Lauraceae, or thelaurels, is a plantfamily that includes thetrue laurel and its closest relatives. This family comprises about 2850 known species in about 45genera worldwide.[4] They aredicotyledons, and occur mainly in warm temperate and tropical regions, especiallySoutheast Asia andSouth America. Many are aromaticevergreen trees or shrubs, but some, such asSassafras, aredeciduous, or include both deciduous and evergreen trees and shrubs, especially in tropical and temperate climates. The genusCassytha is unique in the Lauraceae in that its members areparasiticvines. Many species within the laurel family are highly toxic; however, some, such as theavocado (Persea americana), are edible and widely consumed. Members of the laurel family have played a significant role in the spice trade,[5] particularly genera likeCinnamomum, which produce aromatic oils used extensively in cooking, perfumery, and traditional medicine.[6]
The family has a worldwide distribution in tropical and warm climates. The Lauraceae are important components of tropicalforests ranging from low-lying tomontane. In several forested regions, Lauraceae are among the top five families in terms of the number of species present.[citation needed]
The Lauraceae give their name to habitats known aslaurel forests, which have many trees that superficially resemble the Lauraceae, though they may belong to other plant families such asMagnoliaceae orMyrtaceae. Laurel forests of various types occur on most continents and on many major islands.[citation needed]
Although the taxonomy of the Lauraceae is still not settled, conservative estimates suggest some 52 genera worldwide, including 3,000 to 3,500 species.[7] Compared to other plant families, the taxonomy of Lauraceae still is poorly understood. This is partly due to its great diversity, the difficulty of identifying the species, and partly because of inadequate investment intaxonomic work.[7]
Recent monographs on small and medium-sized genera of Lauraceae (up to about 100 species) have revealed many new species.[7] Similar increases in the numbers of species recognised in other larger genera are to be expected.[7]
The fruits of Lauraceae aredrupes, one-seeded fleshy fruit with a hard layer, theendocarp, surrounding the seed. However, the endocarp is very thin, so the fruit resemble a one-seeded berry.[8] The fruit in some species (particularly in the genusOcotea) are partly immersed or covered in a cup-shaped or deep thickcupule, which is formed from the tube of the calyx where the peduncle joins the fruit; this gives the fruit an appearance similar to anacorn. In someLindera species, the fruit have ahypocarpium at the base of the fruit.[citation needed]
Because the family is so ancient and was so widely distributed on theGondwanasupercontinent, modern species commonly occur inrelict populations isolated by geographical barriers, for instance on islands or tropical mountains. Relict forests retainendemicfauna andflora in communities of great value in inferring thepalaeontological succession andclimate change that followed the breakups of the supercontinents.[citation needed]
Many Lauraceae contain high concentrations ofessential oils, some of which are valued for spices and perfumes. Within the plants, most such substances are components of irritant or toxic sap or tissues that repel or poison manyherbivorous or parasitic organisms.
