{{short description|Seed or dry fruit or infructescence that has hooks or teeth}} {{other uses|Bur (disambiguation)|Burr (disambiguation)}} [[File:Bur Macro BlackBg.jpg|thumb|right|''Geum'' bur]] [[File:Burdock Hooks.jpg|thumb|right|Hooked burs of ''Arctium'' (burdock)]] [[File:Xanthium italicum ENBLA02.JPG|thumb|right|''Xanthium'' bur]] [[File:Bur Macro.jpg|thumb|right|Close-up of a single ''Arctium'' bur]]
A '''bur''' (also spelled '''burr''')<ref name=Cook68>{{Cite book |last=Cook |first=J. Gordon |year=1968 |title=ABC of Plant Terms |publication-place=Watford, Herts |publisher=Merrow |oclc=223208923 }}</ref> is a seed, dry fruit or infructescence that has hooks or teeth. The main function of the bur is to spread the seeds of the bur plant, often through epizoochory. The hooks of the bur are used to latch onto fur or fabric, enabling the bur {{ndash}} which contain seeds {{ndash}} to be transported to another location for dispersal.<ref name=":0" /> Another use for the spines and hooks are physical protection against herbivores.<ref name=":2" /> Their ability to stick to animals and fabrics has shaped their reputation as bothersome.
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 ''Tribulus terrestris'' and ''Xanthium'' species, are often single-stemmed when growing in dense groups, but branch and spread when growing singly.<ref name=":0">{{cite web|url=http://www.dpi.nsw.gov.au/agriculture/pests-weeds/weeds/profiles/californian-burr |title=Noogoora burr, Californian burr, Italian cockleburr and South American burr (''Xanthium'' species)|publisher=New South Wales Department of Primary Industries|date=2009}}</ref> The number of burs per fruit along with the size and shape can vary largely between different bur plants.<ref name=":1">{{Cite journal|last1=Gorb|first1=Elena|last2=Gorb|first2=Stanislav|date=2002-04-01|title=Contact separation force of the fruit burrs in four plant species adapted to dispersal by mechanical interlocking|url=https://www.sciencedirect.com/science/article/pii/S0981942802013815|journal=Plant Physiology and Biochemistry|language=en|volume=40|issue=4|pages=373–381|doi=10.1016/S0981-9428(02)01381-5|bibcode=2002PlPB...40..373G |issn=0981-9428|url-access=subscription}}</ref>
== Function == Containing seeds, burs spread through catching on the fur of passing animals (epizoochory) or machinery as well as by being transported together with water, gravel and grain.<ref name=":0" /> 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.{{cn|date=February 2023}}
Burs serve the plants that bear them in two main ways. * Firstly, burs are spinescent and tend to repel some herbivores, much as other spines and prickles do.<ref name=":2">{{Cite journal|last1=Xu|first1=Qi|last2=Lev-Yadun|first2=Simcha|last3=Sun|first3=Lu|last4=Chen|first4=Zhe|last5=Song|first5=Bo|last6=Sun|first6=Hang|date=2020-04-01|title=Spinescent patterns in the flora of Jiaozi Snow Mountain, Southwestern China|journal=Plant Diversity|language=en|volume=42|issue=2|pages=83–91|doi=10.1016/j.pld.2019.12.002|issn=2468-2659|pmc=7195588|pmid=32373766|bibcode=2020PlDiv..42...83X }}</ref> * 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).<ref name=sf>Magee, M. B. [http://homeguides.sfgate.com/plants-burrs-26416.html Plants With Burrs.] ''San Francisco Chronicle''.</ref> Spinescent plants repel herbivores mechanically by wounding the herbivore's mouth or digestive system. Moreover, burs' mechanical defence can work alongside the color of the bur that can visually warn off herbivores.<ref name=":2" />
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.<ref>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</ref> It also might penetrate a hoof or foot pad or the tires 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'',<ref name="OudtshoornRooyen1999">{{cite book|author1=Karen van Rheede van Oudtshoorn|author2=Margaretha W. van Rooyen|title=Dispersal Biology of Desert Plants|url=https://books.google.com/books?id=bdTfBfsl-DcC&pg=PA95|year=1999|publisher=Springer|isbn=978-3-540-64886-4|pages=95–}}</ref> or shaped like a caltrop as in some species of ''Tribulus'' that have achieved the status of cosmopolitan weeds by sticking to the tires of aircraft.<ref>Holm, L. et al. A geographical atlas of world weeds. Krieger 1991, {{ISBN|978-0894643576}}</ref>
The bur must be able to easily detach from the plant and easily attach to for example the fur of an animal. The ability to spread the seeds depends both on the number of burs that manage to get attached and on force of attachment. The hook span of the bur has been shown to have a large influence on the contact separation force. Some studies have also shown force can increase with the size of the bur, although not all large burs have a high contact separation force. Furthermore, the flexibility of the bur might also influence this force which can increase with stiffness. <ref name=":1" />
