Thorns, spines, and prickles

{{Short description|Sharp plant parts used for defense}} [[File:Thorns on a blackberry branch in Norrkila.jpg|thumb|upright=1.2|Prickles on a blackberry branch]] 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, stiff ends, and generally serve the same function: physically defending plants against herbivory.

==Description== In common language, the terms are used more or less interchangeably, but in botanical terms, thorns are derived from 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),<ref name="Simpson 2010">Simpson, M. G. 2010. "Plant Morphology". In: ''Plant Systematics, 2nd. edition''. Elsevier Academic Press. Chapter 9.</ref><ref name="Judd et al. 2007">Judd, Campbell, Kellogg, Stevens, Donoghue. 2007. "Structural and Biochemical Characters". In: ''Plant Systematics, a phylogenetic approach, third edition''. Chapter 4.</ref><ref name="Turner et al. 2005">[https://books.google.com/books?id=IbKmK2_Tn74C&dq=glossary+thorn+spine+prickle+spinose&pg=PA433 Turner et al. 2005, ''Sonoran Desert Plants, an Ecological Atlas.'' University of Arizona Press.]</ref><ref name="Van Wyk and Van Wyk 2007">[https://books.google.com/books?id=9HOmOElZL6IC&dq=thorn+spine+prickle&pg=PA35 Van Wyk, Van Wyk. 2007. ''How to identify trees in South Africa.'' Struik.]</ref> spines are derived from leaves (either the entire leaf or some part of the leaf that has vascular bundles inside, like the petiole or a stipule),<ref name="Simpson 2010" /><ref name="Judd et al. 2007" /><ref name="Turner et al. 2005" /><ref name="Van Wyk and Van Wyk 2007" /> and prickles are derived from epidermis tissue (so that they can be found anywhere on the plant and do not have vascular bundles inside<ref name="Van Wyk and Van Wyk 2007" />).<ref name="Simpson 2010" /><ref name="Judd et al. 2007" /><ref name="Turner et al. 2005" />

Leaf margins may also have teeth, and if those teeth are sharp, they are called spinose teeth on a spinose leaf margin<ref name="Simpson 2010" /><ref name="Judd et al. 2007" /> (some authors consider them a kind of spine<ref name="Judd et al. 2007" />). 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''<ref name="Simpson 2010" /> (again, some authors call them a kind of spine<ref name="Judd et al. 2007" />). When the leaf epidermis is covered with very long, stiff trichomes (more correctly called bristles in this case;<ref name="Simpson 2010" /> for some authors a kind of prickle<ref name="Judd et al. 2007" />), it may be referred to as a ''hispid vestiture'';<ref name="Simpson 2010" /><ref name="Judd et al. 2007" /><ref name="Turner et al. 2005" /> if the trichomes are stinging trichomes, it may be called a ''urent vestiture''.<ref name="Simpson 2010" />

There can be found also spines or spinose structures derived from roots.<ref name="Jackson 1986" />

==Function== The predominant function of thorns, spines, and prickles is deterring herbivory in a mechanical form. For this reason, they are classified as physical or mechanical defenses, as opposed to chemical defenses.

Not all functions of spines or glochids 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, 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.<ref>{{Cite journal |last1=Agrawal |first1=Anurag A. |last2=Rudgers |first2=Jennifer A. |last3=Botsford |first3=Louis W. |last4=Cutler |first4=David |last5=Gorin |first5=Jessica B. |last6=Lundquist |first6=Carolyn J. |last7=Spitzer |first7=Brian W. |last8=Swann |first8=Alisa L. |date=2000 |title=Benefits and Constraints on Plant Defense against Herbivores: Spines Influence the Legitimate and Illegitimate Flower Visitors of Yellow Star Thistle, Centaurea solstitialis L. (Asteraceae) |journal=The Southwestern Naturalist |volume=45 |issue=1 |pages=1–5 |doi=10.2307/3672545 |jstor=3672545 |bibcode=2000SWNat..45....1A |issn=0038-4909}}</ref>

==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 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.

<gallery mode="packed" heights="200px" style="text-align:left"> File:thorn image.JPG|Smooth, featureless ''Citrus'' 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 1168.jpg|''Carissa bispinosa'' showing characteristic branched thorns. </gallery>

===Spines=== Spines are modified leaves, stipules, 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.<ref name=Bell>Bell, A.D. 1997. ''Plant form: an illustrated guide to flowering plant morphology''. Oxford University Press, Oxford, U.K. [https://books.google.com/books?id=SM3khPHXhKEC&q=adrian+bell preview in google books]</ref>

