Cycad

For the insect, see Cicada.

Cycads /ˈsaɪkædz/ are seed plants with a stout, woody cylindrical trunk with a crown of large, hard, stiff, evergreen and usually pinnate leaves. The species are dioecious, that is, individual plants of a species are either male or female. Cycads vary in size from having trunks only a few centimeters to several meters tall. They typically grow slowly and have long lifespans. They superficially resemble palms or ferns, but are not closely related to either group. Cycads are gymnosperms. Cycads have specialized pollinators, usually a specific beetle, and more rarely a thrips or a moth.

Cycadales Temporal range: Early Permian–Holocene PreꞒ Ꞓ O S D C P T J K Pg N
Cycas rumphii with old and new male strobili.
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Spermatophytes
Clade:	Gymnospermae
Division:	Cycadophyta Bessey 1907: 321.[1]
Class:	Cycadopsida Brongn.[2]
Order:	Cycadales Pers. ex Bercht. & J. Presl
Extant groupings
Cycadaceae Zamiaceae
Synonyms
Cycadofilicales Němejc 1950 Dioales Doweld 2001 Stangeriales Doweld 2001 Zamiales Burnett 1835

Both male and female cycads bear cones (strobili), somewhat resembling conifer cones. Cycads fix nitrogen in association with cyanobacteria living in the plants' roots. Some species are used as narcotics, while in Vanuatu the plant symbolizes peace and appears on the national flag. Cycads all over the world are in decline, with four species on the brink of extinction and seven species having fewer than 100 plants left in the wild.

Description

Cycads are seed plants with a stout, woody, and usually unbranched cylindrical trunk, and a crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious, that is, individual plants of a species are either male or female. Cycads vary in size from having trunks only a few centimeters to several meters tall. They typically grow slowly^[3] and have long lifespans. Because of their superficial resemblance to palms or ferns, they are sometimes mistaken for them, but they are not closely related to either group. Cycads are gymnosperms (naked-seeded), meaning their unfertilized seeds are open to the air to be directly fertilized by pollination, as contrasted with angiosperms, which have enclosed seeds with more complex fertilization arrangements. Cycads have very specialized pollinators, usually a specific beetle, and more rarely a thrips or a moth.^[4]

The leaves are pinnate (shaped like feathers), with a central leaf stalk from which parallel ribs emerge from each side of the stalk, perpendicular to it. The leaves are typically either compound, or have margins so deeply cut as to appear compound. The Australian genus Bowenia and some Asian species like Cycas multipinnata, C. micholitzii and C. debaoensis, have bipinnate leaves, the leaflets each having subleaflets.^[5] The apex of the stem is protected by modified leaves called cataphylls.^[6]

Cycads superficially resemble palms in foliage and plant structure, occur in similar climates, and are often mistaken for them. However, they are so distantly related that they are classified in different phyla. Their similarities are caused by convergent evolution. Differences between cycads and palms include the cones (strobili) of cycads: they are gymnosperms, whereas palms are flowering plants and bear fruit. Both groups' mature foliage look similar, but young emerging cycad leaves – before they unfold and shift into place in the rosette crown – resemble a fiddlehead fern; in contrast, new leaves of palms are just miniature versions of a mature frond. Another difference is in the stem: Both phyla show scarring on their stems – below the rosette, where leaves used to attach – but the scars on a cycad's trunks are helically arranged and small; the scars on palm trunks are a circle, that wraps around the whole stem. The stems of cycads are generally rougher and shorter than those of palms.^[7]

• 
  Growth habit
• 
  Rosette of pinnate leaves around a cylindrical trunk
• 
  Leaves and strobilus of Encephalartos sclavoi
• 
  Bowenia spectabilis: plant with single leaf

Evolution

Fossil record

Petrified cycad fossil, New York Botanical Garden

The oldest probable cycad foliage is known from the latest Carboniferous / early Permian of South Korea and China, such as Crossozamia. Unambiguous fossils are known from the early / middle Permian onwards.^[8] Cycads were uncommon during the Permian.^[9] The two living cycad families diverged from each other at some time between the Jurassic^[10] and Carboniferous.^[11] Cycads are thought to have reached their apex of diversity during the Mesozoic.^[12] Although the Mesozoic is sometimes called "The Age of Cycads", some other groups of distantly related extinct seed plants with similar foliage, such as Bennettitales and Nilssoniales, were considerably more abundant than cycads during the Mesozoic: the "true" cycads were only minor components of Mesozoic vegetation.^[13] The oldest records of the modern genus Cycas are from the Paleogene of east Asia.^[14] Fossils assignable to Zamiaceae are known from the Cretaceous,^[13] with fossils assignable to living genera of the family from the Cenozoic.^[15]

Phylogeny

The two extant families of cycads both belong to the order Cycadales, and are the Cycadaceae and Zamiaceae (including Stangeriaceae). These cycads have changed little since the Jurassic in comparison to some other plant divisions.^[10] Based on genetic studies, cycads are thought to be more closely related to Ginkgo than to other living gymnosperms. They diverged from each other during the early Carboniferous.^[16][17]

External phylogeny[16][17]
Cycads Ginkgo Conifers Anthophytes Bennettitales Gnetales Angiosperms Gymnosperms Cycads Ginkgo Conifers Gnetophytes Angiosperms (flowering plants) Traditional view Modern view

Internal phylogeny[10][15]
Cycads Cycadineae Cycadaceae Cycas Zamiineae Zamiaceae Diooideae Dioon Zamioideae Encephalarteae Macrozamia Lepidozamia Encephalartos Zamieae Bowenia Ceratozamia Stangeria Zamia Microcycas

Taxonomy

Classification of the Cycadophyta to the rank of family.^[18]