==Relevance to humans== Burs are best known as sources of irritation, injury to livestock,<ref name=":0"/> damage to clothing, punctures to tires, and clogging equipment such as agricultural harvesting machinery. Furthermore, because of their ability to compete with crops over moisture and nutrition, bur plants can be labelled as weeds and therefore also be subject to removal. Methods of controlling the spread of bur plants include the use of herbicides, slashing and cultivation among others.<ref name=":0" />
Some have however been used for such purposes as fabric fulling, for which the fuller's teasel is a traditional resource.{{cn|date=February 2023}}
The bur of burdock was the inspiration for hook and loop fastener, also known as Velcro.<ref>{{cite web|url=http://content.time.com/time/nation/article/0,8599,1996883,00.html|title=A Brief History of: Velcro|first=Claire|last=Suddath|date=15 June 2010|access-date=17 October 2018|publisher=Time|via=content.time.com}}</ref>
== Common plants with burs == Common bur-bearing plants include:
* ''Acanthospermum australe'' (''Paraguayan starburr)''<ref>{{Cite web|title=ITIS - Report: Acanthospermum australe|url=https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=36445#null|access-date=2021-11-13|website=www.itis.gov}}</ref> * ''Agrimonia pubescens'' (soft agrimony)<ref>{{Cite web|title=Agrimonia pubescens - Michigan Flora|url=https://michiganflora.net/species.aspx?id=2418|access-date=2021-11-13|website=michiganflora.net}}</ref> * ''Anthriscus caucalis'' (burr chervil)<ref>{{Cite web|title=Anthriscus caucalis|url=https://www.calflora.org/app/taxon?crn=396|url-status=live|access-date=2021-11-13|website=www.calflora.org|archive-url=https://web.archive.org/web/20210305045929/https://www.calflora.org/app/taxon?crn=396 |archive-date=2021-03-05 }}</ref> * ''Arctium lappa'' (greater burdock)<ref>{{Cite web|title=Arctium lappa L. GRIN-Global|url=https://npgsweb.ars-grin.gov/gringlobal/taxon/taxonomydetail?id=3857|access-date=2021-11-13|website=npgsweb.ars-grin.gov}}</ref> * ''Bidens pilosa'' (beggar ticks)<ref name=sf/> * ''Cenchrus longispinus'' (longspine sandbur)<ref>[http://www.ipm.ucdavis.edu/PMG/WEEDS/longspine_sandbur.html ''Cenchrus longispinus''.] University of California IPM.</ref> * ''Circaea lutetiana'' (enchanter's nightshade)<ref>{{Cite web|title=Circaea lutetiana subsp. canadensis (Broadleaf Enchanter's Nightshade) {{!}} North Carolina Extension Gardener Plant Toolbox|url=https://plants.ces.ncsu.edu/plants/circaea-lutetiana-subsp-canadensis/|access-date=2021-11-13|website=plants.ces.ncsu.edu}}</ref> * ''Daucus carota'' (Queen Anne's lace)<ref>{{Cite web|title=Daucus carota (Bee's Nest-Plant, Bird's-Nest, Devil's Plague, Queen Anne's Lace, Wild Carrot) {{!}} North Carolina Extension Gardener Plant Toolbox|url=https://plants.ces.ncsu.edu/plants/daucus-carota/|access-date=2021-11-13|website=plants.ces.ncsu.edu}}</ref> * ''Hylodesmum glutinosum'' (pointed tick-trefoil)<ref>{{Cite web|title=Hylodesmum glutinosum (Muhlenberg ex Willdenow) H. Ohashi & R.R. Mill - Database of Vascular Plants of Canada (VASCAN)|url=http://data.canadensys.net/vascan/taxon/27746|access-date=2021-11-13|website=data.canadensys.net}}</ref> * ''Galium aparine'' (cleavers)<ref>{{cite journal |author1=Gorb, E. |author2=Gorb, S. |year=2002 |title=Contact separation force of the fruit burrs in four plant species adapted to dispersal by mechanical interlocking |journal=Plant Physiology and Biochemistry |volume=40 |issue=4 |pages=373–381 |doi=10.1016/S0981-9428(02)01381-5|bibcode=2002PlPB...40..373G }}</ref> * ''Geum aleppicum'' (yellow avens)<ref>[http://ontariowildflowers.com/main/species.php?id=206] Ontario Wildflowers </ref> * ''Geum canadense'' (white avens)<ref>[http://ontariowildflowers.com/main/species.php?id=204] Ontario Wildflowers </ref> * ''Geum urbanum'' (herb bennet)<ref>{{Cite web|title=Geum urbanum Linnaeus - Database of Vascular Plants of Canada (VASCAN)|url=http://data.canadensys.net/vascan/taxon/8776?lang=en|access-date=2021-11-13|website=data.canadensys.net}}</ref> * ''Osmorhiza claytonii'' (Clayton's sweetroot)<ref>{{Cite PLANTS |symbol=OSCL |taxon=Osmorhiza claytonii |access-date=2021-11-13}}</ref> * ''Phryma leptostachya'' (American lopseed)<ref>{{Cite PLANTS |symbol=PHLE5 |taxon=Phryma leptostachya |access-date=2021-11-13}}</ref> * ''Tribulus terrestris'' (puncturevine)<ref>[http://www.ipm.ucdavis.edu/PMG/WEEDS/puncturevine.html ''Tribulus terrestris''.] University of California IPM.</ref> * ''Xanthium strumarium'' (cocklebur)<ref>[http://www.ipm.ucdavis.edu/PMG/WEEDS/common_cocklebur.html ''Xanthium strumarium''.] University of California IPM.</ref>
==References== {{Reflist}}
== External links == * {{Cite EB1911|wstitle=Bur}}
Category:Lists of plants Category:Plant morphology