Seed plants made up of only three living families, but prominently represented in the Mesozoic. Now of greater interest for their uniqueness than for their ecological or economic importance. Members are scattered around the globe but are restricted to tropical or subtropical climates.
Vegetative Characteristics: Leaves are pinnately compound and distictly palm-like - link to image of Cycas- . Leaf development typically occurs as an uncoiling of a hooked leaf primordium similar to circinate vernation in the ferns. While secondary growth occurs in the stem this does not result in the formation of large amounts of secondary xylem. Apogeotropic roots are formed that enclose mutualistic cyanobacteria that fix nitrogen.
Reproductive Characteristics: Dioecious plants that bear strobili either made up of megasporophylls bearing ovules or of microsporophylls bearing pollen sacs (Cycas is the exception with meagasporophylls arranged in a loose cluster around the apex of the plant). Pollen produces a pollen tube that is haustorial and which delivers flagellated sperm to an egg in an archegonium of the female gametophyte.