Cycad

220px|thumb|Pinnate divided leaves of a cycad, with new growth in the centre in the form of a bud
220px|thumb|Cycads in South Africa

Cycads are seed plants whose fossils are first found in the early Permian period.Willis K.J & McElwain J.C. 2002. The evolution of plants. Chapter 5 Major emergence of the seed plants, and specifically p133/4.

They were very common in the Mesozoic era, the age of the dinosaurs, and are still living, but are now much less common. Only nine genera (about 100 species) of cycads are extant nowadays. In the Lower Jurassic they were part of the forests which became the Jurassic coal measures in part of the world. The fronds could be reached and eaten by some of the typical herbivores of the day.

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.Pinnate: feather-like or multi-divided features arising from both sides of a common axis. 

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). 

These photosynthetic cyanobacteria produce a neurotoxin called BMAA that is found in the seeds of cycads. This is a typical defence against herbivory. the neurotoxin may enter a food chain if the cycad seeds are eaten, and may cause some neurological diseases.

 Notes 

Category:Gymnosperms