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. Sago flour is generally made from true palms – not from the cycad popularly known as "Sago Palm" (Cycas revoluta).
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.
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 as 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.
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.
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.
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.
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.
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.
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).