If you want to know more about Cycads, you have to know first a little more about the group of plants (gymnosperms)
Cycads belong to. If you are only vaguely interested please don't try to read this. If you desperately want to know more about
the most trilling plants on the face of this planet ( living fossils ) Read on and wonder about them. There are more species then
you know! Enjoy it.
I really like to thank the Encyclopaedia Britannica, since most of the informatian of this page came from it.
Gymnosperms
Introduction
Gymnosperms are a group of vascular plants whose seeds
are not enclosed by a ripened ovary (fruit). In 1825 the Scottish
botanist Robert Brown distinguished gymnosperms
from the other major group of seed plants, the angiosperms,
whose seeds are surrounded by an ovary wall. The seeds of many
gymnosperms (literally, "naked seed") are borne in cones and
are not visible. These cones, however, are not the same as fruits.
During pollination, the immature male gametes, or pollen grains,
sift among the cone scales and land directly on the ovules (which
contain the immature female gametes) rather than on elements
of a flower (the stigma and carpel) as in angiosperms. Furthermore,
at maturity, the cone expands to reveal the naked seeds. Gymnosperms
were considered at one time to be a class of seed plants, called
Gymnospermae, but taxonomists now tend to recognize four distinct
divisions of extant gymnospermous plants (Coniferophyta, Cycadophyta,
Ginkgophyta, Gnetophyta) and to use the term gymnosperms only
when referring to the naked-seed habit. Some of the divisions
of gymnosperms are not closely related to others, having been
distinct groups for hundreds of millions of years. Currently,
about 60-70 genera are recognized, with a total of 700-800 species.
Gymnosperms are distributed throughout the world, with extensive
latitudinal and longitudinal ranges.
GENERAL FEATURES
Diversity in size and structure.
Among the gymnosperms are plants with stems that may barely
project above the ground and others that develop into the largest
of trees. Cycads resemble palm trees, with fleshy stems
and leathery, featherlike leaves. The tallest cycads reach 19
metres (62 feet). Zamia pygmaea,
a cycad native to Cuba, has a trunk less than 10 centimetres
(four inches) in height. Of the gnetophytes, Ephedra
(joint fir) is a shrub and some species of Gnetum are
vines, while the unusual Welwitschia has a massive, squat
stem that rises a short distance above the ground. The apex
is about 60 centimetres in diameter. From the edge of the disk-shaped
stem apex arise two leathery, straplike leaves that grow from
the base and survive for the life of the plant. Most gymnosperms,
however, are trees. Of the conifers, the redwoods
(Sequoia) exceed 100 metres in height and, while Sequoiadendron
(giant redwood) is not as tall, the trunk is more massive.
Distribution and abundance.
Although since the Cretaceous period (144 to 66.4 million years
ago) gymnosperms have been gradually displaced by the more recently
evolved angiosperms, they are still successful in many
parts of the world and occupy large areas of the Earth's surface.
Conifer forests, for example, cover vast regions
of northern temperate lands in North America and Eurasia. In
fact, they grow in more northerly latitudes than do angiosperms.
Vascular plants that occur at the highest altitudes are the
gnetophyte Ephedra. Land in the Southern Hemisphere is
rich in conifer forests, which tend to be more abundant at higher
altitudes. Gymnosperms that occupy areas of the world with severe
climatic conditions are adapted to conserving water; leaves
are covered with a heavy, waxy cuticle, and pores (stomata)
are sunken below the leaf surface to decrease the rate of evaporation.
Cycads are distributed throughout the world but are concentrated
in equatorial regions. As a natural population, Ginkgo originally
appeared to have been confined to mountains of southeastern
China; extensive artificial propagation has altered this natural
distribution. Distribution of gymnosperms in the distant past
was much more extensive than at present. In fact, gymnosperms
were dominant in the Mesozoic era (245 to 66.4 million years
ago), during which time some of the modern families originated
(Pinaceae, Araucariaceae, Taxodiaceae).
Importance to humans and ecology.
Some of the oldest living things on earth are gymnosperms. Redwoods live for thousands of years, and some specimens of the
bristlecone pine, found in the White Mountains of California, approach 5,000 years in age.
Gymnospermous plants are widely used as ornamentals. Conifers are often featured in formal gardens and are used for bonsai.
