Root: Encyclopedia II - Root - Types of roots

Root - Types of roots

A true root system consists of a primary root and secondary roots (or lateral roots).

The primary root originates in the radicle of the seedling. During its growth it rebranches to form the lateral roots. Generally, two categories are recognized:

• the taproot: the primary root is prominent and has a single, dominant axis; there are fibrous secondary roots running outward. Usually allows for deeper roots capable of reaching low water tables. Most common in dicots
• the primary root is not dominant: the whole root system is fibrous and branches in all directions. Most common in monocots.

Adventitous roots arise from the stem and not from another root. They usually occur in monocots and pteridophytes, but also in a few dicots, such as strawberry (Fragaria vesca) and white clover (Trifolium repens).

Root - Specialized roots

The roots, or parts of roots, of many plant species have become specialized to serve adaptive purposes besides the two primary functions described in the introduction.

• Aerating roots (or pneumatophores): roots rising above the ground, especially above water such as in some mangrove genera (Avicennia, Sonneratia)
• Aerial roots: roots entirely above the ground, such as in ivy (Hedera helix) or in epiphytic orchids. They function as prop roots or anchor roots.
• Contractile roots: they pull bulbs or corms of monocots deeper in the soil through expanding radially and contracting longitudinally. They show a wrinkled surface.
• Haustorial roots: roots of parasitic plants that can absorb water and nutrients from another plant, such as in mistletoe (Viscum album) and Rafflesia.
• Proteoid roots or cluster roots: dense clusters of rootlets of limited growth that develop under low phosphate or low iron conditions in plants from the following families Betulaceae, Casuarinaceae, Eleagnaceae, Moraceae, Fabaceae and Myricaceae.
• Stilt roots: these are adventitious support roots, common among mangroves. They grow down from lateral branches, branching in the soil.
• Storage roots: these roots are modified for storage of nutrients, such as carrots and beets
• Tubiferous roots: A portion of a root forms into a roundish knob called a (tuber) for food.

Adapted from the Wikipedia article "Types of roots", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki