Thorns, spines, and prickles

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 deterring animals from eating the plant material.

Thorn mullu
A plant with thorns

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),[1][2][3][4] 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),[1][2][3][4] and prickles are derived from epidermis tissue (so that they can be found anywhere on the plant and do not have vascular bundles inside[4]).[1][2][3]

Leaf margins may also have teeth, and if those teeth are sharp, they are called spinose teeth on a spinose leaf margin[1][2] (some authors consider them a kind of spine[2]). 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[1] (again, some authors call them a kind of spine[2]). When the leaf epidermis is covered with very long, stiff trichomes (more correctly called bristles in this case;[1] for some authors a kind of prickle[2]), it may be referred to as a hispid vestiture;[1][2][3] if the trichomes are stinging trichomes, it may be called a urent vestiture.[1]

There can be found also spines or spinose structures derived from roots.[5]

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.[6]

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.

Stem morphology type thorn
Thorns are modified stems and arise from buds
Stem morphology type areole
Cactus areoles; shoot (yellow), spines (green) and glochids (also spines, green and little)
Diferencia espina y aguijón
(A) Thorn or spine
(B) Prickle
Leaf morphology tooth spinose
A spinose tooth in a leaf margin
Leaf morphology apical process spinose-pungent
A spinose apical process

Thorns

Thorns are modified branches or stems. They may be simple or branched.

Thorn image

Smooth, featureless Citrus thorn.

Gymnosporia buxifolia thorn

Gymnosporia buxifolia thorn, its leaves, nodes, and emergence from an axillary bud demonstrating its nature as a branch.

Carissa bispinosa Uniondale 1167

Carissa bispinosa showing characteristic branched thorns.

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.[7]

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.[1] The plants of the cactus family are particularly well known for their dense covering of spines. Cacti often have a particular kind of spine (as found in areoles of Opuntia) called a glochidium or glochid (plural glochidia or glochids), which is very small and deciduous with numerous retrose barbs along its length.[1]

Ocotillothron02262006

The spines of Fouquieria splendens develop from the leaf petioles.

AcaciaXanthophloeaSpines

Stipule spines on Vachellia xanthophloea.

Pereskia grandifolia ies

Areoles and spines of the tree-like Pereskia grandifolia.

Fish hook Cactus Without wool

Spines of Mammillaria balsasoides

With seperate hook

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.[8][9] Technically speaking, many plants commonly thought of as having thorns or spines actually have prickles. Roses, for instance, have prickles.[7]

Wait-a-bit stem

Raised prickles on the stem of Caesalpinia decapetala.

Rose Prickles

Rose prickles.

Solanum viarum 1673056

Prickles on the leaves of Solanum viarum.

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.

Hulst getand blad Ilex aquifolium

Spinose leaf margin in Ilex aquifolium.

Sansevieria trifasciata2

Spinose apical process in Sansevieria.

Galium.aparine

Stiff, sharp trichomes in Galium aparine.

Urtica dioica stinging hair

Stinging trichome in Urtica dioica.

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..."[10]

Cryosophila warscewiczii, thorns of the Silver Star Palm. (11164016416)
"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.[5] The anatomy of crown roots on this species (roots among the bases of the living fronds) also alters during their life.[5] They initially grow upwards and then turn down and finally they, too, become spinous.[5] Lateral roots on these two types of roots, as well as those on the stilt roots on this species, also become spinous.[5] Some authors believe that some of these short spiny laterals have a ventilating function so they are 'pneumorhizae'.[5] Short spiny laterals that may have a ventilating funcion may also be found on roots of Iriartea exorrhiza.[5]

There are also spines that function as pneumorhizae on the palm Euterpe oleracea.[5] 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.[5] 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).[5] 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[5] 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[5] 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.[11][12] However, this suggestion was unsupported by any argument to discount the likelihood that spiny defenses might have been developed as a means of defense in resource-rich environments, where herbivory might have been more intense than in the hypothesized sandy environments.

