A cycad is any member of the phylum Cycadophyta, primitive palm-like shrubs and trees that grow in tropical and sub-tropical regions. They have feathery palm- or fern-like leaves (poisonous in most species) at the top of stout (usually unbranched) stems. In addition to their main roots, they also have special roots containing cyanobacteria that carry out nitrogen fixation. These plants first flourished about 225 million years ago. Most of the 100 or so surviving species are less than 6.1 meters (20 feet) tall.