Cycads /ˈsaɪkædz/ are seed plants with a long fossil history that were formerly more abundant and more diverse than they are today. They typically have a stout and woody (ligneous) trunk with a crown of large, hard and stiff, evergreen leaves. They usually have pinnate leaves. The individual plants are either all male or all female (dioecious). Cycads vary in size from having trunks only a few centimeters to several meters tall. They typically grow very slowly and live very long, with some specimens known to be as much as 1,000 years old. Because of their superficial resemblance, they are sometimes mistaken for palms or ferns, but they are not closely related to either group.
Information related to Cycadophyta
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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 species of beetle. They have been reported to fix nitrogen in association with various cyanobacteria living in the roots (the "coralloid" roots). These photosynthetic bacteria produce a neurotoxin called BMAA that is found in the seeds of cycads. This neurotoxin may enter a human food chain as the cycad seeds may be eaten directly as a source of flour by humans or by wild or feral animals such as bats, and humans may eat these animals. It is hypothesized that this is a source of some neurological diseases in humans.
This is the key to the Cycadophyta:
Pinnae have more than one vein.
Leaves are once-pinnate.
Pinnae have parallel venetion.
Sporophylls in spiral rows.
Sporophylls in vertical rows
Leaflets attach to sides of rachis.
Sporophylls lack a sharp upturned spine
Leaflets do not have an obvious midrib.
Leaves not apically truncate.
Leaves are bi-pinnate.
Leaflets have an obvious midrib.
Leaflets attach to midline of rachis.
- 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
- 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. PMID 22382274.
- 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. PMID 26791617.
- Jones, David L. Cycads of the World: Ancient Plants in Today's Landscape. Smithsonian Books: Reed, Sydney, 2002.
- Walters, T. and Osborne, R. 2004. Cycad Classification Concepts and Recommendations. Wallingford UK: CABI Publishing.
- Whitelock, Loran. The Cycads. Portland: Timber Press, 2002.