Cycads

Ecology and Form

• Palm-like gymnosperms, inhabiting tropical (moist and arid) environments. [Technically palms are cycad-like, since they predate angiosperms by 150 million years!]

Sporophyte

Vegetative features

Stems

• • Usually possess a single trunk 

  • Eustele with a cambium to produce small amounts of wood

  • Manoxylic wood 

  • Largest pith in plant kingdom 

  • Leaf traces that wrap around cortex (called girdling leaf traces) 

Leaves

• • Fronds; large pinnately-compound leaves

  • Many are rigid and tough

  • Possess hypodermis to prevent water loss 

Roots

• • In addition to normal underground roots, they also have roots at the soil surface that look like coral (=coralloid roots) 

  • These roots contain cyanobacteria, and fix nitrogen (see Additional Resources)

Reproductive Structures

• Dioecious (separate male and female plants)

• Large female and male cones found in most species

  • Cycas possesses leaf-like structures that bear seeds (not cone-like) 

  • These are "simple" cones, composed of modified leaves that bear reproductive structures

  • Pollen sacs are born on modified leaves, called microsporophylls

  • Seeds are also born on modified leaves, called megasporophylls

    • micro=small / male; mega = large / female; phyll =leaf; sporophyll = spore-bearing leaf

• Reproductive structures are pollinated by beetles / weevils

Gametophyte

• Female gametophyte creates an archegonium

• Cycads posses the largest sperm cells in the plant world

  • Sperm cells are zoidogamous (sperm swim to egg)

  • Sperm is larger than opening on the neck canal in archegonium. It is assumed that negative pressure inside the archegonium pulls sperm into the chamber to allow fertilization.