|This week's laboratory is on the cycads, a gymnospermous group for
which we have living representatives, the cycadeoids, an extinct Mesozoic
group, and the ginkgophytes, a group characterized by the extant monotypic
genus Ginkgo and numerous fossil relatives.
|Today the cycads represent a relatively small group of about half a
dozen genera (including Cycas, Bowenia, Dioon, Zamia,
Ceratozamia, Encephalartos) that are found worldwide in disjunct
tropical and semitropical areas. This group of plants was much more widespread
during the Mesozoic (Triassic, Jurassic, Cretaceous), which is sometimes
called the "Age of Cycads". The cycads have frequently been compared to
the Cycadeoidales (Bennettitales) which also occurred during the Mesozoic,
and collectively these two groups are sometimes called cycadophytes.Although
they are superficially similar to cycads in growth habit and leaf morphology
they have entirely different reproductive structures. The third group,
the ginkgophytes, today represented by Ginkgo, were also an important
component of the Mesozoic flora.
A. The cycads
|Cycads are typically trees with short trunks and large pinnately compound
frond-like leaves, although some forms are have more slender stems and
occasionally can be rhizomatous and scrambling. They have endarch steles
that produce leaf traces that "girdle" or surround the stele as they diverge
to vascularize the leaf. Girdling leaf traces are characteristic of cycads
and not known in other gymnospermous groups. Cycads typically produce a
small amount of manoxylic wood, with broad parenchymatous rays and often
contain abundant resinous cells and resin canals. To the periphery of the
stem, cycads produce a "leaf armour" consisting of the tightly packed,
helically arranged leaf bases. Look at the live plants, herbarium sheets
and other materials of cycads in the lab.
|Cycads are dioecious and, when fertile, produce a terminally positioned
cone that bears either pollen sacs or ovules. Sometimes the vegetative
apex of a cycad can bifurcate and continue to grow, pushing the cone to
the side. This is called sympodial branching, and is relatively rare among
|In one species, Cycas revoluta, the megasporangiate structures
are not cones, but helically arranged megasporophylls which bear several
seeds pinnately on the modified leaf. Other cycads are characterized by
large cones composed of modified leaves (sporophylls) with peltate heads
that bear two ovules each. In some genera, such as Dioon, the peltate
head may have a small spiny projection on the distal side of the head.
It has been suggested that these variations among the living cycads may
represent an evolutionary series. Look at seed cones in the lab.
|Pollen cones of cycads consist of helically arranged, tightly packed
peltate sporophylls that bear a number of pollen sacs on their surfaces.
Pollen is ovate and monocolpate (with a single elongate germination furrow)
and lacks the air bladders typically seen in pinaceous conifers. The pollen
contains unusual, large top-shaped sperm with helically arranged rows of
flagella. When pollination occurs a branched pollen tube is ruptured by
the swimming sperm on their path to fertilize the egg. This haustorial
pollen tube is quite unlike the straight, rarely branched siphonogamous
pollen tube we encountered in the conifers. You may recall that the conifers
had pollen grains that contained non-motile sperm that were delivered directly
to the pollen chamber of the ovule by their pollen tubes. Cycad pollination
biology is thought to be more primitive. When we get to Ginkgo,
you will see that it too, has motile sperm. Look at the pollen cones
we have in the lab.
B. The fossil record of the cycads:
|Although there has been some suggestion that cycads originated in the
Carboniferous (possibly from the medullosan seed ferns) the first strong
evidence for cycads occurs in the Permian. Mamay (1976) has suggested that
the earliest seed-bearing cycadaceous foliage was Spermopteris,
an entire-margined, strap-shaped leaf with parallel veins very similar
to the vegetative leaf Taeniopteris. Mamay documented the occurrence
of other cycadaceous seed-bearing foliage types in the Permian, including
the genera Phasmatocycas and Archaeocycas, which also have
an entire, or slightly crenulate leaf margin. Since later Mesozoic cycad
vegetative foliage types (Nilssonia, Ctenis, Pseudoctenis)
are pinnately compound, as are modern forms, Mamay suggested an evolutionary
trend toward progressive dissection of cycad fronds.
