Cycad

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Cycadophyta
)
For the insect, see Cicada.

Cycadales
Temporal range:
Early Permian–Holocene
Cycas rumphii with old and new male strobili.
Scientific classification Edit this classification
Kingdom: Plantae
Clade: Tracheophytes
Clade: Spermatophytes
Clade: Gymnospermae
Division: Cycadophyta
Bessey 1907: 321.[2]
Class: Cycadopsida
Brongn.[1]
Order: Cycadales
Pers. ex Bercht. & J. Presl
Extant groupings
Synonyms
  • Cycadofilicales Němejc 1950
  • Dioales Doweld 2001
  • Stangeriales Doweld 2001
  • Zamiales Burnett 1835
Cycads in South Africa

Cycads

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

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

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 rosette of pinnate leaves around a cylindrical trunk

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

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

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]

Taxonomy

The two extant

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]

External phylogeny[13][14] Internal phylogeny[15][16]

Cycads

Ginkgo

Conifers

Anthophytes

Bennettitales

Gnetales

Angiosperms
Gymnosperms

Cycads

Ginkgo

Conifers

Gnetophytes

Angiosperms

(flowering plants)


Traditional view
Modern view
Cycads
Cycadineae
Cycadaceae

Cycas

Zamiineae

Classification of the Cycadophyta to the rank of family.

  • Class Cycadopsida Brongniart 1843
    • Order Cycadales Persoon ex von Berchtold & Presl 1820

Fossil genera

The following extinct cycad genera are known:[17]

  • Amuriella Late Jurassic, Russian Far East (leaf fragments)
  • Androstrobus Triassic to Cretaceous, worldwide (leaf form genus)
  • Antarcticycas Middle Triassic, Antarctica (known from the whole plant)[18]
  • ?Anthrophyopsis Late Triassic, worldwide (leaf form genus, possibly a pteridospermatophyte)[19]
  • Apoldia Triassic-Jurassic, Europe
  • Archaeocycas Early Permian, Texas (leaf with sporophylls)
  • Aricycas Late Triassic, Arizona (leaf form genus)
  • Beania (=Sphaereda), Triassic to Jurassic, Europe & Central Asia (leaf form genus)
  • Behuninia Late Jurassic, Colorado & Utah (fruiting structures)
  • Bucklandia Middle Jurassic to Early Cretaceous, Europe and India (leaf form genus)
  • Bureja Late Jurassic, Russia
  • Cavamonocolpites Early Cretaceous, Brazil (pollen)
  • Crossozamia Early to Late Permian, China (leaf form genus)
  • Ctenis Mesozoic-Paleogene, Worldwide (leaf form genus)
  • Ctenozamites Triassic-Cretaceous, worldwide (leaf form genus)
  • Cycadenia Triassic, Pennsylvania (trunks)
  • Cycadinorachis Late Jurassic, India (rachis)
  • Fascisvarioxylon Late Jurassic, India (petrified wood)
  • Gymnovulites, Latest Cretaceous/earliest Paleocene, India (seed)
  • Heilungia, Late Jurassic to early Cretaceous, Russia & Alaska (leaf form genus)
  • Leptocycas Late Triassic, North Carolina & China (known from the whole plant)[20]
  • Mesosingeria, Jurassic to Early Cretaceous, Antarctica & Argentina (leaf form genus)
  • Michelilloa, Late Triassic, Argentina (stem)
  • ?Nikania, Early Cretaceous, Russia (leaf fragments)
  • ?Nilssonia, Middle Permian to Late Cretaceous, worldwide (leaf form genus) (possibly not a cycad)[21]
  • ?Nilssoniocladus, Early to Late Cretaceous, United States & Russia (stems, likely associated with Nilssonia, possibly deciduous)[22]
  • Palaeozamia, Middle Jurassic, England
  • Paracycas, Middle Jurassic to Late Jurassic, Europe and Central Asia
  • ?Phasmatocycas, Late Carboniferous to Early Permian, Kansas, Texas & New Mexico (leaf with sporophylls)[23]
  • Pleiotrichium, Late Cretaceous, Germany (leaf)
  • Pseudoctenis, Late Permian to Late Cretaceous, worldwide (leaf form genus)
  • Sarmatiella, Late Triassic, Ukraine
  • Stangerites, Late Triassic to Early Jurassic, Virginia and Mexico (leaf form genus)
  • Sueria, Early Cretaceous, Argentina (leaf)
  • Taeniopteris, Carboniferous to Cretaceous, worldwide (polyphyletic leaf form genus, also includes bennettitales and marattiale ferns)

Fossil record

Bowenia spectabilis : plant with single frond in the Daintree rainforest, north-east Queensland
Leaves and strobilus of Encephalartos sclavoi

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]

Petrified cycad fossil, New York Botanical Garden

Distribution

See also: List of cycad species by country

The living cycads are found across much of the

organic material.[citation needed] Some are able to grow in full sun, some in full shade, and some in both.[citation needed] Some are salt tolerant (halophytes).[citation needed
]

Species diversity of the extant cycads peaks at 17˚ 15"N and 28˚ 12"S, with a minor peak at the

tropical
.

Cultural significance

In

nanggaria plant, another symbol of Vanuatu culture, the namele also gives its name to Nagriamel
, an indigenous political movement.

See also

References

  1. ^ Brongniart, A. (1843). Énumération des genres de plantes cultivées au Muséum d'histoire naturelle de Paris.
  2. ^ Bessey, C.E. (1907). "A synopsis of plant phyla". Nebraska Univ. Stud. 7: 275–373.
  3. ^ Dehgan, Bijan (1983). "Propagation and Growth of Cycads—A Conservation Strategy". Proceedings of the Florida State Horticultural Society. 96: 137–139 – via Florida Online Journals.
  4. PMID 33862930
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  6. PMID 26791617
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  7. PMID 35437001
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  8. ^ Davis, Judi (27 June 2018). "Meet Durban's famous cycad family". South Coast Herald. Retrieved 11 September 2022.
  9. PMID 30551676
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  10. ^ Rutherford, Catherine. CITES and Cycads: A User's Guide (PDF). Royal Botanic Gardens, Kew.
  11. ISBN 9781300654537
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  12. ISBN 978-1-4000-5036-9
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  13. ^
    PMID 23315384
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  14. ^
    S2CID 236141481
    .
  15. ^
    S2CID 206535984
    .
  16. ^
    S2CID 14815027
    .
  17. ^ "PBDB". paleobiodb.org. Retrieved 16 March 2024.
  18. doi:10.1016/j.revpalbo.2008.07.005
    .
  19. ISSN 0034-6667
    .
  20. doi:10.1111/j.1759-6831.2010.00079.x
    .
  21. S2CID 3604369
    .
  22. ISSN 0034-6667
    .
  23. ISSN 0002-9122
    .
  24. S2CID 233860955
    .
  25. S2CID 249627815
    .
  26. PMID 37302411
    .
  27. ^
    PMID 28388891
    .
  28. PMID 34520529
    .
  29. ^ Orchard, A.E. & McCarthy, P.M. (eds.) (1998). Flora of Australia 48: 1-766. Australian Government Publishing Service, Canberra.
  30. ^ National Recovery Plan for the MacDonnell Ranges Cycad Macrozamia macdonnellii (PDF) (Report). Department of Natural Resources, Environment, The Arts and Sport, Northern Territory. Retrieved 16 July 2015.
  31. ISBN 9780226044682
    .
  32. ^ "Macrozamia communis", The IUCN Red List of Threatened Species
  33. ^ Dan McGarry (9 April 2018). "A Princely Title". Vanuatu Daily Post.

External links

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