Definitions

cycad

cycad

[sahy-kad]
cycad, any plant of the order Cycadales, tropical and subtropical palmlike evergreens. The cycads, ginkgoes, and conifers comprise the three major orders of gymnosperms, or cone-bearing plants (see cone and plant). The cycads first appeared in the Permian period. They are the most primitive of the living seed-bearing plants and in many ways resemble the ferns. Some have tuberous underground stems, with the crown of leathery, glossy, fernlike leaves springing from ground level; others have a columnar stem, usually 6 to 10 ft (1.8-3.1 m) high (though the corcho of Cuba reaches 30 ft/9.1 m), and are often mistaken for palms. There are 11 genera composed of less than 150 species, some found in very restricted areas. Many cycads (e.g., the fern palm of the Old World tropics and the nut palm of Australia) bear poisonous nutlike seeds. The pith of the coontie (Zamia floridana) yields a starch called Florida arrowroot or sago; the coontie is often called sago palm. Cycads are grown as ornamentals in warm regions and in greenhouses. The cycads are classified in the division Pinophyta, class Cycadopsida.

Any of the palmlike woody plants that constitute the order Cycadales, containing four families: Cycadaceae, Zamiaceae, Stangeriaceae, and Boweniaceae. Cycads have crowns of large, feathery compound leaves and cones at the ends of their branches. Some have tall, unbranched, armourlike trunks; others have partially buried stems with swollen trunks. Slow-growing cycads are used as ornamental conservatory plants, but some survive outdoors in temperate regions. The stems of some cycads yield starch that is edible if thoroughly cooked. The young leaves and seeds of others also are edible.

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Cycads are a group of seed plants characterized by a large crown of compound leaves and a stout trunk. They are evergreen, gymnospermous, dioecious plants having large pinnately compound leaves. They are frequently confused with and mistaken for palms or ferns, but are related to neither, belonging to the division Cycadophyta.

Cycads are found across much of the subtropical and tropical parts of the world. They are found in South and Central America (where the greatest diversity occurs), Mexico, the Antilles, south-eastern United States of America, Australia, Melanesia, Micronesia, Japan, China, Southeast Asia, India, Sri Lanka, Madagascar, and southern and tropical Africa, where at least 65 species occur. Some are renowned for survival in harsh semi-desert climates, and can grow in sand or even on rock. They are able to grow in full sun or shade, and some are salt tolerant. Though they are a minor component of the plant kingdom today, during the Jurassic period they were extremely common.

They have very specialized pollinators and have been reported to fix nitrogen in association with a cyanobacterium living in the roots. This blue-green algae produces a neurotoxin called BMAA that is found in the seeds of cycads.

Origins

The cycad fossil record dates to the Early Permian, 280 mya. There is controversy over older cycad fossils that date to the late Carboniferous period, 300–325  mya. One of the first colonizers of terrestrial habitats, this clade probably diversified extensively within its first few million years, although the extent to which it radiated is unknown because relatively few fossil specimens have been found. The regions to which cycads are restricted probably indicate their former distribution on the supercontinents Laurasia and Gondwana.

The family Stangeriaceae (named for Dr. William Stanger, 1812(?)–1854), consisting of only three extant species, is thought to be of Gondwanan origin as fossils have been found in Lower Cretaceous deposits in Argentina, dating to 70–135 mya. Zamiaceae is more diverse, with a fossil record extending from the Middle Triassic to the Eocene (54–200 mya) in North and South America, Europe, Australia, and Antarctica, implying that the family was present before the break-up of Pangea. Cycadaceae is thought to be an early offshoot from other cycads, with fossils from Eocene deposits (38–54 mya) in Japan and China, indicating that this family originated in Laurasia. Cycas is the only genus in the family and contains 99 species, the most of any cycad genus. Molecular data has recently shown that Cycas species in Australasia and the east coast of Africa are recent arrivals, suggesting that adaptive radiation may have occurred. The current distribution of cycads may be due to radiations from a few ancestral types sequestered on Laurasia and Gondwana, or could be explained by genetic drift following the separation of already evolved genera. Both explanations account for the strict endemism across present continental lines.

