Cycadales description

Families
Cycadaceae
Stangeriaceae
Zamiaceae

Range of the cycads (redrawn from (1)).

Cycadales

Common names

Cycads.

Taxonomic notes

The most primitive extant gymnosperm order, the Cycadales contain (in this treatment) 206 species in the suborders Cycadineae (family Cycadaceae, 47 spp.) and Zamiineae (families Stangeriaceae, 3 spp., and Zamiaceae, 156 spp.).

Due to their largely tropical distribution and tendency toward endemism, some species are undescribed and others, doubtless, undiscovered. The systematics of some genera are poorly understood (1).

Description

Woody, long-lived, unisexual plants. Main roots thickened, fleshy, often tuberous. All species have coralloid roots that are N-fixing due to symbiotic blue-green algae. Stems may be underground or emergent. In most species, leaves are once-pinnate and form a palm-like crown. Leaves of Stangeria are fern-like, while those of Bowenia are bipinnate. Reproductive structure a large cone. The Cycadales and Ginkgoales are unique among seed plants in having motile sperm (1).

Range

Tropical and subtropical latitudes in the Americas, Caribbean, sub-Saharan Africa, Madagascar, eastern India, China, Japan, southeast Asia, Oceania and Australia. Some species are frost-tolerant (1).

Big Tree

With trunks up to 20 m tall and 50 cm in diameter, Lepidozamia hopei is probably the tallest individual cycad. Both Encephalartos laurentianus and Macrozamia dyeri achieve diameters of 100 cm; the former with heights (or lengths, as it is often decumbent) to 15 m, is the most massive of cycads. Moreover, it often grows in clumps.

Oldest

Ages up to 2000 years have been speculated by extrapolating short-term leaf production rate to the number of leaf scars found on the trunk (2).

Dendrochronology

Cycad stem growth is similar to that of monocots, so dendrochronology is impossible.

Ethnobotany

Parts of all plants are poisonous, but many peoples have learned how to cope with this and use the seeds as a food source. Such use is recorded thoughout the range of Cycas (southeast Asia) and has also been made of species of Dioon (Mexico), Encephalartos (Africa) and Macrozamia (Australia). Stem starch (also called sago or arrowroot) has been used as a food source by native peoples in Africa, Florida, Australia, and most of the range of Cycas. It has been extracted on a commercial basis in Florida and New South Wales. Cycad poisons have a cumulative effect on livestock, which has led to efforts (fortunately ineffective, and now abandoned) to eradicate cycads in Australia. The poisons have been used to capture fish in southeast Asia. Historically, cycad poisoning was encountered by European immigrants in many areas. The leaves may be used as a substitute for palm fronds, usually as ornament. Many species are popular ornamentals.

Remarks

Cycads originated in Pangaea during the early Permian and reached their greatest abundance and diversity during the Jurassic, declining sharply during the Cretaceous radiation of the angiosperms. They are found at high paleolatitudes, although their present distribution is largely tropical and subtropical. Permian or Mesozoic fossils are known from Antarctica, Australia, China, Europe, Greenland, India, North America, and South Africa.

Most cycads are threatened, endangered or extinct in the wild. All cycad genera are on Appendix 2 of CITES (the Convention on International Trade in Endangered Species) and many species are on Appendix 1. The principal cause of this condition is excessive collection, often conducted illegally by wealthy collectors and targeted at the rarest taxa. As of 1993, apart from CITES protections (and note that Mexico is not signatory to CITES), the only country actively protecting cycads is South Africa.

Cycad stems are succulent with a structure similar to monocots and enlarge as the plant ages. Branching, when it occurs, is generally adventitious. These bulbils or suckers may be used to vegetatively reproduce cycads, either naturally (as in many species of Encephalartos) or horticulturally. Cycads have the largest apical meristem found in vascular plants, up to 3 mm across. Subterreanean cycads generally have contractile stems, another unique cycad feature, which permits the growing plant to maintain a subterreanean shoot apex.

Many cycads are pollinated by insects, and their pollinators are in all cases from relatively primitive families. The few cycad species that have been studied are reliant on one or two insect species, typically beetles but including the oldest known bee genus (Trigona). Usually, the pollinators mate and lay eggs within the female cone. This evidence suggests that insect pollination predates the evolution of angiosperms and may date to the Permian.

Cones, in members of all genera except Stangeria, produce heat when shedding pollen or receptive to pollination. Temperatures in the cones may exceed ambient levels by 1°C to 17°C. This heating may aid odor production and promote insect activity.

Seeds are dispersed by gravity, water and animals. Gravitational dispersal is largely ineffective. Several species of Cycas have floating seeds that are dispersed long distances across the sea, while most inland species have seeds with a brightly colored sarcotesta (integument) and are dispersed by various animals. Evidently, the sarcotesta may be palatable while the seed is indigestible. In most species, the sarcotesta is nonpoisonous. Seed predation is primarily effected by insects and rodents (1).

Cycads in general are fire-adapted, losing leaves to the blaze but soon restoring them in a growth flush. Cone production, coralloid root activity and foliage production may all be stimulated by frequent fire.

Horticulture: See (1) for extensive comments.

Citations

(1) The current standard work is Jones 1993, a comprehensive and exceptionally well-illustrated work.
(2) Chamberlain 1919.
(3) Stevenson, D. Accessed 6-Jan-1999. A Formal Classification of the Cycads. Web page, address http://www.plantapalm.com/vce/classification.htm.