Seed
Plants (Spermatophytes): Gymnosperms
I. Major Innovations of
Seed Plants (Spermatophytes)
A. Molecular
phylogenetic evidence indicates that extant Spermatophytes (and their
two major clades, the gymnosperms and angiosperms) are monophyletic (phylogenetic
tree of landplants). Gymno
naked, sperm
seed; this refers to the
presence of
ovules that are exposed at the time of fertilization as
opposed
to angiosperms which have ovules enclosed by carpels. Gymnosperms are seed plants
(seeds develop from a
ripened ovule) but do not form fruits - a characteristic only of
angiosperms. As with Lycopodium, Selaginella, and
Equisetum,
sporangia are produced in cone-like structures called strobili.
B. The lycophytes and ferns we
discussed last week have alternation
of generations life cycles. Seed plants (gymnosperms & angiosperms)
have a diplontic
life cycles with a multicellular sporophyte, microgametophyte, and
megagametophyte.
C. Instead of
having sperm that
swim free in external water (as in ferns), seed plants protect
their sperm within the male gametophyte. Another term for this
male gametophyte is pollen
(image
- pine pollen). The sperm cells (or sperm nuclei) are protected
from drying out by the pollen grain that move through the air
and deposits the sperm very near their final destination - the
egg cell.
D. Wood. Secondary xylem and phloem from a vascular
cambium allows larger plants to evolve.
II.
Early Evolution of Spermatophytes
A. The plants that
give us clues about the evolution
of seed plants are all extinct and thus known only from fossils.
These fall into the the
lignophyte clade of the Euphyllophytes (see Figs. 7.8
and 7.12).
The lignophytes have also been called progymnosperms
and
included such fossil taxa as Aneurophyton,
Archaeopteris,
Lyginopteris,
and Medullosa.
These plants were probably
ancestral to the modern gymnosperm groups (cycads, gnetophytes, Ginkgo, cycads and
conifers - below).
B. Archaeopteris.
This plant
had a large trunk and flattened lateral branch systems (Fig. 7.11A).
Although the leaves looked fern-like, it had tracheids in the secondary xylem
that were more like gymnosperms
(the trunk was first named as the organ genus Callixylon; it was
later found joined to the branches and leaves by Beck in
1960). The other progymnosperm Medullosa looks
like a large tree fern.
C. Seed ferns.
Recall that the ferns have
their gamete-producing organs (antheridia, archegonia) located
directly on an unprotected gametophyte. In contrast, some extinct
plants such as Physostoma,
Eurystoma, Genomosperma and Stamnostoma (see Figs. 7.11 D and E
and another reconstruction) developed structures that enclosed the
gametophyte that were the precursors to the ovule. These plants were
called seed ferns such as Emplectopteris.
Some variation in their reproductive structures is shown here. No extant living ferns produce seeds.
D. Probable steps in the evolution of the seed (Fig. 7.11C).
III. Taxonomic survey of extant
gymnosperms
A. Cycadales.
Cycads are the last
survivors of a previously more widespread and diverse group of Triassic
and Jurassic plants that
existed on the supercontinent
Gondwana. The plants are shrubs or palm-like trees with leaves in a tight cluster at the terminal part
of the stem (a rosette). Leaves are pinnately or bipinnately compound.
Male and female cones are produced on separate plants (dioecious).
1.
Cycadaceae.
1 genus Cycas
with 20 species. Cycas is
grown as for food (sago starch) but it can be toxic if large amounts
are ingested.
2. Zamiaceae.
9 genera, 111 species.
- Zamia. This
species, Z. pumila,
occurs in the Bahamas and is the only cycad that occurs in the U.S. Zamia integrifolia
with the megastrobilus. Megastrobilus
broken in half. Microstrobilus.
Zamia pumila
microstrobilus broken to show microsporangia
on underside of the microsporophylls.
- Dioon.
This D.
spinulosum that is cultivated in the PLB greenhouse
produced a large megastrobilus shown here. The strobilus is composed of many spirally
arranged scales,
each with megasporangia on their undersides.
- Encephalartos.
Robust plants with spiny margins on leaflets. On
occassion the E. ferox from the PLB greenhouse produces microstrobili.