Taxonomy

There are about 305 described species, in 10–12 genera and 2–3 families of cycads (depending on taxonomic viewpoint). The classification below, proposed by Dennis Stevenson in 1990, is based upon a hierarchical structure based on cladistic analyses of morphological, anatomical, karyological, physiological and phytochemical data.

The number of species in the clade is low compared to the number of species in most other plant phyla. However, paleobotanical and molecular research indicates that diversity was higher in the history of the phylum. Fossil evidence shows that structural diversity in Mesozoic cycad pollen "considerably exceeds that seen in surviving genera today". The impacts of extinction on diversity are highlighted below. The disparity in molecular sequences is very high between the three main lineages of cycads, implying that genetic diversity in the clade was once high, but this fact has led to major disagreements about the divisions within the Cycadales.

The number of described cycad species has doubled in the past 25 years, mostly due to improved sampling and further exploration. Experts assume there may still be about 100 undescribed species, based on the rate of discovery. These are likely to be in Asia and South America where areas of endemism are highest. Diversity hotspots also occur in Australia, South Africa, Mexico, China and Vietnam, which together account for more than 70% of the world’s cycad species. The taxonomy of the Cycadophyta is, however, now stabilizing.

Cycad systematists reject the biological species concept, as clearly defined cycad species can interbreed and produce fertile offspring; this character is thus not disproportionately weighted when determining species barriers. The phenetic species concept, which states that a species is defined based on overall similarities with other individuals of the same species combined with a significant gap in variation with other species, is also rejected. Most cycad taxonomists agree on a modified version of the evolutionary species concept, termed the ‘morphogeographic’ species concept, which recognises the combined effects of geographical isolation and morphological disparity. Thus the presence of large geographical gaps in cycad distribution has greatly affected the way cycads are classified.Suborder Cycadineae

Family Cycadaceae
Subfamily Cycadoideae
Cycas. About 105  species in the Old World from Africa east to southern Japan, Australia and the western Pacific Ocean islands; type: C. circinalis L.; see also C. pruinosa and C. revolutaSuborder Zamiineae
Family Stangeriaceae
Subfamily Stangerioideae
Stangeria. One species in southern Africa; type: S. eriopus (Kunze) Baillon
Subfamily Bowenioideae
Bowenia. Two species in Queensland, Australia; type: B. spectabilis Hook. ex Hook. f.
Family Zamiaceae
Subfamily Encephalartoideae
Tribe Diooeae
Dioon. 13 species in Mexico and Central America; type: D. edule Lindley
Tribe Encephalarteae
Subtribe Encephalartinae
Encephalartos. About 66 species in southeast Africa; type: E. friderici-guilielmi Lehmann, E. transvenosus (Modjadji cycad)
Subtribe Macrozamiinae
Macrozamia. About 41 species in Australia; type: M. riedlei (Fischer ex Gaudichaud) C.A. Gardner
Lepidozamia. Two species in eastern Australia; type: L. peroffskyana Regel
Subfamily Zamioideae
Tribe Ceratozamieae
Ceratozamia. 26 species in southern Mexico and Central America; type: C. mexicana Brongn.
Tribe Zamieae
Subtribe Microcycadinae
Microcycas. One species in Cuba; type: M. calocoma (Miquel) A. DC.
Subtribe Zamiinae
Chigua. Two species in Colombia; type: C. restrepoi E. Stevenson
Zamia. About 65  species in the New World from Georgia, USA south to Bolivia; type: Z. pumila L.; see also Z. furfuracea

Cycads are most closely related to the extinct Bennettitales, and are also relatively close relatives to the Ginkgoales, as shown in the following phylogeny:

History

Modern knowledge about Cycads began in the 9th century with the recording by two Arab naturalists that the genus Cycas was used as a source of flour in India. Later, in the 16th century, Antonio Pigafetta, Fernao Lopez de Castanheda and Francis Drake found Cycas plants in the Moluccas, where the seeds were eaten. The first report of cycads in the New World was by Giovanni Lerio in his 1576 trip to Brazil, where he observed a plant named ayrius by the indigenous people; this species is now classified in the genus Zamia.

Cycads belonging to the genus Encephalartos were first described by Johann Georg Christian Lehmann in 1834. The name is derived from the Greek articles "en", meaning "in", "cephale", meaning "head", and "artos", meaning "bread".

