The phylum Cycadophyta contains about 140 living species of plants called cycads, with more species present in the fossil record. Cycads have seeds and water-conducting vessels. Cycads belong to a larger group of plants called gymnosperms, which bear seeds in cone-like structures. You'll also see Cycadophyta called a division rather than a phylum.
In gymnosperms such as cycads, seeds don't develop in a plant ovary as they do in flowering plants, but rather on the surface of the reproductive structures. Other gymnosperms include gingko trees, ephedra and its relatives, and pine trees and their relatives. There are three families of living cycads, with many more represented as fossils. Although they superficially resemble palms or ferns, cycads aren't related closely to either, and are probably the most primitive of the seed plants.
Cycads have long, pinnately-divided feathery-looking fronds. Many species have a trunk-like stem. They can become quite tall, with species ranging from 3 to 50 feet tall. Fossil cycads were even taller. Slow-growing cycad stems have a pithy core surrounded by layers of wood. Usually, a single crop of new fronds grows from the center of the leaves each year, with the leaves unfurling from the base as a fern frond does. Cycads have coralloid roots that stay near the soil surface and house symbiotic, nitrogen-fixing cyanobacteria.
When old enough, cycads grow a cone-like structure from the middle of their leaves. Male and female cones grow on separate male and female plants. Male cones produce pollen that contains ciliated mobile sperm that swim down the pollen tube to fertilize the female cones. The only seed plants with swimming sperm are cycads and gingko trees. Pollination is aided primarily by insects, particularly beetles. Once a sperm fertilizes an immature egg, it grows into a round seed, with two seeds at the base of each cone scale in most cycads. Female cones in some species can reach over three feet long and weigh up to 95 lbs.
At the time of the dinosaurs, fossils show that cycads belonging to genera different from those alive today were among the most common plants, with worldwide distribution. Today's cycads live in subtropical and tropical forests of Africa, Central and South America, Asia and Australia . Many prefer sun or partial shade, high humidity and soil with good drainage. Some species live in more arid climates in Australia, Mexico and Africa. Most cannot tolerate freezing weather, although the sago palm (cycas revoluta) native to Japan and China, tolerates light frost when mature.
- Biology of Plants: Peter H. Raven et al.
- Royal Botanic Gardens Sydney: The Cycad Pages: Cycad History
- Palomar College: Wayne's Word: The Five Kingdoms of Life
- Palomar College: Wayne's Word: Plants of Jurassic Park
- Floridata: Cycas Revoluta
- Susquehanna University: Systematic Biology: Diversity of Life: Description of the Phylum Cycadophyta
- Photo Credit PlazacCameraman/iStock/Getty Images
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