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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 A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, st ...
,
root In vascular plants, the roots are the organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plants to grow taller and faster. They are most often below the su ...
s, stems or
buds In botany, a bud is an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of a stem. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately. Buds may be spec ...
with sharp, stiff ends, and generally serve the same function: physically deterring animals from eating the plant material.


Description

In common language the terms are used more or less interchangeably, but in botanical terms, thorns are derived shoots (so that they may or may not be branched, they may or may not have leaves, and they may or may not arise from a bud),Simpson, M. G. 2010. "Plant Morphology". In: ''Plant Systematics, 2nd. edition''. Elsevier Academic Press. Chapter 9.Judd, Campbell, Kellogg, Stevens, Donoghue. 2007. "Structural and Biochemical Characters". In: ''Plant Systematics, a phylogenetic approach, third edition''. Chapter 4.Turner et al. 2005, ''Sonoran Desert Plants, an Ecological Atlas.'' University of Arizona Press.
/ref>Van Wyk, Van Wyk. 2007. ''How to identify trees in South Africa.'' Struik.
/ref> spines are derived from
leaves A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, st ...
(either the entire leaf or some part of the leaf that has vascular bundles inside, like the petiole or a
stipule In botany, a stipule is an outgrowth typically borne on both sides (sometimes on just one side) of the base of a leafstalk (the petiole). Stipules are considered part of the anatomy of the leaf of a typical flowering plant, although in many speci ...
), and prickles are derived from
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rel ...
tissue (so that they can be found anywhere on the plant and do not have vascular bundles inside). Leaf margins may also have teeth, and if those teeth are sharp, they are called spinose teeth on a spinose
leaf margin A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ste ...
(some authors consider them a kind of spine). On a leaf apex, if there is an apical process (generally an extension of the midvein), and if it is especially sharp, stiff, and spine-like, it may be referred to as spinose or as a ''pungent apical process'' (again, some authors call them a kind of spine). When the leaf epidermis is covered with very long, stiff
trichome Trichomes (); ) are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hair on a p ...
s (more correctly called
bristle A bristle is a stiff hair or feather (natural or artificial), either on an animal, such as a pig, a plant, or on a tool such as a brush or broom. Synthetic types Synthetic materials such as nylon are also used to make bristles in items such a ...
s in this case; for some authors a kind of prickle), it may be referred to as a ''hispid vestiture''; if the trichomes are stinging trichomes, it may be called a ''urent vestiture''. There can be found also spines or spinose structures derived from roots.


Function

The predominant function of thorns, spines, and prickles is deterring
herbivory 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 ...
in a mechanical form. For this reason, they are classified as physical or mechanical defenses, as opposed to
chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., w ...
defenses. Not all functions of spines or
glochid Glochids or glochidia (singular "glochidium") are hair-like spines or short prickles, generally barbed, found on the areoles of cacti in the sub-family ''Opuntioideae''. Cactus glochids easily detach from the plant and lodge in the skin, causin ...
s are limited to defense from physical attacks by herbivores and other animals. In some cases, spines have been shown to shade or insulate the plants that grow them, thereby protecting them from extreme temperatures. For example, saguaro cactus spines shade the apical meristem in summer, and in members of the
Opuntioideae Opuntioideae is a subfamily of the cactus family, Cactaceae. It contains 15 genera divided into five tribes. The subfamily encompasses roughly 220–250 species, and is geographically distributed throughout the New World from Canada, to Argentina ...
, glochids insulate the apical meristem in winter. Agrawal ''et al.'' (2000) found that spines seem to have little effect on specialist pollinators, on which many plants rely in order to reproduce.


Definitions and technical distinctions

Pointing or spinose processes can broadly be divided by the presence of vascular tissue: thorns and spines are derived from shoots and leaves respectively, and have vascular bundles inside, whereas prickles (like
rose A rose is either a woody perennial flowering plant of the genus ''Rosa'' (), in the family Rosaceae (), or the flower it bears. There are over three hundred species and tens of thousands of cultivars. They form a group of plants that can be ...
prickles) do not have vascular bundles inside, so that they can be removed more easily and cleanly than thorns and spines.


