Roots play an important role in the structure of the plant. They serve as thee organs. The three major functions of roots are 1) absorption of water and inorganic nutrients, 2) anchoring the plant body to the ground and 3) storage of food and nutrients. Most plants have wide-spreading roots that can absorb water and inorganic nutrients from a larger area of ground.
The growing point is just behind the root tip where cells divide quickly. Another name for the root tip is the root cap, which is made of several layers of cells (the calyptra). It has the task of protecting the root as it's forcing its way into solid earth. These cells are replaced as they wear out. They also act as gravity sensors. The response to gravity is controlled by its chemical called auxin. Behind the root cap is a thick white down consisting of thousands of tiny hairs. This part of the root has the function of absorbing water, with mineral salts dissolved in it, from the soil. Just above the growing point is the zone of elongation. Here, soft new cells take in water, their vacuoles expand and they grow longer, pushing the tip further into the soil.To help anchor the root in the soil, as it grows, millions of tiny tube like growths are fromed called root hairs. They help by sticking to soil particles and taking in water and minerals by osmosis(the movement of water molecules across a partially-permeable membrane down a water potential gradient). Root hairs increase the range of area of roots, allowing th eplant to absorb more water and nutrients. The epidermis of root hairs is thin and usually covered in mucous making it easy for them to stick to soil particles and absorb liquid. The hairs form from the root tips and usually live for about 5-6 weks before being replaced with new hairs.
Xylem and phloem cells are grouped in the roots centre. Xylem cells transports the water and nutrients, that are drawn into the through the root hairs, to the stem and leaves of the plant. Glucose, which is made in the leaves, is the plant's food. It passes down the ploem cells to the growing cells at the tip, where it is used to make cellulose for new cell walls.
Different root types
- Adventitious roots arise out-of-sequence from the more usual root formation of branches of a primary root, and instead originate from the stem, branches, leaves, or old woody roots. They commonly occur in monocots and pteridophytes, but also in many dicots, such as clover (Trifolium), ivy (Hedera), strawberry (Fragaria) and willow (Salix). Most aerial roots and stilt roots are adventitious. In some conifers adventitious roots can form the largest part of the root system.
- Aerating roots (or knee root or knee or pneumatophores or Cypress knee): roots rising above the ground, especially above water such as in some mangrove genera (Avicennia, Sonneratia). In some plants like Avicennia the erect roots have a large number of breathing pores for exchange of gases.
- Aerial roots: roots entirely above the ground, such as in ivy (Hedera) or in epiphytic orchids. They function as prop roots, as in maize or anchor roots or as the trunk in strangler fig.
- Contractile roots: they pull bulbs or corms of monocots, such as hyacinth and lily, and some taproots, such as dandelion, deeper in the soil through expanding radially and contracting longitudinally. They have a wrinkled surface.
- Coarse roots: Roots that have undergone secondary thickening and have a woody structure. These roots have some ability to absorb water and nutrients, but their main function is transport and to provide a structure to connect the smaller diameter, fine roots to the rest of the plant.
- Fine roots: Primary roots usually <2 mm diameter that have the function of water and nutrient uptake. They are often heavily branched and support mycorrhizas. These roots may be short lived, but are replaced by the plant in an ongoing process of root 'turnover'.
- Haustorial roots: roots of parasitic plants that can absorb water and nutrients from another plant, such as in mistletoe (Viscum album) and dodder.
- Propagative roots: roots that form adventitious buds that develop into aboveground shoots, termed suckers, which form new plants, as in Canada thistle, cherry and many others.
- Proteoid roots or cluster roots: dense clusters of rootlets of limited growth that develop under low phosphate or low iron conditions in Proteaceae and some plants from the following families Betulaceae, Casuarinaceae, Elaeagnaceae, 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 food or water, such as carrots and beets. They include some taproots and tuberous roots.
- Structural roots: large roots that have undergone considerable secondary thickening and provide mechanical support to woody plants and trees.
- Surface roots: These proliferate close below the soil surface, exploiting water and easily available nutrients. Where conditions are close to optimum in the surface layers of soil, the growth of surface roots is encouraged and they commonly become the dominant roots.
- Tuberous roots: A portion of a root swells for food or water storage, e.g. sweet potato. A type of storage root distinct from taproot.
