Thorns, spines, and prickles
In
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),[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
There can be found also spines or spinose structures derived from roots.[5]
Function
The predominant function of thorns, spines, and prickles is deterring
Not all functions of spines or
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.
Thorns
Thorns are modified branches or stems. They may be simple or branched.
-
Smooth, featureless Citrus thorn.
-
Gymnosporia buxifolia thorn, its leaves, nodes, and emergence from an axillary bud demonstrating its nature as a branch.
-
Carissa bispinosa showing characteristic branched thorns.
Spines
Spines are modified
Spines are variously described as petiolar spines (as in
-
The spines of Fouquieria splendens develop from the leaf petioles.
-
Stipule spines onVachellia xanthophloea.
-
Areoles and spines of the tree-like Rhodocactus grandifolius.
-
Spines of Mammillaria balsasoides[citation needed]
-
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] 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]
-
Raised prickles on the stem ofCaesalpinia decapetala.
-
Rose prickles.
-
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.
-
Spinose leaf margin in Ilex aquifolium.
-
Spinose apical process in Sansevieria.
-
Stiff, sharp trichomes in Galium aparine.
-
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]
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 function may also be found on roots of Iriartea exorrhiza.[5]
There are also spines that function as pneumorhizae on the palm
Evolution
It has been proposed that thorny structures may have first evolved as a
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),[13]
- a tiny point at the tip of the leaf (mucronate leaves) (Sansevieria),[14]
- leaves fully converted to spines (Opuntia),[15]
- 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
Human uses
Plants bearing thorns, spines, or prickles are often used as a defense against
See also
References
- ^ 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.
- ^ 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.
- ^ a b c d Turner et al. 2005, Sonoran Desert Plants, an Ecological Atlas. University of Arizona Press.
- ^ a b c Van Wyk, Van Wyk. 2007. How to identify trees in South Africa. Struik.
- ^ ISBN 978-94-009-4358-2.
- JSTOR 3672545.
- ^ 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
- ISBN 9781770072404.
- ^ 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.
- ^ 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
- ^ Steve Brill, Evelyn Dean, Identifying and Harvesting Edible and Medicinal Plants (1994), p. 17.
- ^ August Weismann, John Arthur Thomson, Margaret R. Thomson, The Evolution Theory (1904), p. 124.
- ^ Bihrmann.com
- ISBN 0-621-02863-0, 1976
- ISBN 978-0-88192-498-5
- ^ Felson, Marcus (2006) Crime and Nature, p. 288.
- ISBN 978-1-56849-109-7
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
- Texts on Wikisource:
- "Thorn (plant)". Collier's New Encyclopedia. 1921.
- "Thorn". The New Student's Reference Work. 1914.