Spines are variously described as '''petiolar spines''' (as in ''Fouquieria''), '''leaflet spines''' (as in ''Phoenix''), or '''stipular spines''' (as in ''Euphorbia''), all of which are examples of spines developing from a part of a leaf containing the petiole, midrib, or a secondary vein.<ref name="Simpson 2010" /> The plants of the cactus family are particularly well known for their dense covering of spines. Some cacti have also '''glochids''' (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 areoles of ''Opuntia'').<ref name="Simpson 2010" />

<gallery mode="packed" heights="200" style="text-align:left"> File:Ocotillothron02262006.JPG|The spines of ''Fouquieria splendens'' develop from the leaf petioles. File:AcaciaXanthophloeaSpines.JPG|Stipule spines on ''Vachellia xanthophloea''. File:Pereskia grandifolia ies.jpg|Areoles and spines of the tree-like ''Rhodocactus grandifolius''. File:Fish hook Cactus Without wool.jpg|Spines of ''Mammillaria balsasoides''{{citation needed|date=May 2020}} File:With separate hook.jpg|Hooked spine of ''Mammillaria balsasoides'' </gallery>

===Prickles=== Prickles are comparable to hairs but can be quite coarse (for example, rose prickles). They are extensions of the cortex and epidermis.<ref>{{cite book|last=Van Wyk|first=Braam|title=How to Identify Trees in Southern Africa|publisher=Struik|year=2007|edition=illustrated|page=184|isbn=978-1-77007-240-4}}</ref><ref>{{cite web|url=http://www.biologie.uni-hamburg.de/b-online/e05/stachel.htm|title=Cross-Section Through the Prickle of a Rose|last=Sengbusch|first=Peter|date=2003-07-31|access-date=2009-04-27|archive-url=https://web.archive.org/web/20080430190522/http://www.biologie.uni-hamburg.de/b-online/e05/stachel.htm|archive-date=2008-04-30}}</ref> Technically speaking, many plants commonly thought of as having thorns or spines actually have prickles. Roses, for instance, have prickles.<ref name=Bell/> While the position of thorns and spines are known positively to be controlled by phyllotaxis, the positioning of prickles appears to be truly random. If not, then by a phyllotaxis so arcane as to give the appearance of randomness.{{citation needed|date=August 2023}} The largest prickles are found on the trunk and major limbs of ''Ceiba speciosa'' which can be {{cvt|5|cm}} long and diameter.

The genus ''Solanum'' contains a mixture of species with and without prickles. Phylogenetic analysis in 2024 concluded that members of a gene family known for involvement in cytokinin biosynthesis were involved in prickle production.<ref>{{Cite journal |last1=Satterlee |first1=James W. |last2=Alonso |first2=David |last3=Gramazio |first3=Pietro |last4=Jenike |first4=Katharine M. |last5=He |first5=Jia |last6=Arrones |first6=Andrea |last7=Villanueva |first7=Gloria |last8=Plazas |first8=Mariola |last9=Ramakrishnan |first9=Srividya |last10=Benoit |first10=Matthias |last11=Gentile |first11=Iacopo |last12=Hendelman |first12=Anat |last13=Shohat |first13=Hagai |last14=Fitzgerald |first14=Blaine |last15=Robitaille |first15=Gina M. |date=2024-08-02 |title=Convergent evolution of plant prickles by repeated gene co-option over deep time |journal=Science |language=en |volume=385 |issue=6708 |article-number=eado1663 |doi=10.1126/science.ado1663 |pmid=39088611 |issn=0036-8075|pmc=11305333 |bibcode=2024Sci...385o1663S }}</ref>

<gallery mode="packed" heights="200px" style="text-align:left"> File:Wait-a-bit stem.jpg|Raised prickles on the stem of ''Caesalpinia decapetala''. File:Rose Prickles.jpg|Rose prickles. File:Solanum viarum 1673056.jpg|Prickles on the leaves of ''Solanum viarum''. </gallery>

===Other structures=== Other similar structures are spinose teeth, spinose apical processes, and trichomes. Trichomes, 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.

<gallery mode="packed" heights="200px" style="text-align:left"> File:Hulst getand blad Ilex aquifolium.jpg|Spinose leaf margin in ''Ilex aquifolium''. 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''. </gallery>

''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".<ref>Ross, J. H. "A conspectus of the African Acacia species." Series: Memoirs of the Botanical Survey of South Africa, No. 44 Botanical Research Institute, Dept. of Agricultural Technical Services, Pretoria, 1979</ref>