Yews and junipers are often low-growing plants cultivated for ground cover. Conifers are effective windbreaks, especially
those that are evergreen. Cycads are used as garden plants in warmer latitudes, and some may even thrive indoors. Their
leathery green foliage and sometimes colourful cones are striking. Ginkgo is a hardy tree, and although it once approached
extinction, it is now cultivated extensively and survives such challenging habitats as the streets of New York City. Some
gymnosperms are weedy in that they invade disturbed areas or abandoned agricultural land. Pines and junipers are notorious
invaders, making the land unusable.
Most of the commercial lumber in the Northern Hemisphere is derived from the trunks of conifers such as pine, Douglas fir,
spruce, fir, and hemlock. Araucaria, kauri, and Podocarpus are important conifers of the Southern Hemisphere used for
lumber. The wood is straight-grained, light for its strength, and easily worked. Wood of gymnosperms is often called softwood
to differentiate it from the hardwood angiosperms. Wood of angiosperms typically has more kinds of elements than does
softwood of gymnosperms. In addition to its use in building construction, gymnospermous wood is used for utility poles and
railroad ties. Aromatic wood of cedar is frequently used in the construction of closets or clothes chests and apparently repels
cloth-eating moths. Most plywood is gymnospermous. Fibres of conifers make up paper pulp and may occasionally be used
for creating artificial silk or other textiles. Conifers are frequently planted in reforestation projects. Conifer bark is often the
source of compounds involved in the leather tanning industry. Bark is also used extensively as garden mulch.
From conifer resins are derived turpentine and rosin. A hardened form of resin from a kauri (Agathis australis), called copal,
is used in the manufacture of paints and varnishes. Some resins, such as balsam (from hemlock) and dammar (from Agathis)
are used in the preparation of mounting media for microscope slides. Resins may also have medicinal uses. Many types of
amber are derived from fossilized resin of conifers. Commercially useful oils are derived from such conifers as junipers, pines,
hemlock, fir, spruces, and aborvitae. These oils serve as air fresheners, disinfectants, and scents in soaps and cosmetics.
Seeds are often food sources. Pine seeds are a delicacy eaten plain or used as a garnish on bakery products. Seeds of Ginkgo
and cycads may be poisonous unless detoxified. "Berries" (in reality the fleshy cones) of juniper are used to flavour gin.
Earliest gymnosperms.
The earliest recognized group of gymnospermous seed plants are members of the division Pteridospermophyta (pteridosperms,
or seed ferns). These plants originated in the late Devonian and were widespread toward the end of the Paleozoic era (570 to
245 million years ago). Descendants persisted into the Mesozoic. In habit, Paleozoic seed ferns resembled some
progymnosperms in that they were small trees with fernlike leaves (the equivalent of a progymnospermous flattened branch)
bearing seeds. While Paleozoic seed ferns resembled ferns externally, the internal structure was like that of gymnosperms.
Secondary vascular tissues were common in stems of seed ferns. The wood, however, was composed of thin-walled tracheids
and abundant vascular rays, suggesting that stems were fleshy like those of cycads. Pteridosperm seeds were very similar to
those of cycads. Many were large, with an outer, softer seed coat and a harder, inner seed coat. Within an ovule ready for
fertilization was a massive female gametophyte with several archegonia. There has been one report of a pollination droplet in a
Carboniferous pteridosperm ovule and a report of a pollen tube emerging from a pollen grain in the micropyle of a seed-fern
ovule, suggesting that transport of the sperm through a pollen tube (siphonogamy) was in existence as far back as the
Paleozoic. In some pteridosperms the seed was contained within a cupule; some botanists interpret the cupule as a precursor of
an angiosperm carpel. Pollen grains, however, landed directly on the micropyle of the ovule. Pollen-bearing organs were
variable among the pteridosperms; in many cases the microsporangia were elongated and fingerlike and were produced in
clusters or were fused into compound organs. Mesozoic seed ferns are less well defined, and the concept of pteridospermy is
used loosely to refer to plants with fernlike foliage bearing seeds.
Appearance of gymnosperm divisions.
It is generally conceded that from the pteridosperms arose members of the division Cycadophyta. The first cycads appeared in
the Permian period (286 to 245 million years ago). Some of these presumed cycads differ from extant members in that
megasporophylls were undivided, unlike those of Cycas, considered to be primitive among cycads, in which the distal portion
of the megasporophyll may be pinnately divided. Other Permian megasporophylls, from China, are more like those of Cycas.
Cycad remains, especially leaves, are abundant in Mesozoic rocks. For this reason paleobotanists often refer to the Mesozoic
era as the "age of cycads." The earliest well-known cycads appear to have had slender stems, sometimes branched, with leaves
not borne close together, unlike the situation in extant cycads in which leaves are densely crowded at the apex of the plant.
