Plants use either taproot or fibrous root systems to draw moisture and nutrients from the soil, and to anchor themselves against disturbance, such as high winds or grazing animals. Taproot and fibrous root systems grow differently, take different forms and perform some different functions. Under certain conditions, plants with taproot systems can also develop fibrous roots.
Taproots develop quickly from the first root as soon as the seed germinates. Lateral roots grow out from the taproot and spread out through the soil as the plant grows larger. Taproots benefit plants by searching out water deep underground, and anchoring them well during high winds. Although taproots are usually the largest root, some plants, such as many trees, grow taproots while young, then develop larger lateral roots. Some vegetables, such as carrots and turnips, are actually swollen taproots.
Fibrous roots are thin, similar-sized roots growing in a clump. When the seed germinates, the first root dies quickly and is replaced by roots growing from the stem base. Fibrous root systems absorb rainwater as it soaks into the soil surface. As they hold a large volume of soil, they're difficult for grazing animals to uproot, and can help prevent soil erosion. Grasses and cereals, such as wheat, have fibrous roots. Sweet potatoes and onions are swollen fibrous roots.
Taproot and fibrous root systems suit different plant growth patterns. Plants that develop more stems and branches from a single stem need a taproot to supply the stem, drawing water and nutrients from a network of lateral roots. As fibrous root systems develop from the stem base, plants using this system spread out along the soil surface, through structures called stolons or rhizomes, and new roots grow from the bases of the new stems.
Adventitious roots grow from a dissimilar structure. Fibrous roots are adventitious because they grow from stems. Taproots aren't adventitious because they develop from another root, the seed root. However, many plants with taproot systems can also grow adventitious, fibrous roots. Some plants also grow horizontally at the soil surface, and develop roots from new stems, and many taproot plants grow adventitious roots when their stems are cut for propagation.