[[File:Cryosophila warscewiczii, thorns of the Silver Star Palm. (11164016416).jpg|170px|thumb|right|"Root spines" on the trunk of a ''Cryosophila'' species.]] There are also spines derived from roots, like the ones on the trunk of the "root spine palms" (''Cryosophila'' spp.). The trunk roots of ''Cryosophila guagara'' grow downwards to a length of 6–12 cm, then stop growing and transform into a spine.<ref name="Jackson 1986">{{Cite book |last=Jackson |first=M. B. |url=https://books.google.com/books?id=gAv8CAAAQBAJ&pg=PA80 |title=New Root Formation in Plants and Cuttings |date=2012-12-06 |publisher=Springer Science & Business Media |isbn=978-94-009-4358-2 |language=en}}</ref> The anatomy of crown roots on this species (roots among the bases of the living fronds) also alters during their life.<ref name="Jackson 1986" /> They initially grow upwards and then turn down and finally they, too, become spinous.<ref name="Jackson 1986" /> Lateral roots on these two types of roots, as well as those on the stilt roots on this species, also become spinous.<ref name="Jackson 1986" /> Some authors believe that some of these short spiny laterals have a ventilating function so they are 'pneumorhizae'.<ref name="Jackson 1986" /> Short spiny laterals that may have a ventilating function may also be found on roots of ''Iriartea exorrhiza''.<ref name="Jackson 1986" />

There are also spines that function as pneumorhizae on the palm ''Euterpe oleracea''.<ref name="Jackson 1986" /> In ''Cryosophila nana'' (formerly ''Acanthorhiza aculeata''), there are spiny roots; some authors prefer to term these "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.<ref name="Jackson 1986" /> Adventitious spiny roots have also been described on the trunks of dicotyledonous trees from tropical Africa (e.g. Euphorbiaceae, as in ''Macaranga barteri'', ''Bridelia micrantha'' and ''B. pubescens''; Ixonanthaceae, Sterculiaceae), and may also be found protecting perennating organs such as tubers and corms (e.g. ''Dioscorea prehensilis'' -Dioscoreaceae- and ''Moraea'' spp. -Iridaceae- respectively).<ref name="Jackson 1986" /> Short root spines cover the tuberous base of the epiphytic ant-plant ''Myrmecodia tuberosa'' (Rubiaceae), these probably give protection to ants which inhabit chambers within the tuber as they wander over the plant's surface. (Jackson 1986<ref name="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<ref name="Jackson 1986" /> and references therein).

==Evolution== It has been proposed that thorny structures may have first evolved as a defense mechanism in plants growing in sandy environments that provided inadequate resources for fast regeneration of damage.<ref>Steve Brill, Evelyn Dean, ''Identifying and Harvesting Edible and Medicinal Plants'' (1994), p. 17.</ref><ref>August Weismann, John Arthur Thomson, Margaret R. Thomson, ''The Evolution Theory'' (1904), p. 124.</ref>

==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'', ''Crataegus''), * spiky inflorescences (''Tylecodon''),<ref>[https://www.bihrmann.com/caudiciforms/subs/tyl-ret-sub.asp Bihrmann.com]</ref> * a tiny point at the tip of the leaf (mucronate leaves) (''Sansevieria''),<ref>Dyer, R. Allen, "The Genera of Southern African Flowering Plants", Vol 2. {{ISBN|0-621-02863-0}}, 1976</ref> * leaves fully converted to spines (''Opuntia''),<ref>Anderson, Edward F., The Cactus Family, Pub: Timber Press 2001 {{ISBN|978-0-88192-498-5}}</ref> * stipules converted to spines (many ''Vachellia''), * prickles on stems (''Rosa'', ''Erythrina'' and ''Ceiba speciosa''), * urticating (i.e. stinging) hairs, * bristles, and * finely barbed spines called glochids.

Some thorns are hollow and act as myrmecodomatia; others (e.g. in ''Crataegus monogyna'') bear leaves. The thorns of many species are branched (e.g. in ''Crataegus crus-galli'' and ''Carissa macrocarpa'').

==Human uses== Plants bearing thorns, spines, or prickles are often used as a defense against burglary, being strategically planted below windows or around the entire perimeter of a property.<ref>Felson, Marcus (2006) ''Crime and Nature'', p. 288.</ref> They also have been used to protect crops and livestock against marauding animals. Examples include hawthorn hedges in Europe, agaves or ocotillos in the Americas and in other countries where they have been introduced, Osage orange in the prairie states of the US, and ''Sansevieria'' in Africa.<ref>Hunter, J. A. (1993) "Hunter" Publisher: Buccaneer Books,{{ISBN|978-1-56849-109-7}}</ref>{{page needed|date=April 2019}}

== References == {{reflist}}

== Bibliography == {{refbegin}} * 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. {{refend}}

== External links == {{Commons category|Thorns, spines and prickles}} *{{Wikisource-inline|list= **{{Cite Collier's|wstitle=Thorn (plant) |short=x |noicon=x}} **{{Cite NSRW|wstitle=Thorn |short=x |noicon=x}} }}

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