Cycads/ˈsaɪkædz/ are
seed plants that typically have a stout and woody (
ligneous)
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 very slowly[3] and live very long. Because of their superficial resemblance, they are sometimes mistaken for
palms or
ferns, but they are not closely related to either group.
Cycads are
gymnosperms (naked-seeded), 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 species of
beetle. Both male and female cycads bear cones (
strobili), somewhat similar to
conifer cones.
Cycads have been reported to
fix nitrogen in association with various
cyanobacteria living in the roots (the "coralloid" roots).[4] These photosynthetic bacteria produce a
neurotoxin called
BMAA that is found in the
seeds of cycads. This neurotoxin may enter a human food chain as the cycad seeds may be eaten directly as a source of flour by humans or by wild or feral animals such as bats, and humans may eat these animals. It is hypothesized that this is a source of some
neurological diseases in humans.[5][6] Another defence mechanism against herbivores is the accumulation of toxins in seeds and vegetative tissues; through
horizontal gene transfer, cycads have acquired a family of genes (
fitD) from a microbial organism, most likely a fungus, which gives them the ability to produce an insecticidal toxin.[7]
Cycads all over the world are in decline, with four species on the brink of extinction and seven species having fewer than 100 plants left in the wild.[8]
Description
Cycads have a
cylindrical trunk which usually does not
branch. However, some types of cycads, such as Cycas zeylanica, can branch their trunks. The apex of the stem is protected by modified leaves called
cataphylls.[9] Leaves grow directly from the trunk, and typically fall when older, leaving a crown of leaves at the top. The leaves grow in a
rosette form, with new foliage emerging from the top and center of the crown. The trunk may be buried, so the leaves appear to be emerging from the ground, so the plant appears to be a
basal rosette. The leaves are generally large in proportion to the trunk size, and sometimes even larger than the trunk.
The leaves are
pinnate (in the form of bird feathers,
pinnae), 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 (the leaf stalk has
leaflets emerging from it as "ribs"), or have edges (
margins) so deeply cut (
incised) so as to appear compound. The Australian genus
Bowenia and some Asian species of Cycas, like
Cycas multipinnata,
Cycas micholitzii and
Cycas debaoensis, have leaves that are
bipinnate, which means the leaflets each have their own subleaflets, growing in the same form on the leaflet as the leaflets grow on the stalk of the leaf (
self-similar geometry).[10][11]
Confusion with palms
Due to superficial similarities in foliage and plant structure, cycads and
palms are often mistaken for each other. They also can occur in similar climates. However, they belong to different
phyla and as such are not closely related. The similar structure is the product of
convergent evolution.
Beyond those superficial resemblances, there are a number of differences between cycads and palms. For one, both male and female cycads are
gymnosperms and bear cones (strobili), while palms are
angiosperms and so flower and bear fruit. The mature foliage looks very similar between both groups, but the young emerging leaves of a cycad resemble a
fiddlehead fern before they unfold and take their place in the rosette, while the leaves of palms are just small versions of the mature frond. Another difference is in the
stem. Both plants leave some scars on the stem below the rosette where there used to be leaves, but the scars of a cycad are
helically arranged and small, while the scars of palms are a circle that wraps around the whole stem. The stems of cycads are also in general rougher and shorter than those of palms.[12]
The two extant
families of cycads all belong to the order
Cycadales, and are the
Cycadaceae and
Zamiaceae (including
Stangeriaceae). These cycads have changed little since the Jurassic in comparison to some other plant divisions. Five additional families belonging to the
Medullosales became extinct by the end of the Paleozoic Era.
Based on genetic studies, cycads are thought to be more closely related to Ginkgo than other living gymnosperms. Both are thought to have diverged from each other during the early
Carboniferous.[13][14]
Taeniopteris, Carboniferous to Cretaceous, worldwide (polyphyletic leaf form genus, also includes bennettitales and
marattiale ferns)
Fossil record
The oldest probable cycad foliage is known from the latest Carboniferous-Early Permian of South Korea and China, such as Crossozamia. Unambiguous fossils of cycads are known from the Early-Middle Permian onwards.[24] Cycads were generally uncommon during the Permian.[25] The two living cycad families are thought to have split from each other sometime between the Jurassic[15] and Carboniferous.[26] Cycads reached their apex of diversity during the Mesozoic. Although the Mesozoic is sometimes called the "Age of Cycads," the foliage of cycads is very similar to other groups of extinct seed plants, such as
Bennettitales and
Nilssoniales, that are not closely related, and cycads were probably only a minor component of mid-Mesozoic floras, with Bennettitales and Nilsonniales being more abundant than cycads.[27] The oldest records of the modern genus Cycas are from the Paleogene of East Asia.[28] Fossils assignable to Zamiaceae are known from the Cretaceous,[27] with fossils assignable to living genera of the family known from the Cenozoic.[16]
Species diversity of the extant cycads peaks at 17˚ 15"N and 28˚ 12"S, with a minor peak at the
equator. There is therefore not a
latitudinal diversity gradient towards the equator but towards the
Tropic of Cancer and the
Tropic of Capricorn. However, the peak near the northern tropic is largely due to Cycas in Asia and Zamia in the New World, whereas the peak near the southern tropic is due to Cycas again, and also to the diverse genus Encephalartos in southern and central Africa, and Macrozamia in Australia. Thus, the distribution pattern of cycad species with latitude appears to be an artifact of the geographical isolation of the remaining cycad genera and their species, and perhaps because they are partly
xerophytic rather than simply
tropical.
Cultural significance
In
Vanuatu, the cycad is known as namele and is an important symbol of traditional culture. It serves as a powerful
taboo sign,[33] and a pair of namele leaves appears on the
national flag and coat of arms. Together with the
nanggaria plant, another symbol of Vanuatu culture, the namele also gives its name to
Nagriamel, an indigenous political movement.
^Hermsen, Elizabeth J.; Taylor, Edith L.; Taylor, Thomas N. (January 2009). "Morphology and ecology of the Antarcticycas plant". Review of Palaeobotany and Palynology. 153 (1–2): 108–123.
doi:
10.1016/j.revpalbo.2008.07.005.
^Spiekermann, Rafael; Jasper, André; Siegloch, Anelise Marta; Guerra-Sommer, Margot; Uhl, Dieter (June 2021). "Not a lycopsid but a cycad-like plant: Iratinia australis gen. nov. et sp. nov. from the Irati Formation, Kungurian of the Paraná Basin, Brazil". Review of Palaeobotany and Palynology. 289: 104415.
doi:
10.1016/j.revpalbo.2021.104415.
S2CID233860955.