Dichotomous Key/Cycadophyta

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.

Cycas revoluta
Kingdom Plantae
Division Cycadophyta
Class Cycadopsida
Order Cycadales
Family Cycadaceae

Information related to Cycadophyta


Glossary for this page
(all links are to Wikipedia articles)
Bipinnate: Leaf arrangement where the leaflets of a pinnate leaf are also pinnately divided. Also called "twice pinnate".
Midrib: The central, most prominent vein of a leaf or leaflet.
Pinna(e): Leafy segment of a single pinnate blade, sometimes appearing to be an individual leaf.
Pinnule: Ultimate free leaflet of a multi-pinnate leaf.
Rachis: The main stem of a compound leaf.
Spine: A stiff, sharp structure; usually a modified leaf or stem.
Sporophyll: A modified leaf that bears the plant's spores.

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).[1] 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.[2][3]

This is the key to the Cycadophyta:


  • Cycas: Pinnae have a single midrib and lack secondary veins
  • 1: Pinnae have more than one vein


Pinnae have more than one vein.

  • 10: Leaves are bipinnate.
  • 2: Leaves are once-pinnate.


Leaves are once-pinnate.


Pinnae have parallel venetion.

  • 4: Sporophylls in spiral rows.
  • 5: Sporophylls in vertical rows.


Sporophylls in spiral rows.

  • 12: Leaflets attach to midline of rachis.
  • 6: Leaflets attach to sides of rachis.


Sporophylls in vertical rows

  • 11: Leaflets have an obvious midrib.
  • 8: Leaflets lack an easily identifiable midrib or midrib is not obvious


Leaflets attach to sides of rachis.

  • Macrozamia: Sporophylls have a sharp upturned spine.
  • 7: Sporophylls lack a sharp upturned spine


Sporophylls lack a sharp upturned spine

  • Dioon: Sporophylls have broad, flattened apices that overlap.
  • Encephalartos: Sporophylls have blocky, faceted apices that do not overlap.


Leaflets do not have an obvious midrib.


Leaves not apically truncate.


Leaves are bi-pinnate.

  • Bowenia spectabilis: One to ten leaf bearing branches; Largest leaves have ten or less pinnae.
  • Bowenia serrulata: Five to twenty leaf bearing branches; Largest leaves have more than ten pinnae.


Leaflets have an obvious midrib.

  • Chigua bernalii: Pinnae are over 30 centimeters long and less than 20 millimeters wide.
  • Chigua restrepoi: Pinnae are less than 30 centimeters long and over 20 millimeters wide.


Leaflets attach to midline of rachis.

  • Lepidozamia hopei: Pinnae have fifteen to thirty parallel veins; Broadest pinnae fifteen to thirty millimeters wide.
  • Lepidozamia peroffskyana: Pinnae have five to fifteen parallel veins; Broadest pinnae five to fifteen millimeters wide.


  1. 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 
  2. 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. 
  3. 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.