Morphological variation

Spinose structures occur in a wide variety of ecologies, and their morphology also varies greatly. They occur as:

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.[16] They also have been used to protect crops and livestock against marauding animals. Examples include hawthorn hedges in Europe, Agaves 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.[17]

See also

References

General references
  • 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.
Footnotes
  1. ^ a b c d e f g h i j Simpson, M. G. 2010. "Plant Morphology". In: Plant Systematics, 2nd. edition. Elsevier Academic Press. Chapter 9.
  2. ^ a b c d e f g h Judd, Campbell, Kellogg, Stevens, Donoghue. 2007. "Structural and Biochemical Characters". In: Plant Systematics, a phylogenetic approach, third edition. Chapter 4.
  3. ^ a b c d Turner et al. 2005, Sonoran Desert Plants, an Ecological Atlas. University of Arizona Press.
  4. ^ a b c Van Wyk, Van Wyk. 2007. How to identify trees in South Africa. Struik.
  5. ^ a b c d e f g h i j k l 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.ar/books?id=gAv8CAAAQBAJ&pg=PA80
  6. ^ Agrawal, A, A., Rudgers, A, J., Botsford, W, L., Cutler, S., Gorin, B, J., Lundquist, C, J., Spitzer, W, B., & Swann, L, A. (2000). Benefits and Constraints on Plant Defense against Herbivores: Spines Influence the Legitimate and Illegitimate Flower Visitors of Yellow Star Thistle, Centaurea solstitialis L. (Asteraceae). JSTOR, 45(1), 1-5. https://www.jstor.org/stable/3672545. retrieved 2012-03-20
  7. ^ a b Bell, A.D. 1997. Plant form: an illustrated guide to flowering plant morphology. Oxford University Press, Oxford, U.K. preview in google books
  8. ^ Van Wyk, Braam (2007). How to Identify Trees in Southern Africa (illustrated ed.). Struik. p. 184. ISBN 9781770072404.
  9. ^ Sengbusch, Peter (2003-07-31). "Cross-Section Through the Prickle of a Rose". Archived from the original on 2008-04-30. Retrieved 2009-04-27.
  10. ^ 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
  11. ^ Steve Brill, Evelyn Dean, Identifying and Harvesting Edible and Medicinal Plants (1994), p. 17.
  12. ^ August Weismann, John Arthur Thomson, Margaret R. Thomson, The Evolution Theory (1904), p. 124.
  13. ^ Bihrmann.com
  14. ^ Dyer, R. Allen, “The Genera of Southern African Flowering Plants”, Vol 2. ISBN 0-621-02863-0, 1976
  15. ^ Anderson, Edward F., The Cactus Family, Pub: Timber Press 2001 ISBN 978-0-88192-498-5
  16. ^ Marcus Felson, Crime and Nature (2006), p. 288.
  17. ^ Hunter, J. A., "Hunter" Publisher: Buccaneer Books, 1993, ISBN 978-1-56849-109-7

External links

Araya and Paria xeric scrub

The Araya and Paria xeric scrub (NT1301) is an ecoregion in Venezuela that stretches along the Caribbean coast to the west of Trinidad, and that includes Margarita Island and some smaller islands. The ecoregion includes dune herbs, dry thorn scrubs and deciduous forests.

The mountains on Margarita Island hold montane forests similar to those in the mainland coastal range.

The beaches are used for breeding by sea turtles.

Some endemic bird species are threatened with extinction by illegal capture for the pet trade.

Deforestation and over-grazing by goats are also problems.

Domestication

Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the reproduction and care of another group to secure a more predictable supply of resources from that second group.Charles Darwin recognized the small number of traits that made domestic species different from their wild ancestors. He was also the first to recognize the difference between conscious selective breeding in which humans directly select for desirable traits, and unconscious selection where traits evolve as a by-product of natural selection or from selection on other traits. There is a genetic difference between domestic and wild populations. There is also such a difference between the domestication traits that researchers believe to have been essential at the early stages of domestication, and the improvement traits that have appeared since the split between wild and domestic populations. Domestication traits are generally fixed within all domesticates, and were selected during the initial episode of domestication of that animal or plant, whereas improvement traits are present only in a proportion of domesticates, though they may be fixed in individual breeds or regional populations.The dog was the first domesticated vertebrate, and was established across Eurasia before the end of the Late Pleistocene era, well before cultivation and before the domestication of other animals. The archaeological and genetic data suggest that long-term bidirectional gene flow between wild and domestic stocks – including donkeys, horses, New and Old World camelids, goats, sheep, and pigs – was common. Given its importance to humans and its value as a model of evolutionary and demographic change, domestication has attracted scientists from archaeology, paleontology, anthropology, botany, zoology, genetics, and the environmental sciences.

