Division of naked seeded dioecious plants
Cycads
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
[
edit
]
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
[
edit
]
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]
Taxonomy
[
edit
]
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]
External phylogeny
[13]
[14]
|
Internal phylogeny
[15]
[16]
|
|
|
Traditional view
|
Modern view
|
|
|
Classification of the Cycadophyta to the rank of family.
- Class Cycadopsida
Brongniart 1843
- Order Cycadales
Persoon ex von Berchtold & Presl 1820
- Suborder Cycadineae
Stevenson 1992
- Suborder Zamiineae
Stevenson 1992
- Family
Zamiaceae
Horaninow 1834
- subfamily Diooideae
Pilg. 1926
- subfamily Zamioideae
Stevenson 1992
- Tribe Encephalarteae
Miquel 1861
- Tribe Zamieae
Miquel 1861
Fossil genera
[
edit
]
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
marattialean
ferns)
Fossil record
[
edit
]
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]
Distribution
[
edit
]
The living cycads are found across much of the
subtropical
and
tropical
parts of the world, with a few in temperate regions such as in Australia.
[29]
The greatest diversity occurs in
South
and
Central America
.
[
citation needed
]
They are also found in
Mexico
, the
Antilles
, southeastern
United States
,
Australia
,
Melanesia
,
Micronesia
,
Japan
,
China
,
Southeast Asia
,
Bangladesh
,
India
,
Sri Lanka
,
Madagascar
, and
southern
and tropical
Africa
, where at least 65
species
occur. Some can survive in harsh
desert
or semi-desert
climates
(
xerophytic
),
[30]
others in wet
rain forest
conditions,
[31]
and some in both.
[32]
Some can grow in
sand
or even on
rock
, some in oxygen-poor, swampy,
bog
-like soils rich in
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
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
[
edit
]
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.
See also
[
edit
]
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[
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External links
[
edit
]
Extant
life
phyla/divisions by domain
|
---|
|
|
---|
Cycadophyta
| |
---|
Cycadales
| |
---|
Cycadidae
| |
---|
Cycadopsida
| |
---|