Cycad

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Cycad_table_infobox_0

Cycadophyta

Temporal range: Early PermianHolocene PreꞒ O S D C P T J K Pg NCycad_header_cell_0_0_0

Scientific classification CycadopsidaCycad_header_cell_0_1_0
Kingdom:Cycad_cell_0_2_0 PlantaeCycad_cell_0_2_1
Clade:Cycad_cell_0_3_0 TracheophytesCycad_cell_0_3_1
Division:Cycad_cell_0_4_0 Cycadophyta

Bessey 1907: 321.Cycad_cell_0_4_1

Class:Cycad_cell_0_5_0 Cycadopsida

Brongn.Cycad_cell_0_5_1

OrdersCycad_header_cell_0_6_0

Cycad_description_list_0

  • For the insect, see Cicada.Cycad_item_0_0

Cycads /ˈsaɪkædz/ are seed plants with a very long fossil history that were formerly more abundant and more diverse than they are today. Cycad_sentence_0

They typically have a stout and woody (ligneous) trunk with a crown of large, hard and stiff, evergreen leaves. Cycad_sentence_1

They usually have pinnate leaves. Cycad_sentence_2

The species are dioecious, therefore the individual plants of a species are either male or female. Cycad_sentence_3

Cycads vary in size from having trunks only a few centimeters to several meters tall. Cycad_sentence_4

They typically grow very slowly and live very long, with some specimens known to be as much as 1,000 years old. Cycad_sentence_5

Because of their superficial resemblance, they are sometimes mistaken for palms or ferns, but they are not closely related to either group. Cycad_sentence_6

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. Cycad_sentence_7

Cycads have very specialized pollinators, usually a specific species of beetle. Cycad_sentence_8

They have been reported to fix nitrogen in association with various cyanobacteria living in the roots (the "coralloid" roots). Cycad_sentence_9

These photosynthetic bacteria produce a neurotoxin called BMAA that is found in the seeds of cycads. Cycad_sentence_10

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. Cycad_sentence_11

It is hypothesized that this is a source of some neurological diseases in humans. Cycad_sentence_12

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. Cycad_sentence_13

Description Cycad_section_0

Cycads have a cylindrical trunk which usually does not branch. Cycad_sentence_14

Leaves grow directly from the trunk, and typically fall when older, leaving a crown of leaves at the top. Cycad_sentence_15

The leaves grow in a rosette form, with new foliage emerging from the top and center of the crown. Cycad_sentence_16

The trunk may be buried, so the leaves appear to be emerging from the ground, so the plant appears to be a basal rosette. Cycad_sentence_17

The leaves are generally large in proportion to the trunk size, and sometimes even larger than the trunk. Cycad_sentence_18

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. Cycad_sentence_19

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. Cycad_sentence_20

Some species 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). Cycad_sentence_21

Confusion with palms Cycad_section_1

Due to superficial similarities in foliage and plant structure between cycads and palms they are often confused with each other. Cycad_sentence_22

They also can occur in similar climates. Cycad_sentence_23

In reality, they belong to completely different phyla, and are not closely related at all. Cycad_sentence_24

The similar structure is evidence of convergent evolution. Cycad_sentence_25

Beyond those superficial resemblances, there are a number of differences between cycads and palms. Cycad_sentence_26

For one, both male and female cycads bear cones (strobili), while palms are angiosperms and so flower and bear fruit. Cycad_sentence_27

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 never coiled up and instead are just small versions of the mature frond. Cycad_sentence_28

Another difference is in the stem. Cycad_sentence_29

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. Cycad_sentence_30

The stems of cycads are also in general rougher and shorter than those of palms. Cycad_sentence_31

Taxonomy Cycad_section_2

Historical diversity Cycad_section_3

The probable former range of cycads can be inferred from their global distribution. Cycad_sentence_32

For example, the family Stangeriaceae only contains three extant species in Africa and Australia. Cycad_sentence_33

Diverse fossils of this family have been dated to 135 mya, indicating that diversity may have been much greater before the Jurassic and late Triassic mass extinction events. Cycad_sentence_34

However, the cycad fossil record is generally poor and little can be deduced about the effects of each mass extinction event on their diversity. Cycad_sentence_35

Instead, correlations can be made between the number of extant gymnosperms and angiosperms. Cycad_sentence_36

It is likely that cycad diversity was affected more by the great angiosperm radiation in the mid-Cretaceous than by extinctions. Cycad_sentence_37

Very slow cambial growth was first used to define cycads, and because of this characteristic the group could not compete with the rapidly growing, relatively short-lived angiosperms, which now number over 250,000 species, compared to the 1080 remaining gymnosperms. Cycad_sentence_38