Thorns

Thorns are modified branches or stems. They may be simple or branched. File:thorn image.JPG, Smooth, featureless ''
Citrus ''Citrus'' is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, lemons, grapefruits, pomelos, and limes. The genus ''Citrus'' is native to ...
'' thorn. File:Gymnosporia buxifolia thorn.JPG, '' Gymnosporia buxifolia'' thorn, its leaves, nodes, and emergence from an axillary bud demonstrating its nature as a branch. File:Carissa bispinosa Uniondale 1167.jpg, '' Carissa bispinosa'' showing characteristic branched thorns.


Spines

Spines are modified
leaves A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, st ...
,
stipules In botany, a stipule is an outgrowth typically borne on both sides (sometimes on just one side) of the base of a leafstalk (the petiole). Stipules are considered part of the anatomy of the leaf of a typical flowering plant, although in many speci ...
, or parts of leaves, such as extensions of leaf veins. Some authors prefer not to distinguish spines from thorns because, like thorns, and unlike prickles, they commonly contain
vascular tissue Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. The ...
.Bell, A.D. 1997. ''Plant form: an illustrated guide to flowering plant morphology''. Oxford University Press, Oxford, U.K
preview in google books
/ref> Spines are variously described as petiolar spines (as in ''
Fouquieria ''Fouquieria'' is a genus of 11 species of desert plants, the sole genus in the family Fouquieriaceae. The genus includes the ocotillo ('' F. splendens'') and the Boojum tree or cirio ('' F. columnaris''). They have semisucculent stems with thin ...
''), leaflet spines (as in '' Phoenix''), or stipular spines (as in ''
Euphorbia ''Euphorbia'' is a very large and diverse genus of flowering plants, commonly called spurge, in the family Euphorbiaceae. "Euphorbia" is sometimes used in ordinary English to collectively refer to all members of Euphorbiaceae (in deference to t ...
''), all of which are examples of spines developing from a part of a leaf containing the petiole, midrib, or a secondary vein. The plants of the cactus family are particularly well known for their dense covering of spines. Some cacti have also
glochid Glochids or glochidia (singular "glochidium") are hair-like spines or short prickles, generally barbed, found on the areoles of cacti in the sub-family ''Opuntioideae''. Cactus glochids easily detach from the plant and lodge in the skin, causin ...
s (or glochidia, singular glochidium) – a particular kind of spine of different origin, which are smaller and deciduous with numerous retrose barbs along its length (as found in
areole In botany, areoles are small light- to dark-colored bumps on cacti out of which grow clusters of spines. Areoles are important diagnostic features of cacti, and identify them as a family distinct from other succulent plants. Gordon Rowley - W ...
s of ''
Opuntia ''Opuntia'', commonly called prickly pear or pear cactus, is a genus of flowering plants in the cactus family Cactaceae. Prickly pears are also known as ''tuna'' (fruit), ''sabra'', '' nopal'' (paddle, plural ''nopales'') from the Nahuatl word ...
''). File:Ocotillothron02262006.JPG, The spines of '' Fouquieria splendens'' develop from the leaf petioles. File:AcaciaXanthophloeaSpines.JPG, Stipule spines on ''
Vachellia xanthophloea ''Vachellia xanthophloea'' is a tree in the family Fabaceae, commonly known in English as the fever tree. This species of ''Vachellia'' is native to eastern and southern Africa (Botswana, Kenya, Malawi, Mozambique, Somalia, South Africa, Eswati ...
''. File:Pereskia grandifolia ies.jpg, Areoles and spines of the tree-like ''
Rhodocactus grandifolius ''Rhodocactus grandifolius'' (rose cactus; syn. ''Pereskia grandifolia'') is a species of cactus native to eastern and southern Brazil. Like all species in the genus ''Rhodocactus'' and unlike most cacti, it has persistent leaves. It was first ...
''. File:Fish hook Cactus Without wool.jpg, Spines of '' Mammillaria balsasoides'' File:With seperate hook.jpg, Hooked spine of ''Mammillaria balsasoides''


