tractile roots: roots that 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. Coralloid roots: similar to root nodules, these provide nitrogen to the plant. They are often larger than nodules, branched, and located at or near the soil surface, and harbor nitrogen-fixing cyanobacteria. They are only found in cycads. Dimorphic root systems: roots with two distinctive forms for two separate functions Fine roots: typically primary roots 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 common milkweed (Asclepias syriaca), Canada thistle (Cirsium arvense), and many others. Photosynthetic roots: roots that are green and photosynthesize, providing sugar to the plant. They are similar to phylloclades. Several orchids have these, such as Dendrophylax and Taeniophyllum. 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. Root nodules: roots that harbor nitrogen-fixing soil bacteria. These are often very short and rounded. Root nodules are found in virtually all legumes. Stilt roots: adventitious support roots, common among mangroves. They grow down from lateral branches, branching in the soil. Storage roots: roots 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: roots that 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: fleshy and enlarged lateral roots for food or water storage, e.g. sweet potato. A type of storage root distinct from tapro