There is evidence that these earliest cycads were deciduous. Megasporophylls of Mesozoic cycads are essentially like those of
extant cycads. The megasporophyll of the Triassic Palaeocycas is like that of Cycas. Jurassic megasporophylls are like those
of most other cycads. Extant cycads are now limited in geographic distribution to the warmer parts of the earth.
Coexisting with the cycads during the Mesozoic was another group of gymnosperms, the cycadeoids (division
Cycadeoidophyta--sometimes called the Bennettitophyta). Although they are superficially similar in habit to the cycads, with a
squat trunk and often pinnately divided leaves, their reproductive structures were different, and their actual relationship is not
close. Typically seeds were borne on the surface of a fleshy receptacle. Among the seeds were sterile structures, called
interseminal scales, that held the seeds tightly together. Pollen organs were quite similar among the forms in the sense that all
had a whorl of modified leaves (microsporophylls) on which were borne compound microsporangia.
Conifers (division Coniferophyta) appeared first toward the end of the Late Carboniferous epoch (320 to 286 million years
ago). Some of the earliest conifers (class Cordaitopsida) were trees with long, strap-shaped leaves. Trunks were similar to
those of extant conifers, with dense, compact wood; small, thick-walled tracheids; and narrow vascular rays. Reproductive
axes were slender, bearing narrow bracts in the axils of which were small, budlike shoots with helically arranged scales. On
some of the topmost scales were borne elongated microsporangia. Buds on other axes bore ovules instead of microsporangia.
By the late Paleozoic there came into existence another group of extinct conifers, the Voltziales (class Coniferopsida). In
general habit they must have resembled some of the extant araucarias (e.g., Norfolk Island pine), with whorled, flattened
branches bearing helically arranged, needlelike leaves. Reproductive axes were generally homologous with those of the
Cordaitales, but they were more compact, with the bracts on the ovule-bearing axes obscuring the axillary fertile buds. During
the end of the Paleozoic and in the early Mesozoic, these axillary buds underwent further transformation. The sterile,
non-seed-bearing part became flattened, with the scales fused together. The ovule-bearing portion was situated toward the
upper surface (away from the bract). The ovuliferous scale of a conifer seed cone, then, may be interpreted as an axis bearing
bracts in the axils of which are modified woody ovuliferous scales derived from lateral buds.
Modern families of conifers began to appear in the Mesozoic era. Members of the Taxodiaceae, the family to which redwoods
and bald cypress are assigned, appeared first in the Jurassic period. Metasequoia, the dawn redwood, is also a member of this
family. Discovered first as fossils in Miocene (23.7 to 5.3 million years ago) deposits, it was assumed to have become extinct
until it was discovered growing in Szechwan province in China. Its distribution in the late Mesozoic and Tertiary (66.4 to 1.6
million years ago) was throughout the Northern Hemisphere. The plant has since been introduced to a variety of places in the
world.
During late Triassic times there existed a type of conifer (Compsostrobus) that had many features of the Pinaceae. Seed cones
had woody ovuliferous scales subtended by bracts with two ovules on the upper surface of each ovuliferous scale. More
typical pinaceous remains occurred later in the Mesozoic. Coniferophytes were the dominant vegetation just before the
appearance of the angiosperms.
The division Ginkgophyta, represented today by only one living species, Ginkgo biloba, was much more widespread in past
ages. Gymnosperms that were presumed to be ginkgophytes existed as far back as the Permian period. In Mesozoic rocks,
Ginkgo leaves are commonly found throughout the world. The oldest fossil ginkgophytes had leaves much more dissected than
the typical Ginkgo leaf, resembling more closely the leaves found on new growth in extant ginkgoes.
The fossil record of the division Gnetophyta is obscure, and its origin is not clear. Pollen grains similar to those of Ephedra and
Welwitschia are found as far back as the Permian period. Megafossil remains of possible gnetophytan plants occur in Late
Cretaceous (97.5 to 66.4 million years ago) deposits. The plant is unlike any existing one, but venation of the foliage is similar
to that of leaves of Welwitschia. Pollen grains are typically gnetophytan.
Cycadophytes
Although some botanists prefer to restrict
the term cycadophyte to the members of the division Cycadophyta,
three groups of primitive seed plants are discussed here, of
which the seed ferns (division Pteridospermophyta) and cycadeoids
(division Cycadeoidophyta) are represented only by extinct forms.
A third order, Cycadales (cycads), is today represented by 10
or 11 living genera and some 130-160 species.
