Palms

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Palms

The palm family is an ancient group of plants dating from at least the late Mesozoic era, about 85 million years ago. Palms are flowering plants. Flowering plants have been subdivided into two major groups: the monocotyledons which bear only one seed leaf (cotyledon) and the dicotyledons which bear two seed leaves. Palms are among the most diverse of the families of monocotyledons, surpassed in numbers of genera and species only by the orchid, grass and lily families. The palm family contains about 212 genera and 2,800 species. Palmae is the old scientific name for the palm family and is still occasionally used.

Distribution

Palms are widely distributed throughout moist tropical and subtropical regions of the world. They can be found in steamy rain forests, deserts, mangrove swamps, and high mountain thickets. Palms are uncommon in hot, dry regions, however, occurring only where there is a constant source of underground water. The distribution of palms in the tropics is uneven. The greatest diversity of palms is in the eastern tropics of Indo-Malaysia, the Guianas, and Brazil. Africa has relatively few palms with only 16 genera and 116 species; there are fewer palms in all of Africa than on the island of Singapore. The low diversity of palms in Africa is attributed to the dryness of much of the continent.

Palm species are also unequally distributed taxonomically among genera. A small number of genera contain a disproportionately large percentage of palm species. The genus Calamus is the largest with about 400 species in the Old World tropics. In the New World, Chamaedorea is a large genus with about 100 species in southern Mexico, Central America and northern South America. The average number of species per genus is relatively small at 13 and more than half of all the genera have five or fewer species. Seventy-three of the 212 genera of palms consist of only one species that is geographically restricted to a small area, often on an island. The palm floras of the New and Old Worlds are on the whole quite dissimilar.

Very few palms occur in temperate regions. In Europe, for example, there are only two native palms: the dwarf fan-palm (Chamaerops humilis ), which is found in dry sandy and rocky places along parts of the Mediterranean coast, reaching the latitude 44^°N, and the Cretan palm (Phoenix theophrasti ), which is an extremely rare palm found only on the eastern side of Crete. In the Himalayas at 32^°N, the palm Trachycarpus reaches an altitude of 7,875 ft (2,400 m) where snow lasts from November to March. In North America, the genus Serenoa, which includes the palmettoes of the southeastern states, reaches 30^°N. The scarcity of palms in temperate regions where frosts occur is likely related to their mode of growth. Palms generally have only one growing point which is at the tip of the stem or trunk. If this single bud is killed, then the palm dies. It appears that in the vast majority of palms, the growing tip is highly sensitive to frost and that few species have been able to overcome this sensitivity.

Structure

Palms are mostly unbranched shrubs or trees, and are the main tree family within the monocotyleons. Typically the solitary erect stem is crowned by large, persistent leaves that are sheathing at the base. The leaves of Raphia fainifera are the largest of any flowering plant, sometimes reaching more than 65 ft (20 m). Palm leaves are occasionally simple, but usually they are dissected into a fan shape (palmate) or feather shape with many distinct segments that run perpendicular to the main axis of the leaf (pinnately compound). The stem may be very short, so as to appear virtually absent, to 164 ft (50 m) in height. Not all palms have single stems or trunks. Some palms have clustered stems that arise from buds at the base of the initial stem, for example, Phoenix spp. A few unusual species, such as Chrysalidocarpus madagascariensis, have some individuals with solitary straight trunks and other individuals with clustered trunks. Some palms are climbing vines, such as the rattans.

The trunk of palms is very different from the trunk of the conifer or dicotyledonous trees that dominate temperate regions. Conifers, such as pines, spruces, and hemlocks, and dicotyledenous trees, such as maples, oaks, and elms, increase the width of their trunk as they grow by a process called secondary growth. During this process, a ring of specialized cells under the bark of the tree produces new wood toward the center of the tree and other kinds of specialized conducting tissues toward the bark side. Secondary growth is absent in palms. Instead, when a seed germinates, the seedling first grows into an inverted cone whose width matches the full width of the trunk to be grown. Only after this radial growth is completed does the seedling begin to grow vertically, maintaining its width. If a nail is driven into a conifer or dicotylenous tree’s trunk, the tree will grow around it eventually completely embedding the nail within the trunk. A nail driven into the trunk of a palm will remain where driven and not become embedded within the trunk.

Palm flowers are occasionally bisexual, but usually they are unisexual. When unisexual, the flowers of each sex may be on the same plant or, as in humans, only one sex is found per individual. The flowers are small and are generally borne on large, many-branched stems (inflorescences) that are located within the crown or just below it. Flower parts are normally in threes. The pollination biology of palms is not well studied, nevertheless, both wind and insect pollination are common in the family.

Palm fruits are berries, which are simple, fleshy fruits that contain one or more seeds, or drupes, which are simple, fleshy fruits that contain one seed that is surrounded by a bony pit, a stone fruit. The largest fruit in the world comes from the double coconut palm (Lodoicea sechellarum ) and weighs up to 40 lb (18.2 kg).

Economic uses

Within the tropics, the palms rank second in economic importance after the grasses. Palms are used for food, oil, fiber, and as ornamentals.

Food

Dates are the fruit of the date palm (Phoenix dactylifera ) and have been in cultivation for at least 8,000 years. Most dates are grown in Asia Minor and North Africa with an annual production of 2,000,000 metric tons. In the United States dates are grown in Arizona and California. Dates contain 70% carbohydrate but little protein (2%) or fat (2.5%). They have long been an important source of nourishment for nomadic tribes of the Arab world. Dates are usually eaten fresh but can be made into paste. Dates are sometimes mixed with a variety of milk products which increases the protein content.