It is surprising that the cycads are still extant, having been faced with extreme competition and five major extinctions. Cycad_sentence_39

The ability of cycads to survive in relatively dry environments, where plant diversity is generally lower, may explain their long persistence and longevity. Cycad_sentence_40

Origins Cycad_section_4

The cycad fossil record dates to the early Permian, 280 million years ago (mya). Cycad_sentence_41

There is controversy over older cycad fossils that date to the late Carboniferous period, 300–325  mya. Cycad_sentence_42

This clade probably diversified extensively within its first few million years, although the extent to which it radiated is unknown because relatively few fossil specimens have been found. Cycad_sentence_43

The regions to which cycads are restricted probably indicate their former distribution in the Pangea supercontinent before the supercontinents Laurasia and Gondwana separated. Cycad_sentence_44

Recent studies have indicated the common perception of existing cycad species as living fossils is largely misplaced, with only Bowenia dating to the Cretaceous or earlier. Cycad_sentence_45

Although the cycad lineage itself is ancient, most extant species have evolved in the last 12 million years. Cycad_sentence_46

The family Stangeriaceae (named for Dr. William Stanger, 1811–1854), consisting of only three extant species, is thought to be of Gondwanan origin, as fossils have been found in Lower Cretaceous deposits in Argentina, dating to 70–135 mya. Cycad_sentence_47

The family Zamiaceae is more diverse, with a fossil record extending from the middle Triassic to the Eocene (54–200 mya) in North and South America, Europe, Australia, and Antarctica, implying the family was present before the break-up of Pangea. Cycad_sentence_48

The family Cycadaceae is thought to be an early offshoot from other cycads, with fossils from Eocene deposits (38–54 mya) in Japan, China, and North America, indicating this family originated in Laurasia. Cycad_sentence_49

Cycas is the only genus in the family and contains 99 species, the most of any cycad genus. Cycad_sentence_50

Molecular data have recently shown Cycas species in Australasia and the east coast of Africa are recent arrivals, suggesting adaptive radiation may have occurred. Cycad_sentence_51

The current distribution of cycads may be due to radiations from a few ancestral types sequestered on Laurasia and Gondwana, or could be explained by genetic drift following the separation of already evolved genera. Cycad_sentence_52

Both explanations account for the strict endemism across present continental lines. Cycad_sentence_53

Distribution Cycad_section_5

See also: List of cycad species by country Cycad_sentence_54

The living cycads are found across much of the subtropical and tropical parts of the world. Cycad_sentence_55

The greatest diversity occurs in South and Central America. Cycad_sentence_56

They are also found in Mexico, the Antilles, southeastern United States, Australia, Melanesia, Micronesia, Japan, China, Southeast Asia, India, Sri Lanka, Madagascar, and southern and tropical Africa, where at least 65 species occur. Cycad_sentence_57

Some can survive in harsh desert or semi-desert climates (xerophytic), others in wet rain forest conditions, and some in both. Cycad_sentence_58

Some can grow in sand or even on rock, some in oxygen-poor, swampy, bog-like soils rich in organic material. Cycad_sentence_59

Some are able to grow in full sun, some in full shade, and some in both. Cycad_sentence_60

Some are salt tolerant (halophytes). Cycad_sentence_61

Species diversity of the extant cycads peaks at 17˚ 15"N and 28˚ 12"S, with a minor peak at the equator. Cycad_sentence_62

There is therefore not a latitudinal diversity gradient towards the equator but towards the Tropic of Cancer and the Tropic of Capricorn. Cycad_sentence_63

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. Cycad_sentence_64

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. Cycad_sentence_65

Notes: The distribution area on the map should be expanded to include the range of Macrozamia macdonnelliana in the central region of Australia, Zamia boliviana in Bolivia and Mato Grosso, Brazil, Cycas thouarsii on Comoros and Seychelles, and Cycas micronesica on the islands of Guam, Palau, Rota, & Yap. Cycad_sentence_66

Also, the depiction of cycad distribution in Africa, particularly the western boundary, should be improved to show the actual range limits, rather than national borders. Cycad_sentence_67

Cultural significance Cycad_section_6

In Vanuatu, the cycad is known as namele and is an important symbol of traditional culture. Cycad_sentence_68

It serves as a powerful taboo sign, and a pair of namele leaves appears on the national flag and coat of arms. Cycad_sentence_69

Together with the nanggaria plant, another symbol of Vanuatu culture, the namele also gives its name to Nagriamel, an indigenous political movement. Cycad_sentence_70

See also Cycad_section_7

Cycad_unordered_list_1


Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Cycad.