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A bur (also spelled burr) is a seed or dry fruit or infructescence that has hooks or teeth. Some other forms of diaspores, such as the stems of certain species of cactus also are covered with thorns and may function as burs. Bur-bearing plants such as ''Xanthium'' species are often single-stemmed when growing in dense groups, but branch and spread when growing singly.〔(Noogoora burr, Californian burr, Italian cockleburr and South American burr (''Xanthium'' species). ) New South Wales Department of Primary Industries. 2009.〕 == Function == Burs catch on the fur of passing animals or the clothing of people. The hooks or teeth generally cause irritation, and some species commonly cause gross injury to animals, or expensive damage to clothing or to vehicle tires. Burs serve the plants that bear them in two main ways. * Firstly, burs tend to repel some herbivores, much as other spines and prickles do. * Secondly, plants with burs rely largely on living agents to disperse their seeds; their burs are mechanisms of seed dispersal by epizoochory (dispersal by attaching to the outside of animals), of which one form is anthropochory (dispersal by attaching to the outside of humans, usually on their clothing).〔Magee, M. B. (Plants With Burrs. ) ''San Francisco Chronicle''.〕 Most epizoochorous burs attach to hair on the body or legs of the host animal, but a special class of epizoochorous bur is known as the trample-bur (or trample-burr). Several species of ''Tribulus'', ''Harpagophytum'', and ''Grielum'' produce fruit in the form of trample-burs. As the name suggests, they attach themselves to the animal when trampled. They may hook onto the legs of animals as the large hooks of ''Harpagophytum'' do, sometimes causing serious injury, but sometimes hooking onto the leg of say, an ostrich, apparently without causing discomfort.〔Midgley, J.J. and Illing, N. Were Malagasy Uncarina fruits dispersed by the extinct elephant bird? South African Journal of Science 105, November/December 2009〕 It also might penetrate a hoof or foot pad or the tyre of a vehicle, only to be shed after being carried for a considerable time and distance; most ''Tribulus'' and ''Grielum'' species are specialised for such attachment, variously being flat, but with upward-directed spikes as in say, ''Grielum humifusum'', or shaped like a caltrop as in some species of ''Tribulus'' that have achieved the status of cosmopolitan weeds by sticking to the tyres of aircraft.〔Holm, L. et al. A geographical atlas of world weeds. Krieger 1991, ISBN 978-0894643576〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「bur」の詳細全文を読む スポンサード リンク
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