Throughout the 18th-19th centuries, discoveries of several species were reported by numerous naturalist researchers and discoverers traveling throughout the world. One of the most notable researchers of cycads was American botanist C.J. Chamberlain whose work is noteworthy for the quantity of data and the novelty of his approach to studying cycads. His 15 years of travel throughout Africa, the Americas and Australia to observe cycads in their natural habitat resulted in his 1919 publication of The Living Cycads which remains current in its synthesis of taxonomy, morphology and reproductive biology of cycads, most of which was obtained from his original research. His 1940s monograph on the Cycadales, though never published (most likely because of his death) was never used by botanists. There are no other complete works on the cycads.

Uses

The generic name refers to the starch obtained from the stems which was used as food by some indigenous tribes. Tribal people grind and soak the nuts to remove the nerve toxin, making the food source generally safe to eat, although often not all the toxin is removed. In addition, consumers of bush meat may face a health threat as the meat comes from game which may have eaten cycad nuts and carry traces of the toxin in body fat.

There is some indication that the regular consumption of starch derived from cycads is a factor in the development of Lytico-Bodig disease, a neurological disease with symptoms similar to those of Parkinson's disease and ALS. Lytico-Bodic and its potential connection to cycasin ingestion is one of the subjects explored in Oliver Sacks' 1997 book Island of the Colourblind.

Distribution

Overall species diversity peaks at 17˚ 15"N and 28˚ 12"S, with a minor peak at the equator. There is therefore not a latitudinal diversity gradient towards the equator but towards the tropics. However, the peak in the northern tropics is largely due to Cycas in Asia and Zamia in the New World, whereas the peak in the southern tropics is due to Cycas again, and also to the diverse genus Encephalartos in southern and central Africa and Macrozamia in Australia. Thus the distribution pattern of cycad species with latitude appears to be an artifact of the geographical isolation of cycad genera, and is dependent on the remaining species in each genus that did not follow the extinction pattern of their ancestors. Cycas is the only genus that has a broad geographical range and can thus be used to infer that cycads tend to live in the upper and lower tropics. This is probably because these areas have a drier climate with relatively cool winters; while cycads require some rainfall, they appear to be partly xerophytic. Potted specimens are found and thrive in global locations such as Canada, Russia, Finland, Chile.

Speciation

There are no documented cases of sympatric speciation in cycads and allopatry appears to be the most common form of speciation in the group. This is difficult to study as they are long-lived plants, and so natural experiments have been investigated. One example is Cycas seemannii, which occurs only in Fiji, New Caledonia, Tonga and Vanuatu. Genetic diversity within populations was found to be significantly lower than between islands, suggesting that genetic drift is a likely mechanism for speciation, and is probably currently occurring between the isolated populations. Allopatry has also been proposed as the mechanism of speciation in Dioon, which predominantly occurs in Mexico. The many rivers that have shaped the region, and repeated glaciation and consequent disjunction, are thought to have been important in reproductive isolation not only in Dioon but in many other plant and animal taxa. Parapatric speciation may also have occurred, especially as cycads are pollinated by insects rather than by wind. As the range of the species grows, the individuals furthest apart are prevented from interbreeding as insects have relatively small ranges and will not pollinate between these plants. If sympatric speciation has occurred in cycads this would most likely be because of a host shift in pollinators, due to the very fact that cycads are uniformly dioecious.

Extinction

The probable former range of cycads can be inferred from their global distribution. For example, the family Stangeriaceae only contains three extant species, in Africa. Diverse fossils of this family have been dated to 135 mya, indicating that diversity may have been much greater before the Jurassic and late Triassic mass extinction events. However, the cycad fossil record is generally poor and little can be deduced about the effects of each mass extinction event on their diversity.

Instead, correlations can be made between the number of extant gymnosperms and angiosperms. It is likely that cycad diversity was affected more by the great angiosperm radiation in the mid-Cretaceous than by extinctions. Very slow cambial growth was first used to define cycads, and because of this characteristic the group could not compete with the rapidly growing, relatively short-lived angiosperms, which now number over 250,000 species, compared to the 947 remaining gymnosperms. It is surprising that the cycads are still extant, having been faced with extreme competition and five major extinctions. The ability of cycads to survive in relatively dry environments where plant diversity is generally lower, and their great longevity may explain their long persistence.

