Arecaceae, palm trees

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Taxonomy

Taxonomic lineage of Arecaceae (palm family)

cellular organisms - Eukaryota - Viridiplantae - Streptophyta - Streptophytina - Embryophyta - Tracheophyta - Euphyllophyta - Spermatophyta - Magnoliophyta - Liliopsida - commelinids - Arecales - Arecaceae

Taxonomic place

All ~2,800 species of family Arecaceae are regarded as a natural and isolated group within its own order Arecales. Characteristic features of palm trees make it easy ascribing a plant to a family or excluding it despite of superficial similarity with Pandanus (screw pines), Cyclanthaceae (panama-hat family), Cycadopsida (cycads), Aroideae (for example, Philodendroideae) or Agavoideae (agaves and yuccas).

General description

Palms typically have woody self-supporting unbranched trunks growing vertically (rarely plagiotropic - oblique or horizontal). If scadent (climbing), they climb by means of hooked prickles and never by roots. If stem branching is present, it is almost always basal.

The palm stem "woodiness" is not comparable to that of woody dicotyledons with secondary xylem; rather, it depends on an abundance of sclerenchyma and fibers which are often of extreme hardness. The silica contained in stem of many palms may contribute greatly to its hardness.

Palm stems vary widely in height and diameter. Some of the Chamaedorea of Costa Rica have stems 2 m in diameter and less than 25 cm tall, while trunks of the Ceroxylon in Colombia may reach 60 m or more in height while being only 0.5 meters in diameter.

Four models of growth architecture (out of the 23 models in the Hallé-Oldeman system) are identified in palm trees:

1. Holttum's model

   Uncommon: trees are vegetatively unbranched and consist of one hapaxanthic axis (i.e. the vegetative axis ends in a terminal inflorescence) as in Corypha and most Metroxylon spp.

2. Corner's model

   Common: single-stemmed trees with lateral inflorescences.

3. Tomlinson's model

   Commonest: multiple-stemmed palms, each new trunk developing an adventitious root system.

4. Schoute's model

   Rare: trees with apical dichotomy of the vegetative axis, which may be erect as in Hyphaene or horizontal as in Nypa.

The leaf is the most conspicuous and distinctive organ in palms. Leaves are 2-. 3-, or usually many-ranked. Leaf base (at least initially) form a closed tubular sheath that encircles the stem. Leaf blade is well developed with a single midrib or rachis and is usually split partly or completely into leaflets (pinnate leaves) or leaf segments (palmate leaves).

Flowers are sometimes perfect, but usually diclinous by abortion and plants are monoecious or dioecious. Axis in most cases is branched. Trimerous (rarely dimerous or polymerous) flowers with well-developed floral envelope are usually aggregated in 2s, 3s, or more.

Distribution

During the later geologic ages palms grew in most parts of North America. Although they appeared, developed and diversified in the Cretaceous, their fossils are most abundant in the strata of the Tertiary period.

Currently there are two major centers of palms' geographic distribution - tropical America and tropical Asia. There is a minor center in tropical Africa.

The northern geographic limits of modern palms are in the southern United States, southern Europe, Afghanistan and southern Japan, while the southern limit in America is about middle Chile. In other words, the geographic distribution of palms is within 38° north latitude and 37° south latitude in regions with an average temperature of 60° Fahrenheit or more, and a minimum rarely, if ever, below zero.

More than half genera of palms have 5 or fewer species (the average is 13). Thus it is not surprising to find a high degree of endemism. In the South Pacific, each island or island group has its endemic species or genera. Continental Africa is poor in palms, with about 50 species. New Caledonia (30 species) represents an extraordinary rich center of endemism; no less than 17 genera of palms are limited to the island.

There is a marked dissimilarity between the palms flora in the Old and New World with distinct genera existing in one or another. This leaves only the Coryphoideae (for example, Caryota , fishtail palms) and Arecoideae (for example, coconut, Cocos) with essentially pan-tropical distribution. At the genera level only two palms have ranges than span the Atlantic (Elaeis and Raphia).

Ecology and habits

Palms are restricted to tropical and subtropical environments primarily because their vegetative growth is essentially continuous and they lack dormancy mechanisms. Few palms can withstand extended freezing temperatures. The hardiest palm is probably Rhapidophyllum hystrix (needle palm), ranging from central Florida to Alabama and Georgia.

In the Atlantic Coast of United States north of Florida, there are essentially four different palms: cabbage tree, the blue stem, the needle palm, and the saw palmetto, together representing three genera.

Palms occupy a variety of habitats at different altitudes and have strong preference for wet habitats where they may be dominant. About 75% of palms are rainforest species where, despite of usually being a minor component of total forest biomass, they play a diversity of ecological roles.

Habits of palms may be considered in several different contexts. Four basic growth forms are useful for field descriptions. These include three types: shrub palms, acaulescent palms (stemless or apparently so), and climbing palms.

The fixed primary conducting and mechanical ability of the trunk accounts for the fixed crown size. Palms normally remain unbranched above ground simply because they either lack completely any lateral vegetative meristems or such meristems are restricted to the base of the stem. Serenoa is exceptional in that there are vegetative lateral meristems interspersed among the inflorescence axes along the stem, but the axes are usually creeping.

Because palms have little regenerative ability (in the simplest situation one vegetative meristem function throughout the life span) they are very vulnerable to predator attacks - destruction of the one apical meristem destroys the whole tree. The apical meristem is protected primarily by its enclosure within the terminal crown of leaves. In many palms the mechanical protection of the crown is frequently supplemented by prickles.

Most palms are pleonanthic, i.e. each shoot is potentially of unlimited growth and flowers are produced on specialized auxiliary branch systems and the principle axis or axes are not determined by a flower or inflorescence. Senescence and death of the shoot are therefore presumably determined by little known physiological or structural factors. Usually life span of the palm in nature is determined by accident rather than senescence of the shoot. In rainforest palms that reached canopy or isolated palms that grew to adult size and reached reproductive maturity have future life expectancy of about 70 years.

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Pictures of palms in Florida

Saw palmetto, S. repens

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Coconut palm (Cocos nucifera) at GeoChemBio

Taxonomy Brief facts Growth stages Coconut fruit anatomy Growth conditions & ecology References

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References

• Tomlinson PB. Systematics and Ecology of the Palmae. Annual Review of Ecology and Systematics Vol. 10, (1979), pp. 85-107.
• Small JK. Palms of the Continental United States. The Scientific Monthly Vol. 32, No. 3 (Mar., 1931), pp. 240-255.
• Henderson A. Arecaceae. Part I. Introduction and the Iriarteinae. Flora Neotropica, Vol. 53 (May 2, 1990), pp. 1-100.
• Moore HE, Uhl NW. Arecaceae. Major Trends of Evolution in Palms Major Trends of Evolution in Palms. Botanical Review, Vol. 48, No. 1 (Jan. - Mar., 1982), pp. 1-69.
• Carson RB. Arecaceae. The Florida Tropics. Economic Geography Vol. 27, No. 4 (Oct., 1951), pp. 321-339.
• Bennett BC, Hicklin JR. Uses of Saw Palmetto (Serenoa repens, Arecaceae) in Florida. Economic Geography Economic Botany, Vol. 52, No. 4 (Oct. - Dec., 1998), pp. 381-393.
• Cook OF. A Synopsis of the Palms of Puerto Rico. Bulletin of the Torrey Botanical Club, Vol. 28, No. 10 (Oct., 1901), pp. 525-569.

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