Cycadophyta
Leaves and female cone of Cycas revoluta
Scientific classification
Kingdom:
Plantae
Division:
Cycadophyta
Class:
Cycadopsida
Order:
Cycadales

Families

Cycadaceae cycas family
Stangeriaceae stangeria family
Zamiaceae zamia family

Cycads are an ancient 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 unrelated to either, belonging to the division Cycadophyta.

Introduction & overview

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), Australia, the Pacific Islands, Japan, China, India, 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. Sago is made from these plants.

They are 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 fruits of cycads.

Leaves and male cone of Cycas revoluta

Taxonomy

There are 289 species, 11 genera and 3 families of cycads. 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.

Order Cycadales

Suborder Cycadineae
Family Cycadaceae
Subfamily Cycadoideae
Cycas. About 90 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. revoluta
Suborder 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. Ten species in Mexico and Central America; type: D. edule Lindley
Tribe Encephalarteae
Subtribe Encephalartinae
Encephalartos. About 60 species in southeast Africa; type: E. friderici-guilielmi Lehmann, E. transvenosus (Modjadji cycad)
Subtribe Macrozamiinae
Macrozamia. About 30 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. Twelve 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 60 species in the New World from Georgia, USA south to Bolivia; type: Z. pumila L.; see also Z. furfuracea

History

Modern knowledge about Cycads began in the 9th century with the discovery 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". The generic name refers to the starch obtained from the stems which was used as food by some indigenous tribes.

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 a flowing and data rich volume, and which remains current in its synthesis of taxonomy, morphology and reproductive biology of cycads, most of which was obtained from his original research. His 1940's 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.

Conservation

File:Stangeria eriopus1 PD-self.JPG
Stangeria eriopus, an endangered species in southern Africa

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.

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:

Cycad seeds are not CITES regulated. APPENDIX I seeds are treated the same as the plants.

A 1997 the International Union for the Conservation of Nature and Natural Resources (IUCN), now known as the World Conservation Union, reported a list of over 150 threatened Cycad Species throughout the world that are indeterminate, rare, vulnerable, endangered or extinct.

Horticulture

File:Sago.palm.arp.750pix.jpg
A Sago Cycad (Cycas revoluta) growing in England as a houseplant

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 to 3 years (as with some Zamia species), while others in 15 years (as with some Cycas, Australian Macrozamia and Lepidozamia).

References

Category