Conservation

In recent years, many cycads have been dwindling in numbers and may face risk of extinction because of theft and unscrupulous collection from their natural habitats, as well as from habitat destruction.

23% of the 305 extant cycad species are either critically endangered or endangered, and 15% are vulnerable. Thus 38% of cycads are on the IUCN Red List (2004), and the other 62% are in the Least concern or Near Threatened category (i.e. not actually on the Red List), or are data deficient. This value has changed dramatically within the past few years; 46% of cycads were on the 1978 Red List, and this rose to 82% in 1997. This was largely due to the recent discovery of over 150 new species, disagreements about classification, and uncertainty. This has not been helpful for conservation planning for the group.

Zamia in the New World, Cycas in Asia and Encephalartos in Africa are the most threatened genera. This pattern reflects the pressures on species in these regions. At least two species, Encephalartos woodii and Encephalartos relictus (both from Africa), are confirmed extinct in the wild. Cycads are long-lived with infrequent reproduction, and most populations are small, putting them at risk of extinction from habitat destruction and stochastic environmental events. Regionally, Australian cycads are the least at risk, as they are locally common and habitat fragmentation is low. However, land management with fire is thought to be a threat to Australian species. African cycads are rare and are thought to be naturally decreasing due to small population sizes, and there is controversy over whether to let natural extinction processes act on these cycads.

All cycads are in the CITES appendix appearing under the heading Plant Kingdom and under three family names, Cycadaceae, Stangeriaceae and Zamiaceae.

All cycads are CITES APPENDIX II except the following, in APPENDIX I:

  • Cycas beddomei
  • Stangeria eriopus
  • All Ceratozamia
  • All Chigua
  • All Encephalartos
  • Microcycas calocoma

Cycad seeds from species on APPENDIX II are not CITES regulated. APPENDIX I seeds are treated the same as the plants.

Horticulture

Cycads can be cut up into pieces to make new plants, although the most environmentally responsible method is by direct planting of the seeds. Propagation by seeds is the preferred method of growth, and two unique risks to their germination exist. One is that the seeds have no dormancy, so that the embryo is biologically required to maintain growth and development, which means if the seed dries out, it dies. The second is that the emerging radicle and embryo can be very susceptible to fungal diseases in its early stages when in unhygienic or excessively wet conditions. Thus, many cycad growers pre-germinate the seeds in moist, sterile mediums such as vermiculite or perlite. However pre-germination is not necessary, and many report success by directly planting the seeds in regular potting soil. As with many plants, a combination of well-drained soil, sunlight, water and nutrients will help it to prosper. Although, because of their hardy nature, cycads do not necessarily require the most tender or careful treatment, they can grow in almost any medium, including soil-less ones. One of the most common cause of cycad death is from rotting stems and roots due to over-watering.

Some insects, particularly scale insects, some weevils and chewing insects can damage cycads, though the pests are susceptible to insecticides such as the horticulture soluble oil white oil. Sometimes bacterial preparations may be used to control insect infestation on cycads. However, when some of the mature plants prepare for reproduction, the presence of weevils have been shown to help accomplish pollination.

While the cycads have a reputation of slow growth, it is not always well-founded and some actually grow quite fast, achieving reproductive maturity in 2–3 years (as with some Zamia species), while others in 15 years (as with some Cycas, Australian Macrozamia and Lepidozamia).