This photo
shows the microstrobilus removed and some of the microsporophylls with
microsporangia on their undersides. This photo
shows the massive megastrobili forming on E. altensteinii.
B. Ginkgoales.
One genus and species, Ginkgo
biloba - the Maidenhair Tree. This tree,
discovered in China (cultivated for thousands of years), is considered
a living fossil. Leaves, wood, etc. of the living plants are identical
to fossil Ginkgo (Fig. 8.22). The leaves are
alternate or fascicled on short (spur) shoots. They are fan-like with
many dichotomously branched veins. The species is dioecious as shown on
these photos of the ovuliferous
branch and the microsporangiate
branch.
- The pollen is borne in
axillary spike-like clusters
of sporangiophores that look like stamens. In fact, the
entire structure resembles a catkin. Another photo of a younger microsporangiate strobilus.
- There are usually two
naked ovules on the tip of a forked peduncle.
- The seed
(not
the fruit!) is large and drupelike. When the seed
is mature
its fleshy outer layer (integuments) become foul-smelling because of
the presence of butyric acid (the smell of rotten butter
!).
- The inner
part of seed (nucellus and megagametophyte) is edible - used
in Chinese cooking.
The Ginkgo Pages
by Cor Kwant who says he created his site because of his "fascination
and respect for this unique tree, a living fossil, unchanged since the
time of the dinosaurs."
C. Gnetales.
3 families, each with only one genus. These three genera appear quite
different morphologically, so at first do not appear related.
But
molecular evidence clearly shows Gnetales are monophyletic.
- Gnetaceae.
Gnetum
with 35 pantropical species is the most "angiospermous" of the three
genera. It is a dioecious vine or small tree with opposite, simple,
pinnately-veined leaves.
- Habit of Gnetum gnemon
(cultivated at Missouri Bot. Gard.) showing young strobili.
- Megastrobilus
showing ovules with pollination droplets.
- Megastrobilus
with a developed seed at the terminus. Photo of Gnetum urens
- you would think this was an angiosperm with fruits, right?
Wrong!
- Ephedraceae.
Ephedra
(Mormon tea of the W. U.S.) is a genus of about 60 species of shrubs or
woody vines.
- Habit
of E. californica
growing in the desert.
- The stems
are longitudinally ridged and have whorled branches.The leaves are
scale-like, basally fused into a sheath, and deciduous soon after
developing (the stems are then photosynthetic).
- The microstrobili
and microsporangia have a very flower-like appearance For
this reason Gnetales have been referred to as "anthophytes".
- The megastrobilus
has a series of bracts subtending a single ovule. Ovulate
reproductive structures on E. distachya.
- Welwitschiaceae.
Welwitschia
mirabilis
is a large herb found in the Namib deserts of Africa. It produces only
two leathery, straplike leaves its entire life - these get torn and
battered by the wind into gnarled masses. The plants are dioecious,
with the pollen and ovules borne on stalked strobili.
D. Coniferales.
This is the largest of the modern gymnosperm clades. The name conifer comes from the Latin for
cone-bearer and refers to the reproductive structures that produce the
pollen and the seed. Many conifers have evergreen leaves that have
morphological and anatomical adaptations to dry conditions (e.g. thick
cuticle, sunken stomata). Economically, conifers such as pine, spruce,
fir, etc. produce most of the wood used for lumber and paper pulp. Included in the order are the following
families:
- Taxaceae.
5 genera, the most common is Taxus or
(yew). Seeds enclosed by bright red aril. Poisonous
because of taxol, which is also used as an anticancer drug.
- Podocarpaceae.
17 genera with Podocarpus the most
commonly encountered. Shrubs or large trees with simple, narrow or
broad leaves.
- Cupressaceae.
Approximately 30 genera.
- Taxodium
(bald cypress). Habit
of trees in winter condition showing butressed bases and pneumatophores
(knees). Young
microstrobili and old megastrobili with seeds. Tree
with fall foliage. The most spectacular of all members of the genus is
the tule tree, T. mucronatum
from Mexico.
