Cycad

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Cycadophyta
Cycas circinalis.jpg
Cycas circinalis with old and new male cones.
Scientific classification
Kingdom:
Division:
Cycadophyta
Class:
Cycadopsida
Order:
Cycadales

Families

Cycadaceae Cycas family
Stangeriaceae Stangeria family
Zamiaceae Zamia family

Leaves and cone of Encephalartos sclavoi
Pinnate divided leaves of a cycad, with new growth in the centre in the form of a bud
The fossil Cycad Zamites feneonis

Cycads are seed plants whose fossils are first found in the early Permian period.[1]

They were very common in the Mesozoic era, the age of the dinosaurs, and are still living, but are now much less common. Only 9 genera (about 100 species) of cycads are found to be extant nowadays. Cycads are thought to have evolved from order Pteridospermae (seed ferns) of angiosperms, now totally extinct.

They are different from most plants because they have large crowns of compound leaves and a stout trunk. They are evergreen, dioecious plants having large pinnately compound leaves.[2]

They are frequently confused with and mistaken for palms or ferns, but are only distantly related to both, and instead belong to the division Cycadophyta.

Cycads are gymnosperms (naked seeded). Their unfertilized seeds are open to the air to be directly fertilized by pollination. This contrasts with angiosperms, which have enclosed seeds and more complex fertilization arrangements. Cycads have very specialized pollinators, usually a specific species of beetle.

They have been reported to fix nitrogen in association with various cyanobacteria living in the roots (the "coralloid" roots).[3]

These photosynthetic cyanobacteria produce a neurotoxin called BMAA that is found in the seeds of cycads. This neurotoxin may enter a human food chain if the cycad seeds are eaten, and may cause some neurological diseases.[4][5]

Notes[change | change source]

  1. Willis K.J & McElwain J.C. 2002. The evolution of plants. Chapter 5 Major emergence of the seed plants, and specifically p133/4.
  2. Pinnate: feather-like or multi-divided features arising from both sides of a common axis.
  3. Rai, A.N.; Soderback, E.; Bergman, B. (2000), "Tansley Review No. 116. Cyanobacterium-Plant Symbioses", The New Phytologist, 147 (3): 449–481, doi:10.1046/j.1469-8137.2000.00720.x, JSTOR 2588831CS1 maint: uses authors parameter (link)
  4. Holtcamp, W. (2012). "The emerging science of BMAA: do cyanobacteria contribute to neurodegenerative disease?". Environmental Health Perspectives. 120 (3): a110–a116. doi:10.1289/ehp.120-a110. PMC 3295368. PMID 22382274.
  5. Cox, PA, Davis, DA, Mash, DC, Metcalf, JS, Banack, SA. (2015). "Dietary exposure to an environmental toxin triggers neurofibrillary tangles and amyloid deposits in the brain". Proceedings of the Royal Society B. 283 (1823): 20152397. doi:10.1098/rspb.2015.2397. PMC 4795023. PMID 26791617.CS1 maint: uses authors parameter (link)