Palms are one of the most well-known and extensively cultivated plant families. They have had an important role to humans throughout much of history. Many common products and foods are derived from palms, and palms are also widely used in landscaping for their exotic appearance, making them one of the most economically important plants. In many historical cultures, palms were symbols for such ideas as victory, peace, and fertility. Today, palms remain a popular symbol for the tropics and vacations.
Whether shrubs, trees or vines palms are limited to two methods of growth. The common representation is that of a solitary shoot ending in a crown of leaves. This monopodial behavior may be exhibited in prostrate, trunkless and trunk forming members. Some of the common palm trees restricted to solitary growth include Washingtonia and Roystonea. Palms may instead grow in sparse to dense clusters. The trunk will develop an axillary bud at a leaf node, usually near the base, from which a new shoot emerges. The new shoot, in turn, produces an axillary bud and a clustering habit results. Exclusively sympodial genera include many of the rattans, Guihaia, and Rhapis. Several palm genera have both solitary and clustering members. Occasionally, a plant is very often clustering with the occasional solitary member or the converse. These aberrations suggest the habit operates on a single gene.
They have large evergreen leaves that are either palmately ('fan-leaved') or pinnately ('feather-leaved') compound and spirally arranged at the top of the stem. The leaves have a tubular sheath at the base that usually splits open on one side at maturity . The inflorescence is a panicle or spike surrounded by one or more bracts or spathes that become woody at maturity. The flowers are generally small and white, radially symmetric, and can be either uni- or bisexual. The sepals and petals usually number three each and may be distinct or joined at the base. The stamens generally number six, with filaments that may be separate, attached to each other, or attached to the pistil at the base. The fruit is usually a single-seeded drupe , but some genera (e.g. Salacca) may contain two or more seeds in each fruit.
Arecaceae is notable for having the individual trees with the largest seed, largest leaf, largest inflorescence, as well as the tallest individual monocot. The coco de mer (Lodoicea maldivica) has the largest seeds of any plant, 40-50 centimeters in diameter and weighing 15-30 kilograms each. Raffia palms (Raphia spp.), with leaves up to 25 meters long and 3 meters wide, have the largest leaves of any plant. The Corypha species have the largest inflorescence of any plant, up to 7.5 meters tall and containing millions of small flowers. Ceroxylon quindiuense, Colombia's national tree, is the tallest monocot in the world, reaching heights of 60 meters .
The vast majority of palms exist in the tropics. Palms are abundant throughout the tropical regions around the world, and are present in almost every type of habitat in the tropics. Diversity is highest in wet, lowland tropical forests, especially in ecological "hotspots" such as Madagascar, which has more endemic palms than the entire continental Africa. Colombia may have the highest number of palm species in one country.
It is estimated that only 130 palm species grow naturally beyond the tropics, most of which grow in the subtropics. The northernmost native palm is Chamaerops humilis, which reaches 44°N latitude in southern France, where the local Mediterranean climate is milder than other places as far north. The southernmost palm is the Rhopalostylis sapida, which reaches 44°S on the Chatham Islands where an oceanic climate has a similar warming effect . Palms have been known to grow as far north as Ireland.
Palms inhabit a variety of habitats. Over two-thirds of palms live in tropical forests, where some species grow tall enough to form part of the canopy and other shorter palms adapted to shade form part of the understory . Some species form pure stands in areas with poor drainage or regular flooding, including Raphia hookeri which is common in coastal freshwater swamps in West Africa. Other palms live in tropical mountain habitats above 1000 meters, such as those in the genus Ceroxylon native to the Andes. Palms may also live in grasslands and scrublands, usually associated with a water source, and in desert oases such as the Date Palm. A few palms are adapted to extremely basic lime soils, while others are similarly adapted to very acidic serpentine soils .
Palms are a monophyletic group of plants, meaning that the group consists of a common ancestor and all its descendants . Extensive taxonomic research on palms began with botanist H.E. Moore, who organized palms into fifteen major groups based mostly on general morphological characteristics. The following classification, proposed by N.W. Uhl and J. Dransfield in 1987, is a revision of Moore's classification that organizes palms into six subfamilies . A few general traits of each subfamily are listed.