Conversely, some species, though commercially valuable in some countries, are regarded as aggressive invaders in other regions. For example,Cinnamomum camphora, though a valued ornamental and medicinal plant, is so invasive as to have been declared a weed in subtropical forested areas of South Africa.[13]
Lindera melissifolia: This endangered species is native to the southeastern United States, and its demise is associated with habitat loss from extensive drainage ofwetlands for agriculture and forestry.[14]
Lauraceae flowers areprotogynous, often with a complex flowering system to preventinbreeding. The fruits are an important food source for birds, on which somePalaeognathae are highly dependent.[citation needed] Other birds that rely heavily on the fruit for their diets include members of the familiesCotingidae,Columbidae,Trogonidae,Turdidae, andRamphastidae, amongst others. Birds that are specialised frugivores tend to eat the whole fruit and regurgitate seeds intact, thereby releasing the seeds in favourable situations for germination (ornithochory). Some other birds that swallow the fruit pass the seed intact through their guts.[citation needed]
The leaves of some species in the Lauraceae havedomatia in the axils of theirveins. The domatia are home to certainmites. Other lauraceous species, members of the genusPleurothyrium in particular, have asymbiotic relationship with ants that protect and defend the tree. SomeOcotea species are also used as nesting sites by ants, which may live in leaf pockets or in hollowed-out stems.[16]
Defense mechanisms that occur among members of the Lauraceae include irritant or toxic sap or tissues that repel or poison manyherbivorous organisms.[citation needed]
Trees of the family predominate in the world'slaurel forests[citation needed] andcloud forests,[citation needed] which occur in tropical to mild temperate regions of both northern and southern hemispheres. Other members of the family however, occur pantropically in general lowland andAfromontane forest, and in Africa for example there are species endemic to countries such asCameroon,Sudan,Tanzania,Uganda andCongo. Severalrelict species in the Lauraceae occur in temperate areas of both hemispheres. Many botanical species in other families have similar foliage to the Lauraceae due toconvergent evolution, and forests of such plants are calledlaurel forest. These plants are adapted to high rainfall and humidity, and have leaves with a generous layer ofwax, making them glossy in appearance, and a narrow, pointed-oval shape with a 'drip tip', which permits the leaves to shed water despite the humidity, allowing transpiration to continue. Scientific names similar toDaphne (e.g.,Daphnidium,Daphniphyllum)[17] or "laurel" (e.g.,Laureliopsis,Skimmia laureola) indicate other plant families that resemble Lauraceae.[citation needed]
Some Lauraceae species have adapted to demanding conditions in semiarid climates, but they tend to depend on favorableedaphic conditions, for example, perennial aquifers, periodic groundwater flows, or periodically flooded forests in sand that contains hardly any nutrients. Various species have adapted to swampy conditions by growingpneumatophores, roots that grow upward, that project above the levels of periodic floods that drown competing plants which lack such adaptations.[18]
Paleobotanists have suggested the family originated some 174±32 million years ago (Mya), while others[19] do not believe they are older than the mid-Cretaceous. Fossil flowers attributed to this family occur inCenomanian clays (mid-Cretaceous, 90–98 Mya) of the EasternUnited States (Mauldinia mirabilis). Fossils of Lauraceae are common in the Tertiary strata of Europe and North America, but they virtually disappeared from central Europe in the LateMiocene.[20] Because of their unusual fragility, the pollens of Lauraceae do not keep well and have been found only in relatively recent strata.[citation needed]
Deciduous Lauraceae lose all of their leaves for part of the year depending on variations in rainfall. The leaf loss coincides with thedry season in tropical, subtropical, and arid regions.[citation needed]
Laurel wilt disease, caused by the virulent fungal pathogenRaffaelea lauricola, a native ofsouthern Asia, was found in the southeast United States in 2002. The fungus spreads between hosts via a wood-boring beetle,Xyleborus glabratus, with which it has asymbiotic relationship. Several Lauraceae species are affected. The beetle and disease are believed to have arrived in the US via infected solid wood packing material, and have since spread to several states.[21]