|See p. 343 of your text for illustrations of Permian cycadophyte
|Recently however, new cycad remains have been discovered in China that
demonstrate Permian aged cones with megasporophylls strikingly similar
to those of modern Cycas media. You may recall that this
is the one genus of cycads that has helically arranged megasporophylls
rather than a compact seed cone (see p. 341, fig. A in your text).
Look at the photos of Crossozamia in your text on p. 344.
These fossils demonstrate that cycad cones bearing megasporophylls much
like those of modern Cycas media (but borne in a cone) were
present at the same time (Permian) as Mamay's various entire-margined cycad
sporophylls. Gao Zhifeng & Thomas suggest that instead of a simple
reduction series toward dissected foliage and compaction into a cone, two
separate trends occurred in cycad megasporophyll evolution, one toward
the cone compaction and the other that toward less compact inflorescence
of Cycas media. If this interpretation is correct, then modern
Cycas media structure can no longer be considered the primitive
|A number of cycad stems are known from Triassic sediments from several
localities. Leptocycas, a plant from the Upper Triassic of North
Carolina, is known from compressions that show stems and leaf attachment
and provide information about the growth habit of the plant. This plant
was apparently a small, slender tree with a more elongate stem than is
typical in modern cycads. See the reconstruction of Leptocycas
in your text on p. 345.
|Several of the Triassic and younger cycads are known from anatomically
preserved material. These include an anatomically preserved cycad stem
from the Petrified Forest and Antarctica, Antarcticycas. Both of
these fossil stems have typical cycad stem anatomy, including a small amount
of manoxylic wood, a broad pith, and resin canals.
|Cycadaceous foliage such as Nilssonia is a common component
of Mesozoic floras in both the northern and southern hemisphere. See
the figure on p. 347 of your text for a general idea about the structure
of this foliage.
|Along with the cycads a second fossil group, the cycadeoids, produces
what has been called cycadophyte foliage. The cycadeoids (Bennettitales)
are a group of extinct seed plants that bore pinnately compound leaves
that look superficially much like cycad leaves. However, cuticular studies
have demonstrated that these two groups can be distinguished from one another
on the basis of epidermal and stomatal differences.
|Starting with Thomas and Bancroft (1913) in the early part of the century,
researchers began to recognize that some major groups of gymnosperms could
be delimited by structural details of their epidermis and cuticle. Cycads
have epidermal cells with straight margins, thin cuticle, and irregularly
oriented stomata. Generally, cycads, conifers, seed ferns, Ephedra,
Ginkgo, and the angiosperms all have haplochelic stomata, where
the guard cells of the stoma develop from one epidermal initial and the
associated subsidiary cells from another initial. In contrast, cycadeoids,
Gnetum and Welwitschia have syndetochelic stomata (where
guard cells and adjacent subsidiaries come from the same initial). Cycadeoids
are also characterized by epidermal cells patterns with wavy margins, thick
cuticles and cells aligned in rows, and stomata occurring at right angles
to the veins.
|Compare diagrams of a haplochelic stoma of cycads (p. 340 Fig. C)
and syndetochelic stoma of cycadeoids (p. 352 fig. A).
|Cycadeoid foliage is a common component of the Triassic Chinle Formation,
the Triassic rock formation of the Petrified Forest National Park, Arizona.