The cycadophytes encompass a diverse collection of mostly extinct
primitive seed plants that probably had their origins among
the progymnosperms of the Devonian period (408 to 360 million
years ago), possibly among a primitive, long-extinct group of
non-seed-bearing plants, the Aneurophytaceae, in which disposition
of fertile structures and patterns of branching bear some resemblance
to those of seed ferns.
GENERAL FEATURES
Diversity.
Seed ferns.
A number of lines of seed-bearing gymnospermous plants are
discernible among fossil plants of the late Paleozoic era (570
to 245 million years ago) and early to middle Mesozoic era (245
to 66.4 million years ago). Among them a rather loose assemblage
of forms, referred to as seed ferns, or pteridosperms, is well
represented. The Carboniferous period (360
to 286 million years ago) especially has been called the "age
of ferns" because of the abundance of fossilized fernlike leaves.
In time, many of these "ferns" were recognized as seed plants,
and it has been determined that seed ferns were a dominant vegetation
in the late Paleozoic. Seed ferns generally are characterized
as having been slender trees or, in some cases, woody, climbing
vines, but generally with large, fernlike fronds.
Characteristic seed-fern foliage consisted of large compound
leaves composed of second- and sometimes third-order branches
(Figure 6). The latter bore fernlike leaflets, hence the name
seed fern, although they are only remotely related to true ferns.
Seed-fern stems generally possessed variable amounts of soft,
loose wood and relatively large zones of cortex and pith; in
this respect they resembled the stems of cycads and differed
considerably from the stems of conifers, which have compact
wood and relatively small zones of cortex and pith.
Reproductive organs of seed ferns were borne upon the foliage;
single ovules and seeds were borne in place of pinnae, while
male organs often occurred as compound pollen organs composed
of partially or wholly united microsporangia. As in other gymnosperms,
the ovule consisted of one megasporangium within a single integument.
It is believed that, as the reproductive cycle progressed, the
megasporangium, also called the nucellus, probably gave rise
first to a quartet of megaspores. One of these then produced
a large fleshy female gametophyte bearing several archegonia,
each with a single egg. Following pollination and fertilization,
the ovule developed into a seed with an embryo nested in the
fleshy female gametophyte, which served as a food source during
germination and seedling growth.
Cycadeoids.
Although a few groups of pteridosperms persisted
from the late Paleozoic era well into the Mesozoic, the common
cycadophytes of the latter ages were members of the Cycadeoidophyta
(also known as Bennettitophyta). They are well represented in
the later Mesozoic era, well into the Cretaceous period (144
to 66.4 million years ago), by members of the genus Cycadeoidea,
which had rather squat, barrel-shaped, unbranched trunks and
once-pinnate compound leaves (Figure 6). The stems were armoured
with the persistent bases of leaves; internally there was a
thick pith surrounded by a narrow zone of vascular tissue from
which vascular strands extended directly into the leaf bases.
The fossilized trunks of these plants display scattered strobili
among leaf bases of the characteristic armour. Fossil cycadeoids
are widespread but are especially abundant in the Black Hills
region of South Dakota.
Earlier in the Mesozoic era, cycadeoids of a more slender,
branching form, exemplified by Williamsonia, were abundant
(Figure 6). As in Cycadeoidea, the fronds were single
pinnate compound leaves.
The feature that set the cycadeoids apart from other cycadophytes
was the compound strobili, which some, but not all, possessed.
These strobili were composed of both male and female sporophylls,
in some cases subtended by a system of bracts. Although often
described as flowerlike and indeed sometimes depicted as having
a floral, rosette form, cycadeoid "flowers," unlike true flowers
(found in the angiosperms), were composed of sporophylls bearing
"naked" (i.e., gymnospermous) ovules. They are not now
considered to have given rise to any group of the true angiospermous
flowering plants.
Although cycadeoids flourished for millions of years, and must
therefore be considered as a highly successful line of plants,
they eventually became extinct in the Cretaceous period.
Cycads.
The living cycads are for the most part palmlike, cone-bearing
plants, generally of low stature (Figure 6). Although few genera,
species, and individuals exist, they are extremely important
plants in terms of the information that can be gained from studying
them. Their reproduction is very primitive in that they rely
on flagellated, motile male gametes (spermatozoids), a feature
linking them with other plants fertilized by motile flagellated
sperm (zooidogamous), such as ferns, club mosses, and other
vascular cryptogams. Without knowledge of fertilization in the
cycads and Ginkgo, it is highly unlikely that scientists
would have more than remote theories as to the reproductive
modes of seed ferns and other extinct groups of seed plants.
