Download Chapter 22-Gymnosperms Key innovations in the evolution of land

Chapter 22-Gymnosperms
Key innovations in the evolution of land plants were pollen and seeds.
Pollen eliminated the need for water that the sperm could swim in to
reach the archegonium and fertilize the egg cell.
In ferns, lycophytes and bryophytes the flagellate sperm
have to swim to the egg cell.
Some seed plants also produce sperm.
A pollen grain actually consists of several nuclei
and sperm cells.
In association with the development of the ovule,
paleobotanical evidence indicates the evolution of
pollen from approximately 365 Ma.
“Prepollen” which contain morphological
features of spores, such as a trilete scar.
Pollen is distinguishable from spores
Gymnosperms
Key innovations in the evolution of land plants were pollen and seeds.
Packaging the embryo into a seed enables plants to surround the embryo with nutritive tissue, providing for the early
survival of the embryo when it is no longer attached to a gametophyte.
A seed develops from the megasporangium.
Before the megasporangium matures into a seed it is called an ovule.
The ovule is composed of tissue of the megasporangium inside a protective coating called the integument.
The integument has a small hole,
Within the megasporangium, meiosis produces a megaspore which gives rise to the female gametophyte and
ultimately the egg cell is produced in the archegonium.
Fertilization occurs with the growth of the male
gametophyte, derived from the pollen, which produces
either sperm or special nuclei that fuse with the egg.
After fertilization, the embryo develops, the integument
hardens into a seed coat and the micropyle closes.
Key steps in the evolution of seeds.
• Heterospory.
Already found in Selaginella, some fern relatives and also present in progymnosperms.
The transition from plants that were homosporous (one spore size) to heterosporous (two spore sizes) is one
of the most important evolutionary trends in the development of seed-bearing plants.
It is postulated that the larger spores of heterosporous
plants were the precursor ovules, and the smaller spores,
the precursor of pollen.
Key steps in the evolution of seeds.
• Retention of the megaspore in the megasporangium
Reduction in the number of functional megaspores
•The origin of the integument is not known, but it may have
evolved from lobes of leaf-like tissue that surrounded the
megasporangium
Another innovation in gymnosperms is wood.
The presence of wood and seeds defines a lineage that
includes the gymnosperms and angiosperms (flowering
plants)
The extinct plant Archeopteris, which lived about
300 million years ago, represents a group that were
intermediate between the ferns and the
gymnosperms.
Archeopteris could achieve this
height because of the presence
of true wood, with lignified
secondary growth very much like
modern gymnosperms.
Another intermediate group were the “seed
ferns” (also extinct).
Botanists now recognize five living lineages of
seed plants.
Four of these lineages are what we refer to as
gymnosperms.
Cycads
The gymnosperms with the most ancestral
features are the Cycads.
Only 11 genera and about 125 species survive.
These species are almost all found in tropical regions.
One sporophyte produces the females structures (ultimately
ovules and egg cells) and the other sporophyte produces the
male structures (ultimately producing pollen).
In cycads, the seed producing strobuli (or cones)
The pollen strobuli are also large. The pollen is transported to the ovules
by beetles or by wind.
Ginkgo
A single species, Ginkgo biloba, remains of this once
thriving group of plants.
The leaves are deciduous (fall off in winter) and broad,
unlike the leaves of most modern gymnosperm trees.
Like cycads, Ginkgo is dioecious.
The pollen germinates and, like cycads, produces sperm that swim
to the egg cells in ovules.
Hirase Sakugorō
Motorized sperm of Ginkgo biloba
Conifers
Many of these plants possess a distinctive woody seed cone.
The name conifer, means “cone-bearer”
Some conifers do not have the woody cone.
The most familiar of these is the juniper, which
produces a fleshy “berry” that could easily be
confused with an angiosperm fruit.
The Pinaceae includes the Pines, Firs, and Spruces.
These are among the most important forest trees for
wood production.
Pinus, the pines
93 species; most are large, long-lived trees.
The bristlecone pines includes the oldest living organisms
(Pinus longaeva), nearly 5,000 years old.
Pines almost always have needles clustered in groups of
2 to 5 per fascicle.
The needles are oval to nearly triangular in cross section
The woody cones bear the seeds.
The Firs, genus Abies.
Symmetrical trees that are generally smaller than pines.
Each years growth produces a new whorl of branches.
Firs are restricted to cool regions of the Northern Hemisphere,
where there are about 40 species.
The Spruces, genus Picea.
Spruces resemble firs, but the needles are angular in
cross section, rather than flat as in firs.
There are about 40 species of Spruce, also restricted to
theNorthern Hemisphere.
The hemlocks, genus Tsuga.