Prickles

Prickles are comparable to hairs but can be quite coarse (for example, rose prickles). They are extensions of the cortex and
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rel ...
. Technically speaking, many plants commonly thought of as having thorns or spines actually have prickles.
Rose A rose is either a woody perennial flowering plant of the genus ''Rosa'' (), in the family Rosaceae (), or the flower it bears. There are over three hundred species and tens of thousands of cultivars. They form a group of plants that can be ...
s, for instance, have prickles. File:Wait-a-bit stem.jpg, Raised prickles on the stem of ''
Caesalpinia decapetala ''Biancaea decapetala'', commonly known as shoofly, Mauritius or Mysore thorn or the cat's claw, is a tropical tree species originating in India. Introduced range ''B. decapetala'' has been introduced to Fiji, French Polynesia, Hawai‘i, New ...
''. File:Rose Prickles.jpg,
Rose A rose is either a woody perennial flowering plant of the genus ''Rosa'' (), in the family Rosaceae (), or the flower it bears. There are over three hundred species and tens of thousands of cultivars. They form a group of plants that can be ...
prickles. File:Solanum viarum 1673056.jpg, Prickles on the leaves of '' Solanum viarum''.


Other structures

Other similar structures are spinose teeth, spinose apical processes, and trichomes.
Trichome Trichomes (); ) are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hair on a p ...
s, in particular, are distinct from thorns, spines, and prickles in that they are much smaller (often microscopic) outgrowths of epidermal tissue, and they are less rigid and more hair-like in appearance; they typically consist of just a few cells of the outermost layer of epidermis, whereas prickles may include cortex tissue. Trichomes are often effective defenses against small insect herbivores; thorns, spines, and prickles are usually only effective against larger herbivores like birds and mammals. File:Hulst getand blad Ilex aquifolium.jpg, Spinose leaf margin in ''
Ilex aquifolium ''Ilex aquifolium'', the holly, common holly, English holly, European holly, or occasionally Christmas holly, is a species of flowering plant in the family Aquifoliaceae, native to western and southern Europe, northwest Africa, and southwest Asi ...
''. File:Sansevieria trifasciata2.jpg, Spinose apical process in '' Sansevieria''. File:Galium.aparine.jpg, Stiff, sharp trichomes in '' Galium aparine''. File:Urtica dioica stinging hair.jpg, Stinging trichome in ''
Urtica dioica ''Urtica dioica'', often known as common nettle, burn nettle, stinging nettle (although not all plants of this species sting) or nettle leaf, or just a nettle or stinger, is a herbaceous perennial flowering plant in the family Urticaceae. Ori ...
''.
''Spinescent'' is a term describing plants that bear any sharp structures that deter herbivory. It also can refer to the state of tending to be or become spiny in some sense or degree, as in: "... the division of the African acacias on the basis of spinescent stipules versus non-spinescent stipules..." There are also spines derived from roots, like the ones on the trunk of the "Root Spine Palms" (''
Cryosophila ''Cryosophila'' is a genus of medium-sized fan palms that range from central Mexico to northern Colombia. Species in the genus can be readily distinguished from related genera by their distinctive downward-pointing spines on the stem, which ar ...
'' spp.). The trunk roots of ''
Cryosophila guagara ''Cryosophila guagara'' is a species of flowering plant in the family Arecaceae. It is found in Costa Rica and Panama. It is threatened by habitat loss Habitat destruction (also termed habitat loss and habitat reduction) is the process by ...
'' grow downwards to a length of 6–12 cm, then stop growing and transform into a spine.Jackson, M. B. (ed.) 1986. ''New Root Formation in Plants and Cuttings.'' Series ''Developments in plant and soil sciences'' nº 20. Martinus Nijhoff Publishers, a member of the Kluwer Academic Publishers Group. Da ordrecht / Boston / Lancaster. p.80-81. https://books.google.com/books?id=gAv8CAAAQBAJ&pg=PA80 The anatomy of crown roots on this species (roots among the bases of the living fronds) also alters during their life. They initially grow upwards and then turn down and finally they, too, become spinous. Lateral roots on these two types of roots, as well as those on the stilt roots on this species, also become spinous. Some authors believe that some of these short spiny laterals have a ventilating function so they are 'pneumorhizae'. Short spiny laterals that may have a ventilating function may also be found on roots of '' Iriartea exorrhiza''. There are also spines that function as pneumorhizae on the palm ''
Euterpe oleracea Euterpe (; el, Εὐτέρπη, lit=rejoicing well' or 'delight , from grc, εὖ, eû, well + el, τέρπειν, térpein, to please) was one of the Muses in Greek mythology, presiding over music. In late Classical times, she was named muse ...
''. In '' Cryosophila nana'' (formerly ''Acanthorhiza aculeata'') there are spine roots or root spines, some authors may prefer "root spines" if the length of the root is less than 10x the thickness, and "spine roots" if the length is more than 10x the thickness. Adventitious spiny roots have also been described on the trunks of dicotyledonous trees from tropical Africa (e.g. Euphorbiaceae, as in ''
Macaranga barteri ''Macaranga'' is a large genus of Old World tropical trees of the family Euphorbiaceae and the only genus in the subtribe Macaranginae (tribe Acalypheae). Native to Africa, Australasia, Asia and various islands of the Indian and Pacific Oceans, t ...
'', ''
Bridelia micrantha ''Bridelia micrantha'', the mitzeeri or the coastal golden-leaf, is a tree in the family Phyllanthaceae and is native to tropical and southern Africa as well as to the island of Réunion in the Indian Ocean. Description A medium to tall tree ( ...
'' and '' B. pubescens''; Ixonanthaceae, Sterculiaceae), and may also be found protecting perennating organs such as tubers and corms (e.g. ''
Dioscorea prehensilis ''Dioscorea'' is a genus of over 600 species of flowering plants in the family Dioscoreaceae, native throughout the tropical and warm temperate regions of the world. The vast majority of the species are tropical, with only a few species extending ...
'' -Dioscoreaceae- and '' Moraea'' spp. -Iridaceae- respectively). Short root spines cover the tuberous base of the epiphytic ant-plant ''
Myrmecodia tuberosa ''Myrmecodia tuberosa'', the ant plant, is an epiphytic plant. The species has a symbiotic relationship with some ant species where ants use the hollow body of the plant as shelter, defend the plant from other insects, and provide nutrients to ...
'' (Rubiaceae), these probably give protection to ants which inhabit chambers within the tuber as they wander over the plant's surface. (Jackson 1986 and references therein). In many respects, the pattern of spine formation is similar to that which occurs in the development of thorns from lateral shoots. (Jackson 1986 and references therein).