Classification within the Lauraceae is not fully resolved. Multiple classification schemes based on a variety of morphological and anatomical characteristics have been proposed, but none are fully accepted. According to Judd et al. (2007),[22] the suprageneric classification proposed by van der Werff and Richter (1996)[23] is currently the authority. However, due to an array of molecular and embryological evidence that disagrees with the groupings, it is not fully accepted by the scientific community. Their classification is based on inflorescence structure and wood and bark anatomy. It divides Lauraceae into two subfamilies, Cassythoideae and Lauroideae. The Cassythoideae comprise a single genus,Cassytha, and are defined by their herbaceous, parasitic habit. The Lauroideae are then divided into three tribes: Laureae, Perseeae, and Cryptocaryeae.[citation needed]
The subfamily Cassythoideae is not fully supported. Backing has come frommatK sequences of chloroplast genes[24] while a questionable placement ofCassytha has been concluded from analysis of intergenetic spacers of chloroplast and nuclear genomes.[25] Embryological studies also appear contradictory. One study by Heo et al. (1998)[26] supports the subfamily. It found thatCassytha develops anab initio cellular-type endosperm and the rest of the family (with one exception) develops a nuclear-type endosperm. Kimoto et al. (2006)[27] suggestCassytha should be placed in the tribe Cryptocaryeae because it shares a glandular anther tapetum and an embryo sac protruding from the nucellus with other members of the Cryptocaryeae.[citation needed]
The tribes Laureae and Perseeae are not well supported by any molecular or embryological studies. Sequences of thematK chloroplast gene,[24] as well as sequences of chloroplast and nuclear genomes,[25] reveal close relationships between the two tribes. Embryological evidence does not support a clear division between the two tribes, either. Genera such asCaryodaphnopsis andAspidostemon that share embryological characteristics with one tribe and wood and bark characteristics or inflorescence characteristics with another tribe blur the division of these groups.[26] All available evidence, except for inflorescence morphology and wood and bark anatomy, fails to support separate tribes Laureae and Perseeae.[citation needed]
The tribe Cryptocaryeae is partially supported by molecular and embryological studies. Chloroplast and nuclear genomes support a tribal grouping that contains all the genera circumscribed by van der Weff and Richter (1996),[23] as well as three additional genera.[25] Partial support for the tribe is also attained from thematK sequences of chloroplast genes[24] as well as embryology.[28]
Challenges in Lauraceae classification
The knowledge of the species comprising the Lauraceae is incomplete. In 1991, about 25-30% of neotropical Lauraceae species had not been described.[29] In 2001, embryological studies had only been completed on individuals from 26 genera yielding a 38.9% level of knowledge, in terms of embryology, for this family.[28] Additionally, the huge amount of variation within the family poses a major challenge for developing a reliable classification.[23][29]
The adaptation of Lauraceae to new environments has followed a long evolutionary journey which has led to manyspecializations, including defensive or deterrent systems against other organisms.[citation needed]
Alaurel wreath, a round or horseshoe-shapedwreath made of connected laurel branches and leaves, is an ancient symbol of triumph in classical Western culture originating inGreek mythology, and is associated in some countries with academic or literary achievement.[citation needed]
^Anzano, Attilio; de Falco, Bruna; Grauso, Laura; Motti, Riccardo; Lanzotti, Virginia (2022-04-01). "Laurel, Laurus nobilis L.: a review of its botany, traditional uses, phytochemistry and pharmacology".Phytochemistry Reviews.21 (2):565–615.Bibcode:2022PChRv..21..565A.doi:10.1007/s11101-021-09791-z.ISSN1572-980X.
^Schroeder, C. A. (1975). "SOME USEFUL PLANTS OF THE BOTANICAL FAMILY LAURACEAE".California Avocado Society.
^Henderson, Lesley (2001).Alien Weeds and Invasive Plants: A Complete Guide to Declared Weeds and Invaders in South Africa, Including Another 36 Species Invasive in that Region. Plant Protection Research Institute.ISBN978-1-86849-192-6.[page needed]
^Judd, Walter S.; Campbell, Christopher S.; Kellogg, Elizabeth Anne (2008).Plant Systematics: A Phylogenetic Approach. W. H. Freeman.ISBN978-0-87893-407-2.[page needed]
Meisner, C. F. (1864)."Lauraceae".Prodromus Systematis Naturalis Regni Vegetabilis.15 (1):1–260.NAID10007643152.
Mez, Carl Christian (1889). "Lauraceae Americanae Monographice Descripsit".Jahrbuch des Königlichen Botanischen Gartens und des Botanischen Museums zu Berlin.5:1–556.
Rohwer, Jens G. in Kubitzki, K.(Editor) 1993. The Families and Genera of Vascular Plants, Vol.2: K. Kubitzki, J. G. Rohwer & V. Bittrich, 366-390.ISBN3-540-55509-9
Wagner, W. L., D. R. Herbst, and S. H. Sohmer. 1990.Manual of the Flowering Plants of Hawaiʻi. Spec. Publ. 83. University of Hawaii Press and Bishop Museum Press. Bishop Museum. 1854 pp.
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