Look at two representatives of this foliage Otozamites and Podozamites,
in the lab. You can see how this foliage would be easily assigned to
cycad-like plants, based on its similarity to that of living cycads. Cycadophyte
leaves (of both cycads and cycadeoids) are widespread from the Triassic
to the Upper Cretaceous in Western Europe, Greenland, North America, and
a number of southern hemisphere localities.
|The cycadeoids are an extinct group of gymnosperms that are mostly
known from Jurassic to Cretaceous strata in England, Europe, Greenland,
India, the Black Hills of South Dakota, and Mexico. There are two families
within this group, the older family, the Williamsoniaceae from the Upper
Triassic to Jurassic, and the the younger family, the Cycadeoidaceae from
the late Jurassic to the Cretaceous. The Williamsoniaceae are generally
plants with longer, more slender branching stems, while the Cycadeoidaceae
are shorter, unbranched trunks.
|Cycadeoid trunks such as that of Cycadeoidea from the Black
Hills of South Dakota are sometimes called "fossil bee hives". They are
large columnar trunks (up to 2 meters) that bear numerous tightly-appressed
leaves and intermixed, helically arranged cones on the trunks (p. 357 of
the textbook). Look at the specimens of Cycadeoidia trunks we
have in the laboratory. Many of these trunks are permineralized and
provide information about the internal anatomy of the plant. Cycadeoids
bear bisporangiate (or less often, monosporangiate) cones that consist
of a central axis (receptacle) which bears hundreds of tiny stalked ovules
that alternate with scales. Microsporophylls are fused structures that
bear synangiate pollen organs with monocolpate pollen somewhat similar
to that of cycads and Ginkgo. The typically bisporangiate nature
and general organization of the cycadeoid cones has at various times suggested
a potential relationship to the angiosperms. Recent cladistic analyses
of the seed plants (e.g., Crane, Doyle and Donoghue) have revived an interest
in this group in relation to the angiosperm flower. Members of the Williamsoniaceae
had cones that opened, that are sometimes called "flowers" because of their
organization. Cones of the Cycadeoidaceae, with their large, fused microsporophylls,
probably did not open to the external environment. Researchers have suggested
that their primary means of pollination was selfing, although evidence
of insect burrowings suggests they may have also been insect pollinated.
See diagrams and photos in your text on p. 359-360.
|The modern plant Ginkgo biloba is characterized as a
small tree that bears fan-shaped leaves on long and short (spur) shoots.
Ginkgo leaves are quite variable in their morphology and may be
anywhere from entire-margined to highly dissected. They have an open dichotomizing
venation pattern than only occasionally sows reticulations or anastomoses.
In this regard they are markedly different from the leaves of Glossopteris,
a Mesozoic seed fern, in which the interconnections or reticulations form
a major part of the construction of the leaf's venation. In any case, both
Ginkgo and glossopterid venation structure is built on a plan where
the veins are all of a single size order, rather than being in a hierarchy,
such as occurs in dicotyledonous leaves.
|The fertile structures of Ginkgo occur on separate trees, with
lax pollen cones in which microsporophylls bear pollen sacs with simple
monosulcate pollen occurring on one tree (dioecy) and obnoxious ovules
occurring on a separate tree. The ovules are borne, usually in pairs, on
the tips of branch-like structures that have been homologized with megasporophylls.
Ovules are subtended by a "collar" and are obnoxious because they have
a modified integument with a fleshy sarcotesta (the outermost seed coat)
that breaks down to produce a nasty smell as they ripen.
|Look at herbarium sheets and other materials of Ginkgo in
|The fossil record of ginkgophytes extends from at least the Triassic
and perhaps the lower Permian. The earliest ginkgophyte is considered to
be Tricholpitys, a ovulate branching structure which bears ovules
with small collars like those of Ginkgo, and has highly dissected
leaves. There is a general trend in the ginkgophyte leaves toward younger,
broader and more entire leaves, which is consistent with the fossil record.
One of the most common, older foliage types that is highly dissected is
called Sphenobaiera. See p. 389 in your text.
| Foliage of Ginkgo-like plants is common and widespread
throughout the Mesozoic. Unfortunately, however, although there is some
fertile material known in the fossil record, it is not clear whether leaves
of this general type are always indicative of plants with the fertile structures