Coconuts are the fruit of the coconut palm (Cocos nucifera ). About 30 billion coconuts are produced each year, mostly in the Philippines, Indonesia, Sri Lanka, Malaysia, and Mexico. Although coconuts are an important food for some Pacific maritime societies and are sold in markets throughout the world, most of the annual harvest is used for the production of coconut oil, which will be discussed later.

Sago or sago starch is an important source of carbohydrate for many people of the tropics, from Thailand to New Guinea. Sago is derived from the pith (central portion of the trunk) of the sago palm (Metroxylon sagu ), which grows in freshwater swamps that are otherwise useless for cultivated crops. Sago is readily extracted from the trunks and has the added advantage that it can be stored virtually pest-free. The one disadvantage is that sago has little protein. Sago is also extracted from several other species in the genera Arenga and Caryota.

The sap of a variety of palms, including Borassus and Caryota, is fermented to produce palm wine, also called palm toddy. When fermented palm sap is distilled, the liquor known as arrack is produced.

In Southeast Asia, the betel nut, which in fact is the seed of the betel palm (Areca catechu ), is commonly used for its mildly narcotic effect. The betel nut is chewed with a leaf of a local pepper plant and this in combination with a bit of lime makes the mouth and saliva red. With constant use, the teeth of betel addicts turn black. It has been estimated that a tenth of the world population chews betel nuts.

Oil

A number of palms are major sources of edible oils that are refined into cooking oil, margarine, and shortening. Palm oils are also used in the manufacture of candles, soaps, lubricating greases, and stabilizers in plastic and rubber compounds. The African oil palm (Elaeis guineensis ) is the single most important oil-producing palm, having recently surpassed the coconut. The African oil palm is a sun-loving species that grows naturally in a 155 mi (250 km) wide strip along the coast of western Africa from Senegal to Angola, but is now planted throughout much of the tropics. The fruits of the oil palm are 23% oil by weight and most of the oil is in the outer husk, which surrounds the inner stone. The yields from plantations are enormous with one hectare yielding as much as 5 tons of crude palm oil per year. The oil is collected by digesting the fruit to a pulp that is then centrifuged or pressed to separate the oil. The American oil palm (E. oleifera ) is native to Central America and northern South America where it is extensively utilized. The American oil palm is better adapted to wet habitats than the African oil palm and the two species have been interbred to improve plantation stock.

The coconut palm is thought to be native to Polynesia, but it has been propagated and cultivated throughout the tropics for so long that its origin is uncertain. Coconut palms were until recently the major source of palm oil. The majority of the annual coconut harvest goes toward the production of oil. Coconut oil is derived from copra, which is the white flesh inside the seed (coconuts bought in a grocery store have had their large outer husk removed). One coconut tree may yield 198 lb (90 kg) of copra a year and 9-11 gal (35-40 l) of oil. Coconut oil is especially suited for the manufacture of fast-lathering soaps and is also used as a hardening agent in a number of seemingly unrelated products such as cosmetics, margarine and rubber. Coconut meat is of course widely used in baking, cooking, and candy making.

Fiber

Palms produce a variety of useful fibers. Shells of the coconut are covered by tough fibers which are collected by soaking the shells in saltwater to loosen the fibers. The fibers, called coir, are then beaten, washed and combed out. Coir is used as stuffing and woven into mats. Raphia fiber is obtained from the genus Raphia by stripping the surface of young leaves. The best-known fiber palms are the rattans which belong to the large genus Calamus. Rattans are interesting palms in that they are climbers and often vine-like. Unlike many climbers, which use roots to attach themselves to their host or twining behavior to grip onto a stem, the rattans are scramblers that hoist themselves up leaf by leaf as the spiny stem hangs like a rope down to the roots. The hanging stems of many species of rattan are cleaned and split for use. Rattan is widely used for cane work such as basketry.

Ornamentals

Palms are a symbol of the tropics and many have been selected and grown for their beauty. The royal palm (Roystonea regia ) is a tall, elegant palm that is commonly planted along streets and boulevards in cities throughout the tropics and subtropics, including the southern United States. The sentinel palm of southern California (Washingtonia filifera ) is also widely planted as an ornamental. Perhaps the most beautiful of all palms is the lipstick or sealing wax palm (Cyrostachys renda ), which is native to peninsular Malaysia, southern Thailand, Borneo, and Sumatra. The sheathing leaf bases and petioles are brilliant red—a rare color for non-flower tissue in plants. Unfortunately this much coveted palm grows poorly outside of its native range.

Many of the palms have multiple uses, such as the coconut. Throughout the tropics a large number of palm species are used locally and intensively. Many of these species are now threatened. As was noted previously, a large proportion of palm species occupy small, geographically restricted areas and so local intense usage of these can have a devastating impact on their population size and survival, as can land clearance. One of the most destructive practices is the

KEY TERMS

Fiber —A generic term for a variety of strands of plant cells that consist of specialized long, thick-walled cells, technically called fiber cells, or other conducting or strengthening cells.

local harvesting of trees for palm hearts, which are the tasty shoot tips and associated tissue. The shoot tip is the only growing point on a palm stem and so its removal causes the death of the tree, or in branched species the stem. Increased awareness of this problem and of conservation of palms in general is necessary to ensure the biodiversity of these attractive and economically important plants.