Cycads
Cycads
The cycads are a relatively small phylum (Cycadophyta) in the plant kingdom Plantae. The cycads are considered to be gymnosperms, because they bear their seeds naked on modified leaves called sporophylls. In contrast, the evolutionarily more recent angiosperms (flowering plants) bear their seeds inside of ovaries. Cycads grow in tropical and subtropical regions of the world. Cycads are sometimes referred to as”living fossils“because they are very similar to extinct species that were much more abundant several hundreds of million years ago. The foliage of many species of cycads resembles that of palm trees, and plants in the genus Cycas are commonly called”Sago Palms.“However, cycads are only distantly related to the palms, and their similarity is superficial.
General characteristics
Many cycad species are shrub-sized in stature, but some species are 20-60 feet (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 taproot, which can extend as much as 30-40 feet (9-12 m) beneath the soil surface. The function of the taproot 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.
Life cycle
Cycads, like all seed-producing plants, have a dominant diploid sporophyte phase in their life cycle—this is the large, familiar, green plant seen in nature. Cycads and other gymnosperms do not have true flowers and their seeds are borne naked. In the more evolutionarily recent angiosperms (flowering plants), the seed is enveloped by a coat or fruit which originates from the ovary.
All species of cycads are dioecious, meaning the male and female reproductive structures are borne on separate plants. The male reproductive structure, known as an androstrobilus, superficially looks like a large pine cone, though it is much simpler in structure. It consists of many densely packed, modified leaves, known as microsporophylls. Each microsporophyll produces a large quantity of pollen grains on its dorsal surface. The pollen grain is the small, multicellular, male haploid gametophyte phase of the cycad life cycle. The pollen is dispersed by wind or by insects to the gynostrobilus, or the female reproductive structure.
The gynostrobilus of cycads also looks like a large pinecone, but it has a morphology different from the androstrobilus. When a pollen grain lands on the gynostrobilus, it germinates and grows a pollen tube, a long tubular cell that extends to deep within the multicellular, female haploid gametophyte. Then a sperm cell of the pollen grain swims through the pollen tube using its whiplike tail, or flagella, and fertilizes the egg to form a zygote. The zygote eventually develops into an embryo, and then a seed. Cycad seeds are rich in starch and have a pigmented, fleshy outer layer known as the sarcotesta. The seeds are often dispersed by birds or mammals, which eat them for the nutritious sarcotesta, and later defecate the still-viable seed.
It is significant that the cycads have flagellated sperm cells, which is considered a primitive (i.e., ancient) characteristic. Other evolutionarily ancient plants, such as mosses, liverworts, and ferns, also have flagellated sperm cells. More evolutionarily recent plants, such as the flowering plants, do not have flagellated sperm cells. In fact, other than the cycads, only one species of gymnosperm, the gingko, or maidenhair tree (Ginkgo biloba ), has flagellated sperm cells. In other gymnosperms and angiosperms, the sperm is transported directly to the female ovule by a sperm tube.
KEY TERMS
Cyanobacteria (singular, cyanobacterium)— Photosynthetic bacteria, commonly known as blue-green algae.
Diploid— Nucleus or cell containing two copies of each chromosome, generated by fusion of two haploid nuclei.
Gametophyte— The haploid, gamete-producing generation in a plant’s life cycle.
Haploid— Nucleus or cell containing one copy of each chromosome.
Rosette— A radial cluster of leaves, often on a short stem.
Sporophyll— An evolutionarily modified leaf which produces spores.
Sporophyte— The diploid, spore-producing generation in a plant’s life cycle.
Evolution
The earliest cycad fossils are from the Permian period (about 300 million years ago). Paleobotanists believe that cycads evolved from the seed ferns, a large group of primitive, seed-bearing plants with fern-like leaves. The seed ferns originated at least 350 million years ago, and became extinct more than 200 million years ago.
Although cycads are considered to be gymnosperms, because they bear naked seeds which are not enclosed by a fruit, fossil evidence suggests they are not closely related to other gymnosperms, such as the conifers. Therefore, many paleobotanists consider the gymnosperms to be an unnatural grouping of unrelated plants.
Cycads were particularly abundant and diverse during the Mesozoic era, so paleobotanists often refer to the Mesozoic as”the age of cycads.“This is also the era during which dinosaurs were the dominant animals, so zoologists refer to this as”the age of dinosaurs.“Consequently, museum drawings and dioramas which depict recreations of dinosaur life typically show cycads as the dominant plants.
The cycads are no longer a dominant group of plants, and there are only about 200 extant (surviving) species. The flowering plants essentially replaced the cycads as ecologically dominant species on land more than 100 million years ago.
See also Paleobotany.
Resources
BOOKS
Jones, D. C. Cycads of the World. Washington, DC: Smithsonian Institute Press, 1993.
Margulis, L., and K.V. Schwartz. Five Kingdoms. New York: W. H. Freeman and Company, 1988.
OTHER
Royal Botanic Gardens Sydney. ”The Cycad Pages.“<http://plantnet.rbgsyd.nsw.gov.au/PlantNet/cycad/> (accessed November 16, 2006).
Palm and Cycad Societies of Florida, Inc. ”The Cycads: Fossils of the Past“<http://www.plantapalm.com/Vce/intro/fossilspast.htm> (accessed November 16, 2006).
Peter A. Ensminger