• Class Cycadopsida Brongniart 1843
  • Order Cycadales Persoon ex von Berchtold & Presl 1820
    • Suborder Cycadineae Stevenson 1992
      • Family Cycadaceae Persoon 1807
        • Genus Cycas
    • Suborder Zamiineae Stevenson 1992
      • Family Zamiaceae Horaninow 1834
        • subfamily Diooideae Pilg. 1926
          • Tribe Diooeae Schuster
            • Genus Dioon
        • subfamily Zamioideae Stevenson 1992
          • Tribe Encephalarteae Miquel 1861
            • Genus Macrozamia
            • Genus Lepidozamia
            • Genus Encephalartos
          • Tribe Zamieae Miquel 1861
            • Genus Bowenia
            • Genus Ceratozamia
            • Genus Stangeria
            • Genus Zamia
            • Genus Microcycas

Fossil genera

The following extinct cycad genera are known:^[19]

• Amuriella Late Jurassic, Russian Far East (leaf fragments)
• Androstrobus Triassic to Cretaceous, worldwide (leaf form genus)
• Antarcticycas Middle Triassic, Antarctica (known from the whole plant)^[20]
• ?Anthrophyopsis Late Triassic, worldwide (leaf form genus, possibly a pteridospermatophyte)^[21]
• Apoldia Triassic-Jurassic, Europe
• Archaeocycas Early Permian, Texas (leaf with sporophylls)
• Aricycas Late Triassic, Arizona (leaf form genus)
• Beania (=Sphaereda), Triassic to Jurassic, Europe & Central Asia (leaf form genus)
• Behuninia Late Jurassic, Colorado & Utah (fruiting structures)
• Bucklandia Middle Jurassic to Early Cretaceous, Europe and India (leaf form genus)
• Bureja Late Jurassic, Russia
• Cavamonocolpites Early Cretaceous, Brazil (pollen)
• Crossozamia Early to Late Permian, China (leaf form genus)
• Ctenis Mesozoic-Paleogene, Worldwide (leaf form genus)
• Ctenozamites Triassic-Cretaceous, worldwide (leaf form genus)
• Cycadenia Triassic, Pennsylvania (trunks)
• Cycadinorachis Late Jurassic, India (rachis)
• Fascisvarioxylon Late Jurassic, India (petrified wood)
• Gymnovulites, Latest Cretaceous/earliest Paleocene, India (seed)
• Heilungia, Late Jurassic to early Cretaceous, Russia & Alaska (leaf form genus)
• Leptocycas Late Triassic, North Carolina & China (known from the whole plant)^[22]
• Mesosingeria, Jurassic to Early Cretaceous, Antarctica & Argentina (leaf form genus)
• Michelilloa, Late Triassic, Argentina (stem)
• ?Nikania, Early Cretaceous, Russia (leaf fragments)
• ?Nilssonia, Middle Permian to Late Cretaceous, worldwide (leaf form genus) (possibly not a cycad)^[23]
• ?Nilssoniocladus, Early to Late Cretaceous, United States & Russia (stems, likely associated with Nilssonia, possibly deciduous)^[24]
• Palaeozamia, Middle Jurassic, England
• Paracycas, Middle Jurassic to Late Jurassic, Europe and Central Asia
• ?Phasmatocycas, Late Carboniferous to Early Permian, Kansas, Texas & New Mexico (leaf with sporophylls)^[25]
• Pleiotrichium, Late Cretaceous, Germany (leaf)
• Pseudoctenis, Late Permian to Late Cretaceous, worldwide (leaf form genus)
• Sarmatiella, Late Triassic, Ukraine
• Stangerites, Late Triassic to Early Jurassic, Virginia and Mexico (leaf form genus)
• Sueria, Early Cretaceous, Argentina (leaf)
• Taeniopteris, Carboniferous to Cretaceous, worldwide (polyphyletic leaf form genus, also includes bennettitales and marattialean ferns)

Distribution and ecology

See also: List of cycad species by country

Global distribution of cycad genera. Red areas: cycad diversity; Purple dots: sampled fossil localities^[26]

The living cycads are found across much of the subtropical and tropical parts of the world, with a few in temperate regions such as in Australia.^[27] The greatest diversity is in the Americas, but they also grow in China, South and Southeast Asia, Pacific islands, and southern and tropical Africa.^[28][26] Some are xerophytes that can survive in desert or semi-desert climates,^[29] others in wet rain forest conditions,^[30] and some in both.^[31]

Cycads accomodate nitrogen-fixing cyanobacteria in their coralloid roots.^[32] The cyanobacteria produce a neurotoxin, BMAA, that accumulates in the plant's seeds.^[33][34] Another defence against herbivores is the accumulation of toxins in seeds and vegetative tissues; through horizontal gene transfer, cycads have acquired a family of genes (fitD) from a microbe, most likely a fungus, which gives them the ability to produce an insecticidal toxin.^[35]

Interaction with humans

Nuts of Cycas orientis (nyathu) are eaten by the Yolngu in Australia's Arnhem Land. They are harvested on their dry season to leach its poison under water overnight before ground into a paste, wrapped under bark and cooked on open fire until done.^[36] A pair of namele cycad leaves, representing peace, appears on the Flag of Vanuatu.^[37][38] Cycads are used as narcotics in Mexico, where they are among the substances called "peyote", while in South Africa, Encephalartos is used for the same purpose. In both regions, collecting for the drugs market is harming wild cycad populations.^[39] Cycads all over the world are in decline, with four species on the brink of extinction and seven species having fewer than 100 plants left in the wild.^[40][41][42]

See also

• Fossil Cycad National Monument, now withdrawn, in the U.S. state of South Dakota