When a seed germinates the first structure to develop is the root. This allows the plant to anchor into the soil and take up water. The water is then used to convert the stored food (starch) in the seed to a usable form (sugar) that the plant can live on until the leaves develop and the young seedling can make its own food.
As the young plant grows and develops, the roots develop into a complex system that fall into one of two main categories: taproots and fibrous roots.
Taproots
The first root of the plant that develops when the seed germinates is called the primary root. In gymnosperms (evergreen trees) and dicotyledons, this root grows directly downward, forming side or lateral roots along the way. This type of root system – one that develops from a taproot and its branches – is called a taproot system.
Taproots are categorized by their shape:
- Fusiform taproots are swollen in the middle and taper towards the base. The radish is an example.
- Conical taproots are broad at the base and taper to the apex (end). The carrot is an example.
- Napiform taproots resemble a child’s top being very broad at the base and tapering steeply to the apex. The turnip is an example.
Taproots as Storage Organs
Taproots functions to store excess food and water for the rest of the plant. Some examples of plants with taproots that function as storage organs are the dandelion, carrot, beet, and radish.
In biennials (pants that complete their life cycle over a two-year period), such as the sugar beet and carrot, large food reserves accumulate in the taproot during the first year and then are used during the second year to power the production of flowers, fruits, and seeds.
Fibrous Roots
In monocotyledons and some dicotyledons, the primary root is usually short-lived, and the root system develops from the stem as a highly branched network of roots known as a fibrous root system in which on one root is more prominent that the others.
Taproot systems generally penetrate into the soil deeper than fibrous root systems. The shallowness of fibrous root systems and their highly branched nature permits them to cling tenaciously to soil particles around them.
In an intensive study of the fibrous roots of winter rye, H.J. Dittmer found that one plant 20 inches high, consisting of a clump of 80 shoots, had a total of 14 million roots – main roots, secondaries, tertiaries, and quaternaries.
The total surface area of the root system was 4,000 square feet – equal to the floor space of two or three good-sized houses – or 130 times the surface area of the stems and leaves. If laid end to end, these roots would have had a length greater than the distance from New York City to Washington, D.C. or around 600 km (360 miles).
Amazingly, all these roots occupied only about 6 liters (2 cubic feet) of soil. Such extensive root systems found in the family of grasses so thoroughly permeate the soil around them that they not only serve to anchor the plant into ground but they also become ecologically important in holding topsoil so that it does not get washed away by rains.
Adventitious Roots
Adventitious roots often originate in unusual places on a plant – stem, branches, leaves, or old woody roots. Wherever they develop, however, adventitious roots must be connected to vascular tissue (xylem (water) tubes and phloem (food) tubes.
One example of adventitious roots is the prop roots of corn and some tropical trees, such as the banyan. Prop roots originate from the lower stem and grow obliquely downward into the soil or mud where they function to support (prop up) the plant.
Another type of adventitious root is the stilt roots of mangrove trees. These are actually lateral branches that grow downward into the mud where they act as support pillars for the aerial branches.
Swollen lateral tuberous roots, such as the sweet potato, are also adventitious roots.
Many special adaptations of roots are found among epiphytes – plants that grow on other plants but are not parasitic on them. Orchids grow perched high on tree trunks and branches where they attach themselves with roots that secrete a kind of cement. They absorb the water they need directly from the moist air of their steamy rainforest habitat and they fulfill their mineral requirements from leaves that fall and decompose near their roots.
Consider the “flower pot plant.” Some of its leaves form hollow containers – “flower pots” – that collect nitrogenous debris and rainwater. Roots, formed at the node above the modified leaf, grow downward into the pot, from which they absorb water and minerals – a plant that grows roots into itself.
As a seed grows, the root system begins to develop. The genes of the plant, shaped by the forces of evolution and adaptation, determine the type of root system that results.
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