References

  • A Historical Perspective on Cycads from Antiquity to the Present, by Paolo De Luca (Dipartimento di Biologia vegetale and Orto Botanico, Universita di Napoli, via Foria 223, 80139 Napoli, Italia). A historical perspective on cycads from antiquity to the present.
  • Memoirs of the New York Botanical Garden 57: 1-7. 1990. A brief survey of the history of cycads in various cultures.
  • Jones, David L. 2002. Cycads of the World. Smithsonian Institution Press. ISBN 1-56098-220-9. Also published in 2002 as: Cycads of the World: Ancient Plants in Today's Landscape. Reed New Holland, Sydney. ISBN 1-876334-69-X
  • Chamberlain, C.J. (1919). The Living Cycads. University of Chicago Press, Chicago.
  • Chaw, S.-M., Parkinson, C.L., Cheng, Y., Vincent, T.M., & Palmer, J.D. (2000) Seed plant phylogeny inferred from all three plant genomes: Monophyly of extant gymnosperms and origin of Gnetales from conifers. Proceedings of the National Academy of Sciences 97: 4086-4091.
  • Chaw, S.-M., Walters, T.W., Chang, C.-C., Hu, S.-H., & Chen, S.-H. (2005) A phylogeny of cycads (Cycadales) inferred from chloroplast matK gene, trnK intron, and nuclear rDNA ITS region. Molecular Phylogenetics and Evolution 37: 214-234.
  • Chaw, S.-M., Zharkikh, A., Sung, H.-M., Luu, T.-C., & Li, W.-H. (1997) Molecular phylogeny of extant gymnosperms and seed plant evolution: Analysis of nuclear 18s rRNA sequences. Molecular Biology and Evolution 14: 56-68.
  • Donaldson, J. (2003). Chapter 3: Regional Overview: Africa. In Cycads: Status Survey and Conservation Action Plan (ed J. Donaldson), pp. 9-19. IUCN.
  • Donaldson, J. (2004). Saving ghosts? The implications of taxonomic uncertainty and shifting infrageneric concepts in the cycadales for red listing and conservation planning. In Cycad Classification: Concepts and Recommendations (eds T. Walters & R. Osborne), pp. 13-22. CABI, Oxford.
  • Donaldson, J., Hill, K.D., & Stevenson, D.W. (2003a). Chapter 2: Cycads of the World: An Overview. In Cycads: Status Survey and Conservation Action Plan (ed J. Donaldson), pp. 3-8. IUCN.
  • Donaldson, J.S., Dehgan, A.P., Vovides, A.P., & Tang, W. (2003b). Chapter 7: Cycads in trade and sustainable use of cycad populations. In Cycads: Status Survey and Conservation Action Plan (ed J. Donaldson), pp. 39-47. IUCN.
  • Golding, J.S. & Hurter, P.J.H. (2003) A Red List account of Africa’s cycads and implications of considering life-history and threats. Biodiversity and Conservation 12: 507–528.
  • Gonzàlez-Astorga, J., Vovides, A.P., Ferrer, M.M., & Iglesias, C. (2003a) Population genetics of Dioon edule Lindl. (Zamiaceae, Cycadales): biogeographical and evolutionary implications. Biological Journal of the Linnean Society 80: 457-467.
  • Gonzàlez-Astorga, J., Vovides, A.P., & Iglesias, C. (2003b) Morphological and geographic variation in Dioon edule. Botanical Journal of the Linnean Society 141: 465-470.
  • Gregory, T.J. & Chemnick, J. (2004). Hypotheses of the relationship between biogeography and speciation in Dioon (Zamiaceae). In Cycad Classification: Concepts and Recommendations (eds T. Walters & R. Osborne), pp. 137-148. CABI, Oxford.
  • Hill, C.R. (1990) Ultrastructure of in situ fossil cycad pollen from the English Jurassic, with a description of the male cone Androstrobus balmei sp. nov. Review of Palaeobotany and Palynology 65: 165-193.
  • Hill, K.D. (2003). Chapter 4: Regional Overview: Australia. In Cycads: Status Survey and Conservation Action Plan (ed J. Donaldson), pp. 20-24. IUCN.
  • Hill, K.D. (2004). Character evolution, species recognition and classification concepts in the cycadaceae. In Cycad Classification: Concepts and Recommendations (eds T. Walters & R. Osborne), pp. 23-44. CABI, Oxford.
  • Hill, K.D., Stevenson, D.W., & Obsorne, R. (2004). The World List of Cycads. Botanical Review 70: 274-298.
  • Keppel, G., Lee, S.W., & Hodgskiss, P.D. (2002). Evidence for Long Isolation Among Populations of a Pacific Cycad: Genetic Diversity and Differentiation in Cycas seemannii A.Br. (Cycadaceae). Journal of Heredity 93: 133-139.
  • Norstog, K.J. & Nicholls, T.J. (1997) Biology of Cycads Cornell University Press, Ithaca.
  • Walters, T., Osborne, R., & Decker, D. (2004). 'We hold these truths…'. In Cycad Classification: Concepts and Recommendations (eds T. Walters & R. Osborne), pp. 1-11. CABI, Oxford.
  • Whitelock, L.M. (2002) The Cycads. Timber Press, Portland.

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