Pyramidal trees with slender, horizontal branches and
drooping tops.
Cones are small and pendant, like spruce.
The douglas firs, Pseudotsuga
Larches and tamaracks, genus Larix.
The cedars, the genus Cedrus.
Native to North Africa and Asia and used as ornamentals
in North America.
The Cupressaceae includes the junipers,
cypresses and redwoods.
The members of this family are highly variable.
Redwoods and relatives are large trees.
The redwoods are the tallest existing trees, with many
specimens over 300 feet.
Taxaceae includes the yews.
Podocarpaceae and Araucariaceae are different from
other gymnosperms in that they have a single ovule per
cone scale.
These are mainly Southern Hemisphere conifers.
Some are major lumber trees.
Pine Life Cycle
Pines, like all seed plants, are heterosporous.
Pollen is produced in a male strobilus, a series of densely
packed sporophylls that are borne on the lower branches
of the trees.
Each strobilus has a large number
of small microphylls attached
spirally.
The microspore nuclei
divide several times inside
the spore wall to produce
the pollen grain
The ovulate cones are produced from the top branches of the tree.
The megaspore inside the ovule
develops very slowly into the female
gametophyte, a process that takes
just over a year in pines
A sticky pollination drop exudes from the
micropyle.
This drop traps pollen grains, which exude
a chemical signal that causes the rapid
absorption of the liquid by the ovule.
This pulls the pollen into the micropylar
chamber.
Immature and mature female
cones are often seen on the
same tree.
The pollen germinates and slowly generates
a pollen tube that grows toward the egg.
The nuclei of the pollen divide, but without
formation of cell walls.
Finally, 2 sperm nuclei are produced.
One sperm nucleus enters the egg
cell and eventually fuses with the
egg nucleus.
Other Gymnosperms –
the Gnetophytes
Gnetophytes have some characteristics similar to
flowering plants.
Ephedra
Gnetum
Welwitschia
It produces long, leather leaves that trail along the ground, with
a short stem and a very long tap root.
Study outline for Chapter 22-Gymnosperms
Why are pollen and seeds important evolutionary innovations in the evolution of land plants?
Paleobotanical evidence indicates the evolution of pollen from approximately _________ million years ago.
Define megasporangium.
Define integument.
Define micropyle.
Define megaspore.
Define microsporangium.
Define microspore.
Define embryo.
After fertilization, what happens to the integument?
What were some key steps in the evolution of seed plants?
Why was wood a major innovation in gymnosperms and angiosperms?
List general characteristics of Acheopteris.
List general characteristics of seed ferns.
Name the five lineages of seed plants.
What are the four lineages of gymnosperms?
Define monoecious.
Define dioecious.
Gymnosperms- Label the following: cycads; Ginkgo biloba; conifers; gnetophytes. List 3 characteristics for each
picture.
Study outline for Chapter 22-Gymnosperms
Define conifer.
Name the 5 families of conifers.
Which conifer families are found mostly in the Northern Hemisphere?
Which conifer families are found predominantly in the Southern Hemisphere?
Define fascicle.
Distinguish between the following genera in the family Pinaceae: Pinus (pines); Albies (firs); Picea (spruce); Tsuga
(hemlocks); Pseudotsuga (douglas firs); Larix (larch and tamarack); and Cedrus (cedars).
Distinguish between the following genera in the family Cupressaceae: Cupressus (cypress); Taxodium (bald cypress);
and Juniperus (junipers).
What are the general characteristics of yews (Taxus) in the family Taxaceae?
Label the following terms on the figure provided:
female cone (ovule-bearing cone)
male cone (pollen-bearing cone)
female gametophyte
male gametophyte
Egg
pollen grain
pollen tube
zygote
embryo
Integument
sporophyte
micropyle
seed coat
Study outline for Chapter 22-Gymnosperms
Matching-Gymnosperms
___ Pinaceae
A.
Gymnosperm lineage with single living species, Ginkgo biloba
___ Cupressaceae
B.
Family that includes junipers, cypresses, and redwoods
___ Conifers
C.
Key reproductive innovation for life on land
___ gnetophytes
D.
Family that includes the yews
___ gymnosperms
E.
“Naked seed” plants
___ Taxaceae
F.
Family that includes pines, firs, and spruces
___ dioecious
G.
Mostly widely known and economically important gymnosperms
___ cycads
H.
Group that includes 3 living genera – Ephedra, Gnetum, and
Welwitchia
I.
Palm-like gymnosperms that were prominent in the Mesozoic era
J.
Having separate male and female plants
___ pollen
___ ginkgophytes
What are the 3 lineages included in the gnetophytes?
List characteristics of Gnetum.
List characteristics of Welwitschia.
List characteristics of Ephedra.