- When the primary root, which is a direct prolongation of the radicle, remains distinct throughout the life of the plant and gives off lateral branches, it is termed as tap root E.g. mustard
- However, in many plants, the root grows from any other part of plant other than original root system, it is called adventitious root system or fibrous root system. E.g. onion, grasses
- Besides their usual function of anchorage and absorption, they may also be suitably modified to carry out several other functions.
Modifications of tap root:
- In many plants, the tap root becomes swollen and assumes various forms, which happens due to the storage of food.
- The modifications of tap root may be of the following types:
1. Fusiform: Here the root tapers gradually at both the ends but is swollen in the middle. E.g. Radish
2. Napiform: When the shape of the primary root becomes almost spherical and tapers abruptly downward, it is said to be napiform. E.g. Turnip
3. Conical: It is cone like in structure. E.g. carrot
4. Tuberous: The root is swollen but has no specific shape. E.g. mirabilis
Modifications of adventitious root:
- Adventitious root may also be modified to carry on the storage of food, mechanical support and other vital functions.
- Some of the main modifications are as follows:
1. Tuberous: The swollen root is shapeless and occurs singly. E.g. Sweet potato
2. Fasciculated: When several tuberous roots occur in a cluster arising from the same place. E.g. Aspapragus, Dahlia
3. Beaded or moniliferous root: Here the adventitious roots have swollen regions at frequent intervals. E.g. Portulaca, Momordica, Yam (Diascorea)
4. Prop: They are pillar like structures hanging vertically downward from the aerial branches. E.g. Banyan tree
5. Stilt: These are short roots growing obliquely from near the base of the main stem and provide additional support. E.g. Maize, Sorghum, Sugarcane, Screwpine
6. Parasitic: Parasitic plants develop roots which penetrate into the host tissue and suck or absorb the nutrients. E.g. Cuscuta
7. Assimilatory: in some plants, adventitious roots arise from the aerial stem, become green and photosynthetic. Such roots are known as assimilatory roots. E.g. Tinospora, Trapa
8. Nodulose: The root gets swollen at or near the apex. E.g. Mango, Ginger (Curcuma)
9. Climbing or clinging root: They arise from the nodes or internodes and secrete a sticky juice to gain hold of substratum. E.g. Betelvine
10. Respiratory roots: They are developed on floating branches and are soft, light, spongy and colorless and help in storing air for facilitating respiration. E.g. Jussiaea repens
11. Epiphytic roots: These are green hanging roots, surrounded by a spongy tissue called velamen which is hygroscopic and helps in absorbing moisture from the surrounding air. E.g. Orchids