Cycad

Cycads Template:IPAc-en—constituting the division Cycadophyta—are seed plants with a stout, woody cylindrical trunk with a crown of large, hard, stiff, evergreen and usually pinnate (feather-shaped) 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.

Both male and female cycads bear cones (strobili) containing their seeds, 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 each with 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<ref name="Dehgan 1983">Template:Cite journal</ref> 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.<ref name="Cai 2018">Template:Cite journal</ref>

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.<ref>Template:Cite book</ref> The apex of the stem is protected by modified leaves called cataphylls.<ref>Template:Cite journal</ref>

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.<ref name="Tudge 2006">Template:Cite book</ref>

• 
  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 fossil of Cycadeoidea marylandica

The oldest probable cycad foliage is known from the latest Carboniferous / early Permian (around 300 million years ago) of South Korea and China, such as Crossozamia. Unambiguous fossils are known from the early / middle Permian onwards.<ref name="Spiekermann 2021">Template:Cite journal</ref> Cycads were uncommon during the Permian.<ref name="Gomankov 2022">Template:Cite journal</ref> The two living cycad families diverged from each other at some time between the Carboniferous<ref name="Coiro Allio 2023">Template:Cite journal</ref> and the Jurassic.<ref name=Nagalingum2011/> Cycads are thought to have reached their apex of diversity during the Mesozoic.<ref name="Coiro 2024">Template:Cite journal</ref> 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.<ref name="Coiro 2017">Template:Cite journal</ref> The oldest records of the modern genus Cycas are from the Paleogene of east Asia.<ref name="Liu Lindstrom 2022">Template:Cite journal</ref> Fossils assignable to Zamiaceae are known from the Cretaceous,<ref name="Coiro 2017"/> with fossils assignable to living genera of the family from the Cenozoic.<ref name=Condamine-2015/>

Phylogeny

The two extant families of cycads both belong to the order Cycadales, and are the Cycadaceae and Zamiaceae (including Stangeriaceae). These cycads may have changed little since the Jurassic in comparison to some other plant divisions, but are by no means "living fossils" as they have continued to evolve.<ref name="Nagalingum2011"/> 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.<ref name="Wu 2013">Template:Cite journal</ref><ref name="Stull 2021">Template:Cite journal</ref>

External phylogeny

The Cycads have traditionally been thought to be basal within the seed plants. A more modern view is that they are Gymnosperms.<ref name="Wu 2013"/><ref name="Stull 2021"/>

Template:Clade Template:Clade Template:Small Template:Small

Internal phylogeny

The Cycads are in two clades, the Cycadaceae and the Zamiaceae.<ref name="Nagalingum2011">Template:Cite journal</ref><ref name="Condamine-2015">Template:Cite journal</ref>

Template:Clade

Taxonomy

Classification of extant Cycadophyta to genus:<ref>Template:Cite web</ref>

• 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

Distribution and ecology

Global distribution of cycad genera. Red areas: cycad diversity; Purple dots: sampled fossil localities<ref name="Coiro Allio Mazet 2023"/>

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.<ref name="Orchard1998">Template:Cite book</ref> The greatest diversity is in the Americas, but they also grow in China, South and Southeast Asia, Pacific islands, and southern and tropical Africa.<ref>Template:Cite web</ref><ref name="Coiro Allio Mazet 2023">Template:Cite journal</ref> Some are xerophytes that can survive in desert or semi-desert climates,<ref name="Northern Territory">Template:Cite report</ref> others in wet rain forest conditions,<ref name="Bermingham Dick 2005">Template:Cite book</ref> and some in both.<ref name="Redlist">Template:Cite report</ref>

Cycads accommodate nitrogen-fixing cyanobacteria in their coralloid roots.<ref name="Rai Soderback 2000">Template:Cite journal</ref> The cyanobacteria produce a neurotoxin, BMAA, that accumulates in the plant's seeds.<ref name=Holtcamp>Template:Cite journal</ref><ref name="Cox and Davis">Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref>

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.<ref>Template:Cite book</ref> A pair of namele cycad leaves, representing peace, appears on the Flag of Vanuatu.<ref>Template:Cite web</ref><ref name="VIMB">Template:Cite web</ref> 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.<ref>Template:Cite web</ref> 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.<ref name="Davis 2018">Template:Cite news</ref><ref>Template:Cite news</ref><ref name="BSSA">Template:Cite news</ref> The Fossil Cycad National Monument was a protected area in South Dakota from 1922, containing many fossils of Cycadeoidea.<ref>Template:Cite web</ref> After vandalism of the exposed fossils, the status was withdrawn in 1957.<ref name="Hughes">Template:Cite web</ref>

External links

• Palm and Cycad Societies of Australia
• Cycad Society of South Africa