Research on cycad reproduction is also providing information
on the early origins of insect pollination, long thought to
have evolved along with the relatively more recent angiosperms,
or flowering plants.
Distribution and abundance.
Seed-fern fossils are found in both the Northern and
Southern hemispheres, but many more have been described from
Europe and North America than from other regions, primarily
because many of the paleobotanical studies are concentrated
there. Pteridosperms have been identified in Australia and India
in recent years. In both hemispheres, seed ferns are common
in coal measures, from which it may be inferred that, ecologically,
they were plants of warm humid climates.
Abundant fossils of cycadeoids and cycads have been discovered
and described from the Mesozoic era. The oldest remains of undisputed
cycads date from the Triassic period, 245 to 208 million years
ago (e.g., Leptocycas, Antarcticycas), but some
problematic forms (e.g., Primocycas, Archaeocycas)
are of Paleozoic age. Most Mesozoic cycads resembled extant
genera (e.g., Cycadites, Pseudocycas, Cycadospadix),
and some are referred to present genera (e.g., Macrozamia
zamoides, Zamia coloradensis). Fossil forms have been found
in many places where they are now extinct (for example, Greenland,
Antarctica, Alaska, Argentina, France, Austria), testifying
to much milder climates in now temperate and even subarctic
regions.
Ten genera of cycads are widely recognized. There are three
endemic Australian genera--Macrozamia (14 species), Lepidozamia
(two species), and Bowenia (two species); four American
and Caribbean genera--Microcycas (one species), Zamia
(about 35 species), and Ceratozamia and Dioon
(10 species each); and two African genera--Encephalartos
(about 40 species) and Stangeria (one species). The genus
Cycas, with about 24 species, is the
most wide-ranging, extending from eastern Australia westward
across the Pacific and Indian oceans to Madagascar and the east
coast of South Africa. In addition to the above well-known genera,
a recent collection of cycad specimens from northwestern Colombia
included a new genus now described under the name Chigua.
Chigua reveals features hitherto undescribed in any American genus
or species, for the specimens, which in most respects resemble
Zamia, are unique in having leaflets
with midribs and lateral veins, a characteristic formerly known
only in Stangeria.
Ecology and habitats.
Cycads are plants of subtropical habitats, where they occupy
a variety of ecological situations ranging from rain forests,
to mesophytic savannas, to near-desert scrublands. Now nowhere
abundant in nature, wild populations of cycads in many regions
are endangered. Only recently have Australian cycads been removed
from the noxious weeds list (because of certain toxic properties
dangerous to cattle) to a protected status.
NATURAL HISTORY
Sporophyte phase.
As in other gymnosperms, the large, woody plant is the sporophyte phase of the
life cycle and typically is diploid in chromosome number. All
cycads may be called "functional conifers," for all species
bear strobili; these strobili are of a simple type, unlike those
of true conifers, which bear more complex, compound strobili.
It is not considered that this feature of cycads indicates anything
other than a parallelism in evolution.
Cycad males and females are morphologically alike except for
their sporophylls. Male sporophylls (microsporophylls) are spatulate
organs bearing large pollen sacs (microsporangia) in clusters
(sori) on their lower (abaxial) surfaces (Figure 7). Up to 200
cubic centimetres of pollen are produced by a single cone of
Cycas rumphii, and some other species produce similar
volumes. It was once estimated that one pollen cone of Encephalartos
produced seven billion pollen grains having a total volume of
about 300 cubic centimetres. While this enormous production
would seem to be consistent with a system of wind dispersal,
observations and controlled experiments strongly suggest that
in most, or perhaps all, cycads, insect pollen vectors are necessary
for effective pollination of ovules.
The Mexican cycad Zamia furfuracea,
for example, is pollinated by a small snout weevil, Rhopalotria mollis,
which lays its eggs and completes its reproductive cycle in
male cones. Emerging adults then carry pollen to female cones
and pollination of ovules and subsequent fertilization of eggs
occurs.
Gametophyte phase.
As in all other gymnosperms, male and female sporophytes of
cycads produce, respectively, male and female gametophytes. The male
gametophyte phase of the life cycle begins in the microsporangium
with meiotic production of tetrads of microspores followed by
the division of each haploid microspore into a three-celled
pollen grain. Because it is multicellular, the pollen grain is considered
to be an immature male gametophyte, but its further development
into a sexually mature organism occurs only after it has been
shed from the microsporangium and transported as a pollen grain
to a megasporophyll--specifically, to an ovule, within which
the male gametophyte grows to maturity.