- Sequoiadendron
(giant sequoia). Habit
of tree. Photo
of branch, bark and megastrobilus.
- Metasequoia glyptostroboides (dawn
redwood). Like Ginkgo,
a living fossil! First known from fossils, the living plant
was
discovered in China in the 1940s. Plants on our campus came
from
the original seedlings distributed through the Missouri Botanical
Garden. Mega-
and microstrobili. Note that the needles are
arranged opposite
on the stem, not alternate (like Taxodium).
- Juniperus
(juniper
or cedar). Shoots
showing dimorphic needles and young microsporangia. Shoots
showing megastrobili. Mature
microstrobilus of J.
rigida.
- Araucariaceae. 3
genera.
- Pinaceae. 10 genera.
- Abies,
such as A. concolor
(white fir), A. magnifica
(red fir), A. balsamea
(balsam fir).
- Larix (larch), e.g.
L. laricina.
- Picea,
such as P. engelmannii
(Englemann's spruce), P. abies
(Norway spruce), P. glauca
(white spruce), P. pungens
(blue spruce).
- Pinus.
The largest genus in the family (100 species). The
"soft pines" include the eastern U.S. white pine P. strobus
and several western species.
The "hard" pines differ from the soft in that their cones
have a
sharp projection from the megastrobilate cone scale called an umbo.
The photo
shows a variety of western U.S. pine cones. Some pines require fire for the
cone to open, such as this P. attenuata
(knobcone pine) - the cone is then termed serotinous.
Pinus longaeva
(bristlecone pine) is one of the oldest tree species on earth.
- Tsuga
(hemlock) such as the eastern hemlock T. canadensis
- Pseudotsuga, e.g. P. menziesii
(Douglas fir).
IV.
The Pine Life Cycle (diagram)
A. Begin with the mature pine tree.
Pine
needles are actually the leaves that occur in clusters (fascicles).
The plant produces both male and female cones (microstrobili
and megastrobili).
The ovule
is actually a specialized female sporangium (megasporangium) enclosed
by integuments. The ovule develops on an ovuliferous scale that
is subtended by a bract.
B. When the female
cone is very small and young, meiosis takes place within the
megasporangium. The female gametophyte (megagametophyte)
develops an (archegonium)
which is composed of just a few cells forming a wall and an egg
cell. In addition, the megagametophyte also consists of a nutritive
tissue called the nucellus. A pore-like opening between the integuments
is called the micropyle. By the time the female cone
is mature, pollination and fertilization
have already occurred and the seed is developing from an ovule.
C. The male cone produces pollen
sacs (microsporangia) and meiosis produces a tetrad of
microspores
which mature into (pollen grains).
One pollen grain forms from each microspore. The mature pollen
grain is the male gametophyte (microgametophyte) which is composed
of only three cells (prothallial, generative and tube cells).
The pollen grain has wings on each side that aid in dispersal
through the air.
D. Pollination occurs when wind carries
pollen to the micropylar opening of the ovule (diagram, photo). Some gymnosperms
have pollination droplet here that the pollen grains adhere to.
Upon evaporation, the pollen is drawn into the micropyle. Once
there, they form a pollen tube.
E. Fertilization occurs when the sperm
nucleus migrates down the pollen tube, into the archegonium, and
finally fuses with the egg nucleus. In some gymnosperms (e.g.
cycads) the sperm are complete, motile cells that actively swim.
In others, free nuclei are released into the archegonium from
the pollen tubes.
F. Several eggs may be fertilized at
once
and may begin developing as independent embryos at the apex of
long suspensor cells. Usually, only one embryo survives within
the mature ovule.
G. The mature ovule is the seed. It
consists
of an outer testa (seed coat - developed from the integument),
a thin nucellar layer (used up), a ripened megagametophyte, and
an embryo. The female gametophyte act as the nutritive tissue
for the seedling (sometimes referred to incorrectly as endosperm
which is found only in angiosperms). Embryos may have two to many
cotyledons.
- SIUC / College of Science / Plants and Society
- URL:
http://www.plantbiology.siu.edu/PLB304/Lecture08Gymnos/Gymnos.html
- Last updated: 06-Feb-09 / dln
