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General Characteristics

Many cycad species are shrub-sized in stature, but some species are 20-60 ft (6-18 m) tall at maturity. The cycads typically have an unbranched central stem, which is thick and scaly. Most species grow relatively slowly and have a large, terminal rosette of leaves. The leaves of most species are compound, in that they are composed of numerous small leaflets. Cycad leaves remain green for 3-10 years, so they are considered evergreen. Many cycad species, though short in stature, have a thick tap root which can extend as much as 30-40 ft (9-12 m) beneath the soil surface. The function of the tap root is to take up water from deep beneath the surface. Cycads also produce coralloid (coral-like) roots, which grow near the surface and are associated with symbiotic cyanobacteria. In a process known as nitrogen fixation, the cyanobacteria take in atmospheric nitrogen gas (N2) and transform it to ammonia (NH3), a chemical form that can be used by the plant as a nutrient. In reciprocation, the plant provides habitat and carbohydrates to the cyanobacteria. The cycads are the only gymnosperms known to form symbiotic relationships with cyanobacteria.

There are about 200 species of cycads in the world. They are endemic to tropical and subtropical regions, and are found in Central America, South America, Africa, Asia, and Australia. The greatest richness of cycad species is in Mexico and Central America. Zamia integrifolia is the only species of cycad native to the United States and is found in Florida and Georgia. Several foreign cycad species are grown as ornamental plants in Florida and elsewhere in the southern United States.

The stems and seeds of most cycads are very rich in starch. In earlier times, the Seminole Indians of Florida used Zamia as an important food source. They dried and then ground up the starchy stem of Zamia to make a flour which they called "coontie." In India, the stem of another cycad, Cycas circinalis, is still used to make Sago flour. However, cycads are of little economic importance today, except as ornamental plants.

Additional topics

Science EncyclopediaScience & Philosophy: Cyanohydrins to Departments of philosophy:Cycads - General Characteristics, Life Cycle, Evolution