Concurrently with pollen development, the ovule differentiates,
and at the time of pollination it consists of a large megasporangium
(nucellus) enclosed within a fleshy integument. At this time
an opening at its distal end (the micropyle) permits pollen
to enter the ovule. Over the next three to five months, the
male gametophyte develops into a haustorial pollen tube, which
eventually penetrates the nucellus and partially projects into
an archegonial chamber.
Meanwhile in the nucellus, a single megaspore mother cell undergoes
meiosis, forming a tetrad of haploid megaspores, only one of
which survives to divide mitotically many times and form a large
fleshy female gametophyte. The female gametophyte grows at the
expense of nucellar tissue but remains enclosed within its remains.
At its micropylar end, this gametophyte develops from one to
many archegonia (commonly one to six in most cycads
and up to 100 in Microcycas, only
five or six of which are functional). Each archegonium is composed
of a quartet of neck cells beneath which is a large egg. This
egg is the largest known in the plant kingdom, being about three
millimetres in length.
The development of male and female gametophytes is synchronized,
and during the final week or so before fertilization, the male
gametophyte forms large, multiflagellated spermatozoids. In
all species but Microcycas, there is just one pair of
sperm per pollen tube; in Microcycas, spermatozoids number
about 12 to 16 per tube. What actually triggers sperm release
into the archegonial chamber is unknown, but when it happens,
the spermatozoids quickly move through the archegonial neck
into the egg cytoplasm. One sperm loses its flagellature, and
fusion of egg and sperm nuclei takes place. Subsequently, the
zygote forms a single large embryo, other eggs meanwhile aborting.
In the Florida cycad, Zamia pumila, the
reproductive cycle occurs over a period of about 14 months,
cones first becoming visible in October, pollination occurring
in December, fertilization taking place in late May and early
June, and embryogenesis and seed maturity being completed the
following December. Similarly slow reproduction is typical also
for other genera and species.
As far as is known, cycad seeds have no dormant period or after-ripening
requirements and in some cases actually begin germinating while still
attached to sporophylls. Possibly some germination inhibitors
are present in the outer fleshy layer of the seed, because its
removal often accelerates germination, and treatment with scarifying
agents also may enhance germinability. The germinating embryo
remains attached to the female gametophyte for as long as two
years, absorbing nutrition through its cotyledons, which remain
embedded in the female gametophyte. The seedling rapidly develops
a flesh taproot and root growth.
The outer layer of the seeds of Cycas circinalis and
C. rumphii are thick and somewhat fibrous,
and experiments which show them to be capable of long immersion
in brine suggest that long-distance dispersal by ocean currents
may account for the presence of these species on remote Pacific
islands. Little is known of natural seed dispersal of other
cycads, but their bright red seed coats suggest a visual signal
to animals. Mockingbirds, squirrels, and coatis are reported
to disperse them. In general, however, cycad seeds, which are
rather heavy, are poorly dispersed, and most germinate at the
base of the parent, where they often languish and die.
FORM AND FUNCTION
Stem.
Stems of cycads are characteristically short
and stout, and while most genera have some species with subterranean,
tuberlike stems, a majority of species are arborescent (Figure 6).
The taller cycads include Microcycas calocoma (up to
10 metres high), Macrozamia moorei (up to 18 metres),
Dioon spinulosum (up to 16 metres), Lepidozamia hopei
(up to 18 metres), and Encephalartos altensteinii (up
to 20 metres), but most of the arborescent (treelike) species
have trunks only two to three metres high. The stems of most
arborescent species are covered with an armour composed of the
hardened leaf and cataphyll bases, but internally they are rather
soft and fleshy, with a thick parenchymatous cortex, a large
pith, and scanty woody tissue (Figure 8). In most cycads, the
woody tissue is on the order of five to 10 millimetres wide,
but Dioon spinulosum has an exceptional
amount of wood, in some specimens up to 10 centimetres wide.
This may constitute evidence of the primitive nature of the
genus, because seed ferns also generally had stems with considerably
more wood than those of most living cycads. Even in Dioon, there is
no evidence of annual growth rings, so that age estimates must
rely on other evidence, most often on counts of the whorls of
leaf scars, which can be related to annual or biennial production
of new leaf flushes. On this basis, it has been estimated that
some cycads (notably Dioon and Macrozamia) may
be as much as 1,000 years old; however, it is doubtful that
most cycads are that old.
