Cycad

Cycads have a rosette of pinnate leaves around a cylindrical trunk

Cycads have a cylindrical trunk which usually does not branch. However, some types of cycads, such as Cycas zeylanica, can branch their trunks. The apex of the stem is protected by modified leaves called cataphylls.[9] Leaves grow directly from the trunk, and typically fall when older, leaving a crown of leaves at the top. The leaves grow in a rosette form, with new foliage emerging from the top and center of the crown. The trunk may be buried, so the leaves appear to be emerging from the ground, so the plant appears to be a basal rosette. The leaves are generally large in proportion to the trunk size, and sometimes even larger than the trunk.

The leaves are pinnate (in the form of bird feathers, pinnae), 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 (the leaf stalk has leaflets emerging from it as "ribs"), or have edges (margins) so deeply cut (incised) so as to appear compound. The Australian genus Bowenia and some Asian species of Cycas, like Cycas multipinnata, Cycas micholitzii and Cycas debaoensis, have leaves that are bipinnate, which means the leaflets each have their own subleaflets, growing in the same form on the leaflet as the leaflets grow on the stalk of the leaf (self-similar geometry).[10][11]

Due to superficial similarities in foliage and plant structure, cycads and palms are often mistaken for each other. They also can occur in similar climates. However, they belong to different phyla and as such are not closely related. The similar structure is the product of convergent evolution.

Beyond those superficial resemblances, there are a number of differences between cycads and palms. For one, both male and female cycads are gymnosperms and bear cones (strobili), while palms are angiosperms and so flower and bear fruit. The mature foliage looks very similar between both groups, but the young emerging leaves of a cycad resemble a fiddlehead fern before they unfold and take their place in the rosette, while the leaves of palms are just small versions of the mature frond. Another difference is in the stem. Both plants leave some scars on the stem below the rosette where there used to be leaves, but the scars of a cycad are helically arranged and small, while the scars of palms are a circle that wraps around the whole stem. The stems of cycads are also in general rougher and shorter than those of palms.[12]

The two extant families of cycads all 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. Five additional families belonging to the Medullosales became extinct by the end of the Paleozoic Era.

Based on genetic studies, cycads are thought to be more closely related to Ginkgo than other living gymnosperms. Both are thought to have diverged from each other during the early Carboniferous.[13][14]

Classification of the Cycadophyta to the rank of family.

• 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

Extinct cycad genera:

• Antarcticycas Middle Triassic, Antarctica (known from the whole plant)[17]
• Ctenis Mesozoic-Paleogene, Worldwide (leaf form genus)
• Pseudoctenis (leaf form genus)

Bowenia spectabilis : plant with single frond in the Daintree rainforest, north-east Queensland

Leaves and strobilus of Encephalartos sclavoi

The oldest probable cycad foliage is known from the latest Carboniferous-Early Permian of South Korea and China, such as Crossozamia. Unambiguous fossils of cycads are known from the Early-Middle Permian onwards.[18] Cycads were generally uncommon during the Permian.[19] The two living cycad families are thought to have split from each other sometime between the Jurassic[15] and Carboniferous.[20] Cycads reached their apex of diversity during the Mesozoic. Although the Mesozoic is sometimes called the "Age of Cycads," the foliage of cycads is very similar to other groups of extinct seed plants, such as Bennettitales and Nilssoniales, that are not closely related, and cycads were probably only a minor component of mid-Mesozoic floras, with Bennettitales and Nilsonniales being more abundant than cycads.[21] The oldest records of the modern genus Cycas are from the Paleogene of East Asia.[22] Fossils assignable to Zamiaceae are known from the Cretaceous,[21] with fossils assignable to living genera of the family known from the Cenozoic.[16]

Petrified cycad fossil, New York Botanical Garden

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.[23] The greatest diversity occurs in South and Central America. They are also found in Mexico, the Antilles, southeastern United States, Australia, Melanesia, Micronesia, Japan, China, Southeast Asia, Bangladesh, India, Sri Lanka, Madagascar, and southern and tropical Africa, where at least 65 species occur. Some can survive in harsh desert or semi-desert climates (xerophytic),[24] others in wet rain forest conditions,[25] and some in both.[26] Some can grow in sand or even on rock, some in oxygen-poor, swampy, bog-like soils rich in organic material. Some are able to grow in full sun, some in full shade, and some in both. Some are salt tolerant (halophytes).

Species diversity of the extant cycads 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 Tropic of Cancer and the Tropic of Capricorn. However, the peak near the northern tropic is largely due to Cycas in Asia and Zamia in the New World, whereas the peak near the southern tropic 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 the remaining cycad genera and their species, and perhaps because they are partly xerophytic rather than simply tropical.[lower-alpha 1][lower-alpha 2]

In Vanuatu, the cycad is known as namele and is an important symbol of traditional culture. It serves as a powerful taboo sign,[27] and a pair of namele leaves appears on the national flag and coat of arms. Together with the nanggaria plant, another symbol of Vanuatu culture, the namele also gives its name to Nagriamel, an indigenous political movement.

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

Wikimedia Commons has media related to Cycadophyta.

• "Palm Trees, Small Palms, Cycads, Bromeliads and tropical plants". Archived from the original on 20 May 2014. Site with thousands of large, high quality photos of cycads and associated flora. Includes information on habitat and cultivation. (Site is dead.)
• "The Cycad Pages". Royal Botanic Gardens Sydney. Archived from the original on 29 March 2021.
• "Gymnosperm Database: Cycads". Archived from the original on 10 August 2010.
• "Fairchild Tropical Botanic Garden". Archived from the original on 23 February 2015. One of the largest collections of cycads in the world in Florida, U.S.A.
• "Main Page". Palm and Cycad Societies of Australia (PACSOA). Retrieved 18 June 2023.
• "Welcome". The Cycad Society of South Africa. Retrieved 18 June 2023.
• David Dalton (1997). "Part I. The range of organisms that can fix nitrogen". Nitrogen Fixation. Retrieved 18 June 2023. Cycad nitrogen fixation
• "Cycad toxicity". The Cycad Pages. Royal Botanic Gardens Sydney. Archived from the original on 17 July 2020.
• Lauren Kessler (28 August 2005). "The Cult of the Cycads". The New York Times Magazine. Archived from the original on 4 June 2022. Magazine article on cycad collectorship and cycad smuggling.