A bur (also spelled burr) is a seed or 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 which contain seeds to be transported to another location for dispersal. Another use for the spines and hooks are physical protection against herbivores. Their ability to stick to animals and fabrics has shaped their reputation as bothersome. Some other forms of

diaspores In botany, a diaspore is a plant dispersal unit consisting of a seed or spore plus any additional tissues that assist dispersal. In some seed plants, the diaspore is a seed and fruit together, or a seed and elaiosome. In a few seed plants, the di ...

, 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. The number of burs per fruit along with the size and shape can vary largely between different bur plants.

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. 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 are

spinescent In plant morphology, thorns, spines, and prickles, and in general spinose structures (sometimes called ''spinose teeth'' or ''spinose apical processes''), are hard, rigid extensions or modifications of leaves, roots, stems or buds with sharp, s ...

and tend to repel some

herbivore A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpart ...

s, 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).Magee, M. B
Plants With Burrs.
''San Francisco Chronicle''. 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. 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. 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'', 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. 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.

Relevance to humans

Burs are best known as sources of irritation, injury to livestock, 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. Some have however been used for such purposes as fabric fulling, for which the

fuller's teasel ''Dipsacus fullonum'', syn. ''Dipsacus sylvestris'', is a species of flowering plant known by the common names wild teasel or fuller's teasel, although the latter name is usually applied to the cultivated variety ''D. fullonum'' var. ''sativus' ...

is a traditional resource. The bur of

burdock ''Arctium'' is a genus of biennial plants commonly known as burdock, family Asteraceae. Native to Europe and Asia, several species have been widely introduced worldwide. Burdock's clinging properties, in addition to providing an excellent mecha ...

was the inspiration for hook and loop fastener, also known as Velcro.

Common plants with burs

Common bur-bearing plants include: * '' Acanthospermum australe'' (''Paraguayan starburr)'' * '' Agrimonia pubescens'' (soft agrimony) * '' Anthriscus caucalis'' (burr chervil) * '' Arctium lappa'' (greater burdock) * '' Bidens pilosa'' (beggar ticks) * ''

Cenchrus longispinus ''Cenchrus longispinus'' is a species of grass, also known as spiny burr grass or gentle Annie. Its fruits are clumped into " burrs" with sharp, barbed spines that can penetrate the hides and mouth of grazing animals. They can also become lodged ...

'' (longspine sandbur) * ''

Circaea lutetiana ''Circaea lutetiana'', known as broad-leaved enchanter's nightshade, is a plant in the evening primrose family, Onagraceae. The genus name comes from the enchantress Circe of Greek mythology and the specific designation is derived from Lutetia, ...

'' (enchanter's nightshade) * ''

Daucus carota ''Daucus carota'', whose common names include wild carrot, European wild carrot, bird's nest, bishop's lace, and Queen Anne's lace (North America), is a flowering plant in the family Apiaceae. It is native to temperate regions of the Old Wor ...

'' (Queen Anne's lace) * ''

Hylodesmum glutinosum ''Hylodesmum glutinosum'' is a species of flowering plant in the family (biology), family Fabaceae. Common names include large tick-trefoil, clustered-leaved tick-trefoil, large-flowered tick-clover, pointed tick-trefoil, beggar's lice and pointe ...

'' (pointed tick-trefoil) * ''

Galium aparine ''Galium aparine'', with common names including cleavers, clivers, catchweed and sticky willy among others, is an annual, herbaceous plant of the family Rubiaceae. Names ''Galium aparine'' is known by a variety of common names in English. They ...

'' (cleavers) * ''

Geum aleppicum ''Geum aleppicum'', commonly called yellow avens or common avens is a flowering plant native to most of the temperate Northern Hemisphere, from eastern Europe across Asia and North America. It is a herbaceous perennial plant growing to 1 m ta ...

'' (yellow avens) * '' Geum canadense'' (white avens) * '' Geum urbanum'' (herb bennet) * ''Osmorhiza'' ''claytonii'' (Clayton's sweetroot) * '' Phryma leptostachya'' (American lopseed) * '' Tribulus terrestris'' (puncturevine) * '' Xanthium strumarium'' (cocklebur)''Xanthium strumarium''.
University of California IPM.

References

External links

* {{Cite EB1911, wstitle=Bur Lists of plants Plant morphology