Among birds, the major domestic species today is the chicken, important for meat and eggs, though economically valuable poultry include the turkey, guineafowl and numerous other species. Birds are also widely kept as cagebirds, from songbirds to parrots.

The longest established invertebrate domesticates are the honey bee and the silkworm. Terrestrial snails are raised for food, while species from several phyla are kept for research, and others are bred for biological control.

The domestication of plants began at least 12,000 years ago with cereals in the Middle East, and the bottle gourd in Asia. Agriculture developed in at least 11 different centres around the world, domesticating different crops and animals.

List of poisonous plants

Poisonous plants are plants that produce toxins that deter herbivores from consuming them. Plants cannot move to escape their predators, so they must have other means of protecting themselves from herbivorous animals. Some plants have physical defenses such as thorns, spines and prickles, but by far the most common type of protection is chemical. Over millennia, through the process of natural selection, plants have evolved the means to produce a vast and complicated array of chemical compounds in order to deter herbivores. Tannin, for example, is a defensive compound that emerged relatively early in the evolutionary history of plants, while more complex molecules such as polyacetylenes are found in younger groups of plants such as the Asterales. Many of the known plant defense compounds primarily defend against consumption by insects, though other animals, including humans, that consume such plants may also experience negative effects, ranging from mild discomfort to death.

Many of these poisonous compounds also have important medicinal benefits. The varieties of phytochemical defenses in plants are so numerous that many questions about them remain unanswered, including:

Which plants have which types of defense?

Which herbivores, specifically, are the plants defended against?

What chemical structures and mechanisms of toxicity are involved in the compounds that provide defense?

What are the potential medical uses of these compounds?These questions and others constitute an active area of research in modern botany, with important implications for understanding plant evolution and for medical science.

Below is an extensive, if incomplete, list of plants containing one or more poisonous parts that pose a serious risk of illness, injury, or death to humans or domestic animals. There is significant overlap between plants considered poisonous and those with psychotropic properties, some of which are toxic enough to present serious health risks at recreational doses. It is also important to remember that there is a distinction between plants that are poisonous because they naturally produce dangerous phytochemicals, and those that may become dangerous for other reasons, including but not limited to infection by bacterial, viral, or fungal parasites, the uptake of toxic compounds through contaminated soil or groundwater, and/or the ordinary processes of decay after the plant has died; this list deals exclusively with the former. Many plants, such as peanuts, also produce compounds that are only dangerous to people who have developed an allergic reaction to them, and with a few exceptions, those plants are not included here (see list of allergens instead). Despite the wide variety of plants considered poisonous, human fatalities caused by poisonous plants – especially resulting from accidental ingestion – are rare in the developed world.

Paraguana xeric scrub

The Paraguana xeric scrub (NT1313) is an ecoregion in Venezuela to the north and east of Lake Maracaibo along the Caribbean coast.

The region holds flora and fauna adapted to the very dry conditions of the coastal dunes and inland areas of bush, scrub, briars and cacti.

There are several endangered species of animals and birds.

Efforts at protecting the environment have been ineffective.

Most of the original trees have been cut down, dunes are being destabilized by loss of vegetation, scrub is replaced by farmland and vegetation is destroyed by grazing goats.

Trichome

Trichomes ( or ), from the Greek τρίχωμα (trichōma) meaning "hair", 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 plant is an indumentum, and the surface bearing them is said to be pubescent.

Western Pennsylvania English

Western Pennsylvania English, known more narrowly as Pittsburgh English or popularly as Pittsburghese, is a dialect of American English native primarily to the western half of Pennsylvania, centered on the city of Pittsburgh, but potentially appearing as far north as Erie County and Limestone, New York (north of Bradford), as far east as Sunbury, Pennsylvania, as far west as Youngstown, Ohio, and as far south as Clarksburg, West Virginia. Commonly associated with the white working class of Pittsburgh, users of the dialect are colloquially known as "Yinzers".

Languages

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.