip but partly
thickened in Beet.
|6||Thin secondary roots are present from where the taproot starts.||The thin thread-like secondary roots are only present at the apical|
region of the root.
Features of modified storage roots
- The storage root mainly stores starch and water.
- Starch is stored in the form of amylase and amylopectin sugar which is broken down to release glucose. The glucose is oxidized to release energy.
- This can be used in adverse conditions when water and food supply is reduced and nutrients can be mobilized.
- The reservoir is also used to provide energy during the growing season.
- The roots are underground and are hidden from herbivores like goats, cows, etc. Thus it’s an adaptation technique.