1. The tap root system bearing secondary, tertiary and other lateral roots ramify more and more in the soil and anchor the plant very firmly.
2. Root hairs absorb water and mineral salts from the soil and send it to the upper part of the plant.
KINDS OF ROOTS:
A. Tap root system:
A root system which develops from the radicle is called tap root system, serves the normal functions such as anchorage and absorption.
B. Adventitious root system:
The root developing from any portion of the plant other than the radicle is known as adventitious root. It may develop from the base of the stem or from any node or inter node of the branch, or even from the leaf. They serve the normal functions such as anchorage, absorption and conduction of solutes and water. Adventitious roots carrying on normal functions arc of the following types.
(i) Fibrous roots:
Cluster of fiber like roots developed from the base of the plumule or from its lower nodes arc known as fibrous roots. Such types of roots are seen in rice, maize, etc.
(ii) Foliar roots:
These roots develop directly from the leaf, from the petiole, or the vein. Ex. Petiole of rubber plant and leaf margin of Bryophyllum.
(iii) Adventitious roots:
These roots develop from the nodes and sometimes from the internodes of the stem, e.g. Oxalis. Stem cuttings of rose, sugarcane, China rose, marigold, etc., give rise to such type of roots.
Modification of roots:
Roots may be modified to different shapes in order to perform special functions besides their normal functions. Root modification is found both in tap root and adventitious root.
A. Tap root modification:
Tap root system is modified into different forms to carry out special function such as storage of food such a root is known as modified tap root. It is differentiated into following types:
It is a modified tap root. The primary root is swollen in the middle while both the ends gradually taper forming a spindle shaped structure, e.g., Radish (Raphanus sativus).
The hypocotyls region is considerably swollen, becoming almost spherical and then abruptly tapering towards the lower end, e.g., Turnip (Brassica campestris var. rapa) and beet (Beta vulgaris).
The roots swell, becoming broad at the base and gradually taper towards apex forming a cone like structure, e.g., Carrot (Daucus carota).
(iv) Tuberous or tubercular:
In this case, the root is thick and fleshy but does not form any definite shape, e.g., Mirabilis jalapa.
These roots grow vertically up and come out of the water or marshy soil like conical spikes. They occur in large numbers around the tree trunk. Such roots are provided with numerous pores (breathing pores) through which air is taken for respiration, e.g., Rhizophora.
B. Adventitious root modification
Roots produced from any portion of the plant other than the radicle is called adventitious roots. It may be changed to different forms for special functions such as storage, mechanical support and vital functions.
(a) For storage of food:
(i) Tuberous root: In this case, adventitious roots grow from the nodes of the running stem. These roots swell and get modified to irregular forms, known as tuberous. These are produced singly, e.g., Sweet potato (Ipomoea batatas).
(ii) Fasciculate root:
In Dahlia, Asparagus the tuberous roots are developed from base of the stem! in clusters.
(iii) Nodulose root:
The underground stem produces adventious roots. Some of the slender roots become suddenly swollen near the apex forming nodule like structures, e.g.] Mango-ginger (Curcuma amada).
(iv) Moniliform or Beaded root:
Swellings are found in the root at frequent intervals. It gives rise to a beaded or moniliform appearance, e.g., Basella, Momordica, Vitis trifolia, Dioscorea alata and many grasses.
(v) Annulated roots:
In this type, the root has a series of ring-like swellings appearing as if formed by number of discs placed one above another, e.g., Cephalis ipecacuanha.
(b) For mechanical support:
(i) Prop roots: The plant has a tap root system. The horizontal branches of the stem give rise to aerial roots that hang vertically downwards. On reaching the ground, these grow and act like pillars providing mechanical support to the plant, e.g., Ficus bengalensis.
(ii) Stilt roots: The plants like screwpine have its own root system, but the anchorage is not very strong. Therefore, it develops adventious roots from near the base of the stem which grow obliquely downwards, act like stilt and provide mechanical support (Pandanus). The maize plant also produces adventitious roots from the lower nodes which also act in a similar way.
(iii) Climbing roots:
These are found in some climbers. Aerial roots are developed from the nodes. These roots twine around and clasp the support and help in climbing, e.g., Betel (Piper betel), Piper nigrum and Pothos, etc.
(iv) Clinging roots:
These roots are seen in orchids, which are developed from the stem base and enter the crevices of the support to fix the epiphyte. It provides mechanical support to the plant.
(v) Root buttresses:
Large trees have great plank-like roots that radiate from the base of the stem. These consist of partly root and partly stem, e.g., Bombax .
(c) For vital functions:
(i) Sucking roots or haustoria:
Cuscuta, a leafless, parasite, non-Greek plant always grows in association with other plants. At the point of attachment with the host plant it produces some knob like roots known as haustoria which penetrate into the host plant and draw nourishment.
(ii) Epiphytic roots:
Orchids are the plants which grow perched on branches of trees. They firmly attach themselves to the trunk of suitable trees by special roots known climbing roots. The epiphytes also have aerial absorbing roots. They remain fleshy and grow along with the clinging roots. They have a spongy vela men tissue which absorbs atmospheric moisture. They also have chlorophyll and perform photosynthesis.
(iii) Floating roots:
These roots are found in aquatic plants like Jussiaea. These are adventitious roots developing at the nodes of the floating branches. The roots are spongy and colorless. They remain above the water level. Being soft and spongy they store air which help the plant in floating. They also facilitate respiration.
(iv) Assimilatory roots:
Normally roots are non-green in color. But the adventitious roots developed from the branches of Tinospora cord folia being green in color carry out photosynthesis and are called assimilatory roots. These roots are slender, long and in hanging state.