Coryphoideae is the most diverse subfamily and is a paraphyletic group, meaning that all members of the group share a common ancestor but the group does not include all the ancestor's descendants. Most palms in this subfamily have palmately lobed leaves and solitary flowers with three, sometimes four carpels. The fruit normally develops from only one carpel. Subfamily Calamoideae includes the climbing palms such as rattans. The leaves are usually pinnate; derived characters (synapomorphies) include spines on various organs, organs specialized for climbing, an extension of the main stem of the leaf bearing reflexed spines, and overlapping scales covering the fruit and ovary. Subfamily Nypoideae contains only one genus and one species, Nypa fruticans, which has large pinnate leaves. The fruit is unusual in that it floats, and the stem is dichotomously branched, also unusual in palms. Subfamily Ceroxyloideae has small to medium-sized flowers that spirally arranged, with a gynoecium of three joined carpels. Arecoideae is the largest subfamily with six diverse tribes containing over 100 genera. All tribes have pinnate or bipinnate leaves and flowers arranged in groups of three, with a central pistillate and two staminate flowers. Phytelephantoideae is a monoecious subfamily. Members of this group have distinct monopodial flower clusters. Other distinct features include a gynoecium with five to ten joined carpels, and flowers with more than three parts per whorl. Fruits are multiseeded and have multiple parts .
Currently, few extensive phylogenetic studies of Arecaceae exist. In 1997, Baker et al. explored subfamily and tribe relationships using chloroplast DNA from 60 genera from all subfamilies and tribes. The results strongly showed that Calamoideae is monophyletic, and that Ceroxyloideae and Coryphoideae are paraphyletic. The relationships of Arecoideae are uncertain but it is possibly related to Ceroxyloideae and Phytelephantoideae. However, hybridization has been observed among Orbignya and Phoenix species, and using chloroplast DNA in cladistic studies may produce inaccurate results due to maternal inheritance of the chloroplast DNA. Chemical and molecular data from non-organelle DNA, for example, could be more effective for studying palm phylogeny .
Arecaceae is the first modern family of monocots that is clearly represented in the fossil record. Palms first appear in the fossil record around 80 million years ago, during the late Cretaceous Period. The first modern species, such as Nypa fruticans and Acrocomia aculeata, appeared 69-70 million years ago, confirmed by fossil Nypa pollen dated to 70 million years ago. Palms appear to have undergone an early period of adaptive radiation. By 60 million years ago, many of the modern, specialized genera of palms appeared and became widespread and common, much more widespread than their range today. Because palms separated from the monocots earlier than other families, they developed more intrafamilial specialization and diversity. By tracing back these diverse characteristics of palms to the basic structures of monocots, palms may be valuable in studying monocot evolution.
Like many other plants, palms have been threatened by human intervention and exploitation. The greatest risk to palms is destruction of habitat, especially in the tropical forests, due to urbanization, wood-chipping, mining, and conversion to farmland. Palms rarely reproduce after such great changes in the habitat, and palms with a small habitat range are most vulnerable to them. The harvesting of heart of palm, a delicacy in salads, also poses a threat because it is derived from the inner core of the plant and thus harvesting kills the plant. The use of rattan palms in furniture has caused a major population decrease in these species that has negatively affected local and international markets as well as biodiversity in the area. The sale of seeds to nurseries and collectors is another threat, as the seeds of popular palms are sometimes harvested directly from the wild. At least 100 palm species are currently endangered, and nine species have reportedly recently become extinct.
However, several factors make palm conservation more difficult. Palms live in almost every type of habitat and have tremendous morphological diversity. Most palm seeds lose viability quickly, and they cannot be preserved in low temperatures because the cold kills the embryo. Using botanical gardens for conservation also presents problems, since they can only house a few plants of any species or truly imitate the natural setting. There is also the risk of cross-pollination, which leads to hybrid species.