Evolution

It has been proposed that thorny structures may have first evolved as a
defense mechanism In psychoanalytic theory, a defence mechanism (American English: defense mechanism), is an unconscious psychological operation that functions to protect a person from anxiety-producing thoughts and feelings related to internal conflicts and o ...
in plants growing in sandy environments that provided inadequate resources for fast regeneration of damage.


Morphological variation

Spinose structures occur in a wide variety of ecologies, and their morphology also varies greatly. They occur as: * sharpened branches (e.g. in '' Carissa'', ''
Citrus ''Citrus'' is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, lemons, grapefruits, pomelos, and limes. The genus ''Citrus'' is native to ...
'', ''
Crataegus ''Crataegus'' (), commonly called hawthorn, quickthorn, thornapple, Voss, E. G. 1985. ''Michigan Flora: A guide to the identification and occurrence of the native and naturalized seed-plants of the state. Part II: Dicots (Saururaceae–Cornacea ...
''), * spiky inflorescences (''
Tylecodon ''Tylecodon'' is a genus of succulent plants in the family Crassulaceae, native to southern Africa. Until the late 1970s all these plants were included in the genus '' Cotyledon'', but in 1978 Helmut Toelken of the Botanical Research Institute, ...
''), * a tiny point at the tip of the leaf (mucronate leaves) ('' Sansevieria''), * leaves fully converted to spines (''
Opuntia ''Opuntia'', commonly called prickly pear or pear cactus, is a genus of flowering plants in the cactus family Cactaceae. Prickly pears are also known as ''tuna'' (fruit), ''sabra'', '' nopal'' (paddle, plural ''nopales'') from the Nahuatl word ...
''), * stipules converted to spines (many '' Vachellia''), * prickles on stems (''
Rosa Rosa or De Rosa may refer to: People *Rosa (given name) * Rosa (surname) *Santa Rosa (female given name from Latin-a latinized variant of Rose) Places *223 Rosa, an asteroid * Rosa, Alabama, a town, United States * Rosa, Germany, in Thuringia, ...
'', ''
Erythrina ''Erythrina'' is a genus of plants in the pea family, Fabaceae. It contains about 130 species, which are distributed in tropical and subtropical regions worldwide. They are trees, with the larger species growing up to in height. The generic n ...
'' and ''
Ceiba speciosa ''Ceiba speciosa'', the floss silk tree (formerly ''Chorisia speciosa''), is a species of deciduous tree that is native to the tropical and subtropical forests of South America. It has several local common names, such as ''palo borracho'' (in S ...
''), * urticating (i.e. stinging) hairs, * bristles, and * finely barbed spines called
glochid Glochids or glochidia (singular "glochidium") are hair-like spines or short prickles, generally barbed, found on the areoles of cacti in the sub-family ''Opuntioideae''. Cactus glochids easily detach from the plant and lodge in the skin, causin ...
s. Some thorns are hollow and act as myrmecodomatia; others (e.g. in ''
Crataegus monogyna ''Crataegus monogyna'', known as common hawthorn, one-seed hawthorn, or single-seeded hawthorn, is a species of flowering plant in the rose family Rosaceae. It is native to Europe, northwestern Africa, and West Asia, but has been introduced i ...
'') bear leaves. The thorns of many species are branched (e.g. in ''
Crataegus crus-galli ''Crataegus crus-galli'' is a species of hawthorn known by the common names cockspur hawthorn and cockspur thorn. It is native to eastern North America from Ontario to Texas to Florida, and it is widely used in horticulture. It is thought to be t ...
'' and '' Carissa macrocarpa'').


