Cycad

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 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.

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. The apex of the stem is protected by modified leaves called cataphylls.

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.

File:Cycads, Limpopo, South Africa (2417726335).jpg|Growth habit File: Cycad leaves semicircle.jpg|Rosette of pinnate leaves around a cylindrical trunk File:Cycad cone.jpg|Leaves and strobilus of Encephalartos sclavoi File:Bowenia spectabilis.JPG|Bowenia spectabilis: plant with single leaf

Evolution

Fossil record

The oldest probable cycad foliage is known from the latest Carboniferous to early Permian (around 300 million years ago) of South Korea and China, such as Crossozamia. Unambiguous fossils are known from the early to middle Permian onwards. Cycads were uncommon during the Permian. The two living cycad families diverged from each other at some time between the Carboniferous and the Jurassic. 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. The oldest records of the modern genus Cycas are from the Paleogene of east Asia. Fossils assignable to Zamiaceae are known from the Cretaceous, with fossils assignable to living genera of the family from the Cenozoic.

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. 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.

External phylogeny

The Cycads have traditionally been thought to be basal within the seed plants. A more modern view is that they are Gymnosperms.

|}

Internal phylogeny

The Cycads are in two clades, the Cycadaceae and the Zamiaceae.

Taxonomy

Classification of extant Cycadophyta to genus:

• 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

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. The greatest diversity is in the Americas, but they also grow in China, South and Southeast Asia, the Pacific islands, and southern and tropical Africa. Some are xerophytes that can survive in desert or semi-desert climates, others in wet rain forest conditions, and some in both.

Cycads accommodate nitrogen-fixing cyanobacteria in their coralloid roots. The cyanobacteria produce a neurotoxin, BMAA, that accumulates in the plants' seeds, possibly as a defense against herbivores.

Cycads have acquired a family of genes (fitD) from a microbe, most likely a fungus, via horizontal gene transfer, which gives them the ability to produce an insecticidal toxin.

Relationship with humans

Nuts of Cycas orientis (nyathu) are eaten by the Yolngu in Australia's Arnhem Land. They are harvested during the dry season and soaked in water overnight before being ground into a paste, wrapped under bark, and cooked on an open fire. A pair of namele cycad leaves, representing peace, appears on the Flag of Vanuatu. 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. 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. The Fossil Cycad National Monument was a protected area in South Dakota from 1922, containing many fossils of Cycadeoidea. After vandalism of the exposed fossils, the status was withdrawn in 1957.

References

References

1. Bessey, C.E.. (1907). "A synopsis of plant phyla". Nebraska University Studies.
2. Brongniart, A.. (1843). "Énumération des genres de plantes cultivées au Muséum d'histoire naturelle de Paris".
3. Dehgan, Bijan. (1983). "Propagation and Growth of Cycads—A Conservation Strategy". Proceedings of the Florida State Horticultural Society.
4. (2018-09-10). "Beetle Pollination of Cycads in the Mesozoic". Current Biology.
5. "CITES and Cycads: A user's guide". [[Royal Botanic Gardens, Kew]].
6. (2018). "Does plant size influence leaf elements in an arborescent cycad?". [[Biology (journal).
7. Tudge, Colin. (2006). "The Tree". Crown Publishers.
8. (June 2021). "Not a lycopsid, but a cycad-like plant: ''Iratinia australis'' gen. nov. et sp. nov. from the Irati formation, Kungurian of the Paraná basin, Brazil". [[Review of Palaeobotany and Palynology]].
9. Gomankov, A.V.. (June 2022). "Cycads in the Permian of thе Subangara region". [[Paleontological Journal]].
10. (2023-06-11). "Reconciling fossils with phylogenies reveals the origin and macroevolutionary processes explaining the global cycad biodiversity". [[New Phytologist]].
11. (2024-03-14). "Disparity of cycad leaves dispels the living fossil metaphor". [[Communications Biology]].
12. (December 2017). "Eobowenia gen. nov. from the Early Cretaceous of Patagonia: indication for an early divergence of Bowenia?". BMC Evolutionary Biology.
13. (2022-01-28). "Not that young: Combining plastid phylogenomic, plate tectonic and fossil evidence indicates a Palaeogene Diversification of Cycadaceae". Annals of Botany.
14. (January 2013). "Chloroplast phylogenomics indicates that ''Ginkgo biloba'' is sister to cycads". Genome Biology and Evolution.
15. (July 19, 2021). "Gene duplications and phylogenomic conflict underlie major pulses of phenotypic evolution in gymnosperms". Nature Plants.
16. (2011). "Recent synchronous radiation of a living fossil". Science.
17. (17 April 2015). "Origin and diversification of living cycads: a cautionary tale on the impact of the branching process prior in Bayesian molecular dating". BMC Evolutionary Biology.
18. "''Cycadophyta'' Bessey".
19. (1998). "Flora of Australia". Australian Government Publishing Service.
20. "Global Distribution of Cycadales".
21. (2023). "Reconciling fossils with phylogenies reveals the origin and macroevolutionary processes explaining the global cycad biodiversity". New Phytologist.
22. "National Recovery Plan for the MacDonnell Ranges Cycad ''Macrozamia macdonnellii''". Department of Natural Resources, Environment, The Arts and Sport, Northern Territory.
23. (2005). "Tropical Rainforests: Past, present, and future". University of Chicago Press.
24. "The IUCN Red List of Threatened Species". [[International Union for Conservation of Nature]].
25. (2000). "Tansley Review No. 116. Cyanobacterium-Plant Symbioses". The New Phytologist.
26. Holtcamp, W.. (2012). "The emerging science of BMAA: do cyanobacteria contribute to neurodegenerative disease?". Environmental Health Perspectives.
27. (2015). "Dietary exposure to an environmental toxin triggers neurofibrillary tangles and amyloid deposits in the brain". Proceedings of the Royal Society B.
28. (2022). "The Cycas genome and the early evolution of seed plants". Nature Plants.
29. (2017). "Midawarr: Harvest: The Art of Mulkun Wirrpanda and John Wolseley". National Museum of Australia Press.
30. Oslievskyi, Yevhenii. "Human Relationships with Cycads: Cycads in Vanuatu: Tabu mo Kot".
31. (19 May 2023). "The Vanuatu Flag: An Emblem of Unity, Tradition, and Prosperity". VIMB.
32. Bonta, Mark. "Cycads: Cycads in as Narcotics".
33. Davis, Judi. (27 June 2018). "Meet Durban's famous cycad family". South Coast Herald.
34. "Protecting cycads through microdots". Botanical Society of South Africa.
35. "Cycad Project". Botanical Society of South Africa.
36. "Fossil Cycad National Monument". [[National Park Service]].
37. "Fossil Cycad National Monument: A Case of Paleontological Resource Mismanagement".