Species of Macrozamia, Encephalartos, and Cycas often
develop additional cylinders of vascular tissue, apparently
formed from vascular cambia originating in the cortex. The result
is a condition in which concentric rings of xylem and phloem
are present, often two or three, but in exceptional cases, as
many as 14. The xylem of cycad seedlings and that of some subterranean
stems (Stangeria, Zamia) is composed of scalariform tracheids;
in older stems, the tracheids exhibit primitive, multiseriate,
bordered pits.
Another feature of those cycad stems in which terminal cones
are produced is the presence of "cone domes" in the pith (Figure
8A). In longitudinal sections, the pith appears partitioned
horizontally at intervals by vascular tissue. Each cone dome
represents the displacement of a cone axis to one side as a
result of the initiation and growth of the new vegetative apex.
The cycad stem grows from the tip (apically); the only lateral
buds and branches are those unusually placed (adventitious)
stems, whose buds arise by regeneration after the apical growth
tissue (meristem) has been destroyed or as a result
of wounding. Apical dominance and lack of branching bring about
an apparent single-stemmed (monopodial) growth form,
so that older plants become quite palmlike. This appearance,
however, is deceptive, because in more than half the genera
the apical meristem is converted from a vegetative to a reproductive
function in that it is transformed into a strobilus (cone).
A new vegetative meristem arises to one side of the cone meristem;
subsequent growth and enlargement further displace the cone
or cones to the side, so that the monopodial appearance is maintained
even though the type of growth is actually sympodial (Figure
8). Only members of three genera (Macrozamia, Lepidozamia,
Encephalartos) have cones initiated to the side and are
truly monopodial; the remaining eight are considered sympodial.
Cycads have such thick stems that rearrangements of internal
vascular connectives are not externally apparent. The cycad
trunk is about as thick at its crown as at its base, thus furthering
the resemblance to palms. Such stems, termed pachycaulous, result as
in palms from activity of a primary thickening meristem
(PTM) lateral to the apical meristem (Figure 8), which produces
much greater increments of cortical parenchyma than would result
if only an apical meristem were present. This is an important
difference between cycadophytes and coniferophytes, for in the
latter there is no PTM and the stem at its apical end is relatively
smaller than at its base.
A further characteristic of cycad stems not occurring in cycadeoids,
seed ferns, or coniferophytes is the presence of girdling leaf
traces. In cacad stems, the vascular strands follow a circuitous
route to the leaf bases, which is clearly seen in cross sections
of stems. Girdling leaf traces are an important means of distinguishing
between cycad and cycadeoid fossils (Figure 8).
Roots.
Cycad seedlings initially form a stout, fleshy taproot that persists
in subterranean forms for many years but is augmented by secondary
roots which also are quite thick and fleshy.
The taproots, larger secondary roots, and, in some cases, underground
stems, have contractile elements in the pith and cortex that
draw the stem more deeply into the ground.
Branch roots are of two kinds: long-branching geotropic roots
and short-branching apogeotropic roots, which are referred to
as coralloid because of their irregular, beady
appearance. The coralloid roots contain symbiotic cyanobacteria
(blue-green algae), which fix nitrogen and, in association with
root tissues, produce such beneficial amino acids as asparagine
and citrulline.
The taproot does not persist long in arborescent cycads but
is replaced by large adventitious roots, which obscure the basic
taproot system of the seedling. In all cycads, young roots are
diarch with a parenchymatous cortex and an outer cover of epidermal
scales. In this aspect they also resemble seed ferns. Older
roots become triarch or tetrarch, eventually developing substantial
amounts of wood and an outer covering of periderm.
Leaves.
The leaves of cycads are for the most part once-pinnately
compound; however, in the genus Bowenia, the leaves are
bipinnate and quite fernlike. Stangeria also
has fernlike leaves, and before cones were found to be associated
with them the plant was described as a fern in the genus Lomaria.
Stangeria leaves and those of the recently described Chigua
are unique in possessing pinnately veined leaflets with midribs
and side veins. Cycas pinnae also have midribs,
but these lack side veins altogether. Pinnae of all other cycads
have dichotomously branching, more or less parallel veins. The
size of the cycad leaf is variable; Zamia pygmaea,
the smallest cycad, has leaves about 20-30 centimetres long,
while some species of Macrozamia, Lepidozamia, Ceratozamia, and
Cycas have leaves three metres in length.
