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, sometimes at or below ground level

    • The stem or trunk is frequently called a caudex

  • Slightly woody stem: eustele with a cambium to produce small amounts of wood (manoxylic wood)

  • Largest pith (central area with parenchyma cells) in plant kingdom 

  • Leaf traces that wrap around cortex, called girdling leaf traces

Leaves

• • Fronds: large pinnately compound leaves

  • Many are rigid, tough, and spine-tipped

  • 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.