Cycad
Cycads —constituting the division Cycadophyta—are seed plants with a stout, woody cylindrical trunk with a crown of large, hard, stiff, evergreen and usually pinnate leaves. The species are dioecious, that is, individual plants of a species are either male or female. Cycads vary in size from having trunks only a few centimeters to several meters tall. They typically grow slowly and have long lifespans. They superficially resemble palms or ferns, but are not closely related to either group. Cycads are gymnosperms. Cycads have specialized pollinators, usually a specific beetle, and more rarely a thrips or a moth.
Both male and female cycads bear strobili, somewhat like conifer cones, containing their pollen and seeds, respectively. Cycads fix nitrogen in association with cyanobacteria living in their roots. Some species are used as narcotics, while in Vanuatu the plant symbolizes peace and appears on the national flag. Cycads all over the world are in decline, with four species on the brink of extinction and seven species each with fewer than 100 plants left in the wild.
Description
Cycads are seed plants with a stout, woody, and usually unbranched cylindrical trunk, and a crown of large, hard, stiff, evergreen and pinnate leaves. The species are dioecious, that is, individual plants of a species are either male or female. Cycads vary in size from having trunks only a few centimeters to several meters tall. They typically grow slowly and have long lifespans. Because of their superficial resemblance to palms or ferns, they are sometimes mistaken for them, but they are not closely related to either group.Cycads are gymnosperms, meaning their unfertilized seeds are open to the air to be directly fertilized by pollination, as contrasted with angiosperms, which have enclosed seeds with more complex fertilization arrangements. Cycads have very specialized pollinators, usually a specific beetle, and more rarely a thrips or a moth.
The leaves are pinnate, with a central leaf stalk from which parallel ribs emerge from each side of the stalk, perpendicular to it. The leaves are typically either compound, or have margins so deeply cut as to appear compound. The Australian genus Bowenia and some Asian species like Cycas multipinnata, C. micholitzii and C. debaoensis, have bipinnate leaves, the leaflets each having subleaflets. The apex of the stem is protected by modified leaves called cataphylls.
Cycads superficially resemble palms in foliage and plant structure, occur in similar climates, and are often mistaken for them. However, they are so distantly related that they are classified in different phyla. Their similarities are caused by convergent evolution. Differences between cycads and palms include the cones of cycads: they are gymnosperms, whereas palms are flowering plants and bear fruit. Both groups' mature foliage look similar, but young emerging cycad leaves – before they unfold and shift into place in the rosette crown – resemble a fiddlehead fern; in contrast, new leaves of palms are just miniature versions of a mature frond. Another difference is in the stem: Both phyla show scarring on their stems – below the rosette, where leaves used to attach – but the scars on a cycad's trunks are helically arranged and small; the scars on palm trunks are a circle, that wraps around the whole stem. The stems of cycads are generally rougher and shorter than those of palms.
Evolution
Fossil record
The oldest probable cycad foliage is known from the latest Carboniferous to early Permian of South Korea and China, such as Crossozamia. Unambiguous fossils are known from the early to middle Permian onwards. Cycads were uncommon during the Permian. The two living cycad families diverged from each other at some time between the Carboniferous and the Jurassic. Cycads are thought to have reached their apex of diversity during the Mesozoic. Although the Mesozoic is sometimes called "The Age of Cycads", some other groups of distantly related extinct seed plants with similar foliage, such as Bennettitales and Nilssoniales, were considerably more abundant than cycads during the Mesozoic: the "true" cycads were only minor components of Mesozoic vegetation. The oldest records of the modern genus Cycas are from the Paleogene of east Asia. Fossils assignable to Zamiaceae are known from the Cretaceous, with fossils assignable to living genera of the family from the Cenozoic.Phylogeny
The two extant families of cycads both belong to the order Cycadales, and are the Cycadaceae and Zamiaceae. These cycads may have changed little since the Jurassic in comparison to some other plant divisions, but are by no means "living fossils" as they have continued to evolve. Based on genetic studies, cycads are thought to be more closely related to Ginkgo than to other living gymnosperms. They diverged from each other during the early Carboniferous.External phylogeny
The Cycads have traditionally been thought to be basal within the seed plants. A more modern view is that they are Gymnosperms.Internal phylogeny
The Cycads are in two clades, the Cycadaceae and the Zamiaceae.Taxonomy
Classification of extant Cycadophyta to genus:- Class Cycadopsida Brongniart 1843
- * Order Cycadales Persoon ex von Berchtold & Presl 1820
- ** Suborder Cycadineae Stevenson 1992
- *** Family Cycadaceae Persoon 1807
- **** Genus Cycas
- ** Suborder Zamiineae Stevenson 1992
- *** Family Zamiaceae Horaninow 1834
- **** subfamily Diooideae Pilg. 1926
- ***** Tribe Diooeae Schuster
- ****** Genus Dioon
- **** subfamily Zamioideae Stevenson 1992
- ***** Tribe Encephalarteae Miquel 1861
- ****** Genus Macrozamia
- ****** Genus Lepidozamia
- ****** Genus Encephalartos
- ***** Tribe Zamieae Miquel 1861
- ****** Genus Bowenia
- ****** Genus Ceratozamia
- ****** Genus Stangeria
- ****** Genus Zamia
- ****** Genus ''Microcycas''
Distribution and ecology
Cycads accommodate nitrogen-fixing cyanobacteria in their coralloid roots. The cyanobacteria produce a neurotoxin, BMAA, that accumulates in the plants' seeds, possibly as a defense against herbivores.
Cycads have acquired a family of genes from a microbe, most likely a fungus, via horizontal gene transfer, which gives them the ability to produce an insecticidal toxin.