In cross section, the pinnae of most cycads are rather thick
and sclerophyllous. The stomata are sunken and are of the type
known as haplocheilic; that is, the guard cells arise directly
from the mother cell, as contrasted with the syndetocheilic
type, in which the guard cells are one division removed from
the mother cell. The haplocheilic type is found in living conifers,
pteridosperms, cycads, Ginkgo, and some others but not
in the Cycadeoidea.
Sporophylls and strobili.
Cycads are universally
dioecious. Male plants produce pollen by leaf homologues called
microsporophylls, and female plants produce
ovules by leaf homologues known as megasporophylls (Figure 7).
In all cycads, the microsporophylls are arranged spirally about
a cone axis; in all cycads but Cycas, megasporophylls are similarly
arranged. Megasporophylls of Cycas do not form a true
cone but are arranged in two to three whorls at the stem apex.
Later the stem resumes vegetative growth, and the megasporophylls
then are interposed between whorls of foliar leaves and cataphylls;
the usual arrangement is two to three whorls of leaves, then
several whorls of cataphylls, followed by megasporophylls, but
variations in this sequence are not unusual.
The megasporophyll of the Asian Cycas revoluta
is considered to most typify the ancestral seed-fern condition.
Each megasporophyll consists of a stalk, a fertile portion bearing
two to six ovules, and an expanded terminal blade having fringelike
"pinnae" (Figure 7). An evolutionary series of plant forms probably
led toward the biovulate, peltate megasporophylls of such forms
as Encephalartos, Ceratozamia, Microcycas, and Zamia
(Figure 7). Microsporophylls similarly vary among cycads; those
of Cycas are the more leaflike, those of Zamia
less so (Figure 7). Microsporangia, which are found on the abaxial
surface of microsporophylls, are usually numerous--several hundred
in Cycas, several dozen in Zamia--and arranged
in small clusters of two to five. They are the equivalent of
sori of ferns and of pteridosperms. The cycad microsporangium
resembles a clamshell, being somewhat flattened with an elongate
suture (Figure 7).
CLASSIFICATION
Many botanists believe that extant gymnosperms represent at
least two evolutionary lineages: one that leads to the extant
conifers, taxads, and possibly Ginkgo and the Gnetales;
and another that leads to the cycadophytes, represented today
by seed ferns, cycadeoids, cycads, and perhaps others. Cycadophytes
probably had their origins among the Devonian progymnosperms
(Progymnospermopsida), although the particular antecedents are
unknown.
DIVISION CYCADOPHYTA
Gymnospermous plants possessing compound leaves, ovules have
1 integument; seeds borne on either the foliage or megasporophylls;
soft, loose wood contains scalariform tracheids and tracheids
with multiseriate bordered pits; stem cross sections show wide
zones of pith and cortex.
DIVISION PTERIDOSPERMOPHYTA
(seed ferns)
Primitive, primarily Paleozoic, primarily small
trees or woody vines; large compound fronds; leaf-borne ovules;
and microsporangia more or less united as synangia; subdivided
into several groups; two orders, Caytoniales and Glossopteridales,
persisted into Cretaceous, the latter sometimes included with
pteridosperms, but commonly ranked separately and thought to
be closely related to certain primitive angiosperms.
DIVISION CYCADEOIDOPHYTA (BENNETTITOPHYTA)
Mesozoic; common and cycadlike; differ from
cycads in having direct leaf traces, in sometimes being monoecious,
and sometimes having bisexual cones.
DIVISION CYCADOPHYTA
Late Paleozoic? to the present; woody, coniferous
plants with compound leaves, simple cones; flagellate motile
male gametes; stout, fleshy stems; 4 families currently are
recognized.
Family Cycadaceae
Generally restricted to species of Cycas;
foliar, multiovulate megasporophylls arranged in an indeterminate
strobilus; pinnae with a single midrib but lacking lateral,
branch veins; 24 species defined.
Family Zamiaceae
Singly pinnate compound leaves, bearing leaflets
with parallel, dichotomously branching veins (Chigua,
if included, would be an exception); simple cones; female cones
with biovulate megasporophylls; a total of about 112 species
includes Macrozamia, Lepidozamia, Ceratozamia, Encephalartos,
Zamia, Microcycas, and Dioon.
Family Stangeriaceae
Fernlike leaves bearing pinnae with a prominent
midrib and numerous dichotomously branching lateral veins; simple
cones; female cones with biovulate megasporophylls; includes
only Stangeria paradoxa, a southern African
cycad.
Family Boweniaceae
Differ from other cycads in possessing bicompound
leaves; one genus, Bowenia, with 2
species.
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