The Palm Specialist Group of the World Conservation Union (IUCN) began in 1984 and has performed a series of three studies in order to find basic information on the status of palms in the wild, utilization of wild palms, and palms under cultivation. Two projects on palm conservation and utilization supported by the World Wildlife Fund took place from 1985-1990 and 1986-1991, in the American tropics and southeast Asia respectively. Both studies produced a large amount of new data and publications on palms. Preparation of a global action plan for palm conservation began in 1991, supported by the IUCN, and was published in 1996 .
Human use of palms is as old or older than human civilization itself, starting with the cultivation of the Date Palm by Mesopotamians and other Middle Eastern peoples 5000 years or more ago. Date wood, pits for storing dates, and other remains of the Date Palm have been found in Mesopotamian sites. The Date Palm had a tremendous effect on the history of the Middle East. W.H. Barreveld wrote:
Arecaceae has great economic importance including coconut products, oils, dates, ivory nuts, carnauba wax, rattan cane, and raffia.
The type member of Arecaceae is the Areca palm, the fruit of which, the betel nut, is chewed with the betel leaf for intoxicating effects. Also belonging to the family of the Arecaceae are the Date Palm, harvested for its edible fruit; Rattans, whose stems are used extensively in furniture and baskets; and the Coconut. Palm oil is an edible vegetable oil produced by the oil palms in the genus Elaeis. Several species are harvested for heart of palm, a vegetable eaten in salads. Palm sap is sometimes fermented to produce palm wine or toddy, an alcoholic beverage common in parts of Africa, India, and the Philippines . The Palm Sunday festival uses palm leaves, usually from the Date Palm, hence the name. Dragon's blood, a red resin used traditionally in medicine, varnish, and dyes, may be obtained from the fruit of Daemonorops species. Coir is a coarse water-resistant fiber extracted from the outer shell of coconuts, used in doormats, brushes, mattresses, and ropes. Some indigenous groups living in palm-rich areas use palms to make many of their necessary items and food. Sago, for example, a starch made from the pith of the trunk of the Sago Palm Metroxylon sagu, is a major staple food for lowland peoples of New Guinea and the Moluccas. Palm leaves are also valuable to some peoples as a material for thatching or clothing.
Today, palms are valuable as ornamental plants and are often grown along streets in tropical and subtropical cities, and also along the Mediterranean coast in Europe. Farther north, palms are a common feature in botanical gardens or as indoor plants. Few palms tolerate severe cold, however, and the majority of the species are tropical or subtropical. The three most cold-tolerant species are Trachycarpus fortunei, native to eastern Asia, and Rhapidophyllum hystrix and Sabal minor, both native to the southeastern United States . For more details, see hardy palms.
The southeastern state of South Carolina is nicknamed the Palmetto State after the Cabbage Palmetto, logs from which were used to build the fort at Fort Moultrie. During the American Revolutionary War they were invaluable to those defending the fort, because their spongy wood absorbed or deflected the British cannonballs. Some palms can be grown as far north as Maryland, Arkansas, southern Ohio and even up along the Pacific coast to Oregon, Washington and British Columbia, where ocean winds have a warming effect. There have even been known species of transplanted palms that have survived as far north as southern New Jersey. The Chinese Trachycarpus fortunei is being grown experimentally on the Faroe Islands at 62°N, with young plants doing well so far.
The palm branch was a symbol of triumph and victory in pre-Christian times. The Romans rewarded champions of the games and celebrated military successes with palm branches. Early Christians used the palm branch to symbolize the victory of the faithful over enemies of the soul, as in the Palm Sunday festival celebrating the triumphal entry of Jesus into Jerusalem. In Judaism, the palm represents peace and plenty, and is one of the Four Species of Sukkot; the palm may also symbolize the Tree of life in Kabbalah. Today, the palm, especially the Coconut, remains a symbol of the stereotypical tropical island paradise . Palms appear on the flags and seals of several places where they are native, including those of Haiti, Guam, Florida and South Carolina.
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