Human uses

Plants bearing thorns, spines, or prickles are often used as a defense against
burglary Burglary, also called breaking and entering and sometimes housebreaking, is the act of entering a building or other areas without permission, with the intention of committing a criminal offence. Usually that offence is theft, robbery or murd ...
, being strategically planted below windows or around the entire perimeter of a property. They also have been used to protect crops and livestock against marauding animals. Examples include
hawthorn Hawthorn or Hawthorns may refer to: Plants * '' Crataegus'' (hawthorn), a large genus of shrubs and trees in the family Rosaceae * ''Rhaphiolepis'' (hawthorn), a genus of about 15 species of evergreen shrubs and small trees in the family Rosace ...
hedges in Europe,
agave ''Agave'' (; ; ) is a genus of monocots native to the hot and arid regions of the Americas and the Caribbean, although some ''Agave'' species are also native to tropical areas of North America, such as Mexico. The genus is primarily known for ...
s in the Americas and in other countries where they have been introduced,
Osage orange ''Maclura pomifera'', commonly known as the Osage orange ( ), is a small deciduous tree or large shrub, native to the south-central United States. It typically grows about tall. The distinctive fruit, a multiple fruit, is roughly spherical ...
in the prairie states of the US, and '' Sansevieria'' in Africa.Hunter, J. A., "Hunter" Publisher: Buccaneer Books, 1993,


See also

*
Areole In botany, areoles are small light- to dark-colored bumps on cacti out of which grow clusters of spines. Areoles are important diagnostic features of cacti, and identify them as a family distinct from other succulent plants. Gordon Rowley - W ...
*
Glochid Glochids or glochidia (singular "glochidium") are hair-like spines or short prickles, generally barbed, found on the areoles of cacti in the sub-family ''Opuntioideae''. Cactus glochids easily detach from the plant and lodge in the skin, causin ...
*
Trichome Trichomes (); ) are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hair on a p ...
*
Plant defense against herbivory Plant defense against herbivory or host-plant resistance (HPR) describes a range of adaptations evolved by plants which improve their survival and reproduction by reducing the impact of herbivores. Plants can sense being touched, and they can ...


References


Bibliography

* Simpson, M. G. 2010. "Plant Morphology". In: ''Plant Systematics, 2nd. edition''. Elsevier Academic Press. Chapter 9. * Judd, Campbell, Kellogg, Stevens, Donoghue. 2007. "Structural and Biochemical Characters". In: ''Plant Systematics, a phylogenetic approach, third edition''. Chapter 4. *Esau, K. 1965. ''Plant Anatomy'', 2nd Edition. John Wiley & Sons. 767 pp. *Llamas, K. A. 2003. ''Tropical Flowering Plants''. Timber Press, Portland. 423 pp.


External links

* {{DEFAULTSORT:Thorns, Spines, And Prickles Plant morphology Biological defense mechanisms Hazards of outdoor recreation