Figure 1. Casts of Nypa
Photo courtesy and copyright University
Figure 2. Permineralized Nypa
Photo courtesy and copyright University
The earliest palm fossils are leaves of Sabal magothiensis and stems of Palmoxylon cliffwoodensis from the Late Cretaceous, about 80 million years agowhen Gondwana had started to break up and the pieces were drifting apart. These early leaf records place palms among the earliest recognizable modern families of flowering plants. By the middle of the Maastrichtian, some 69 million years ago, the first recognizable modern species appeared. One of these, Nypa fruticans or the mangrove palm (Figs. 1-2), is identified by its distinctive pollen. This species did and still does live in mangrove habitats that lend themselves to fossil formation; thus, it is not surprising that early fossils of such a species are present. Another extant genus represented in the fossil record as pollen is Acrocomia.
By the beginning of the Eocene Epoch, nearly 60 million years ago, palms were widespread and abundant. A diversity of generaincluding Phoenix, Sabal, Serenoa, Livistona, Trachycarpus, and Oncospermaexisted in many places in which palms do not occur today (or in which these genera do not currently grow), such as parts of the United States, Canada, India, Europe, and China. These genera include members of groups considered primitive and specialized within the family and appear to represent an early burst of radiation and diversification.
Palms are now divided into six subfamilies: Coryphoideae, Calamoideae, Nypoideae, Ceroxyloideae, Arecoideae, and Phytelephantoideae. Members of the subfamilies are diverse and their interrelationships are not clear. Some of the botanical differences between the subfamilies are as follows:
Coryphoideae Costapalmate or palmate leaves, either induplicate (usually) or entire (rarely); flowers solitary or clustered, but not occurring in triads (groups of three).
Calamoideae Spines on many organs (stems, petioles, leaves); tubular inflorescence bracts, small brachts for each flower, and flowers in dyads (groups of two); fruit with closely overlapping scales and a specialized three-part gynoecium; climbing organs (cirrus and flagellum).
Nypoideae Monotypic subfamily (only one genus and one species) distinguished by a unique erect inflorescence bearing a terminal head of female flowers and lateral spikes of male flowers; prostrate, dichotomously branching stem.
Ceroxyloideae Pinnate, reduplicate leaves; several peduncular brachts covering the inflorescence; flowers arranged in spirals or rows and usually unisexual, with a specialized syncarpous, triovulate gynoecium; relatively small fruit usually developing from only one of three carpels.
Arecoideae Characterized by unisexual flowers in triads (with one female flower surrounded by two male flowers) or clusters derived from such triads.
Phytelephantoideae Characteristic large female flowers borne in a head or short peduncle, with multi-part flowers containing numerous stamens with centrifugal development; fruit extremely heavy and many-seeded; may not have other palm relatives.
Some of the structural specialization of the subfamilies relates to the demands of gigantism and an unbranched habit. Underlying such requirements are reproductive structures and a vascular system that are characteristic of all monocotyledons. Thus, the study of palms may be valuable in interpreting monocotyledonous evolution. It has been suggested that the palms, like the other families in the Arecidae, may represent a fragment of an early monocotyledonous radiation. However, the Palmae was separated earlier than the other families and has developed more specialized features, although within its own special constraints.
References for this chapter:
Encyclopedia Britannica Online
Stewart, L. 1994. A Guide to Palms & Cycads of the World. Angus & Robertson, Sydney, Australia.
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