Root plant to fuel the process. In return, the plant takes nitrogen compounds produced from ammonia by the bacteria. The term root crops refers to any edible underground plant structure, but many root crops are actually stems, such as potato tubers. Edible roots include cassava, sweet potato, beet, carrot, rutabaga, turnip, parsnip, radish, yam and horseradish. Spices obtained from roots include sassafras, angelica, sarsaparilla and licorice. Sugar beet is an important source of sugar. Yam roots are a source of estrogen compounds used in birth control pills. The fish poison and insecticide rotenone is obtained from roots of "Lonchocarpus" spp. Important
Cluster root primary rootlets, and also form secondary rootlets on the primary rootlets. Some Proteaceae, such as "Banksia" and "Grevillea", are valued by the horticulture and floriculture industries. In cultivation, only slow-release low-phosphorus fertilizers should be used, as higher levels cause phosphorus toxicity and sometimes iron deficiency, leading to plant death. Crop management should minimise root disturbance, and weed control should be via slashing or contact herbicides. Many plants with proteoid roots have economic value. Cultivated crops with proteoid roots include "Lupinus" and "Macadamia". 5. Lambers, H. & Poot, P. (eds) 2003. Structure and Functioning of Cluster Roots and Plant Responses to