Chapter 22-Gymnosperms

Key innovations in the evolution of land plants were pollen and seeds.

In ferns, lycophytes and bryophytes the flagellate sperm

have to swim to the egg cell.

Some seed plants also produce sperm.

Pollen eliminated the need for water that the sperm could swim in to

reach the archegonium and fertilize the egg cell.

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.

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.

Pollen is distinguishable from spores

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.

A seed develops from the megasporangium.

Before the megasporangium matures into a seed it is called an ovule.

The integument has a small hole,

After fertilization, the embryo develops, the integument

hardens into a seed coat and the micropyle closes.

The ovule is composed of tissue of the megasporangium inside a protective coating called the integument.

Within the megasporangium, meiosis produces a megaspore which gives rise to the female gametophyte and

ultimately the egg cell is produced in the archegonium.

Key steps in the evolution of seeds.

• Heterospory.

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.

Already found in Selaginella, some fern relatives and also present in progymnosperms.

It is postulated that the larger spores of heterosporous

plants were the precursor ovules, and the smaller spores,

the precursor of pollen.

•The origin of the integument is not known, but it may have

evolved from lobes of leaf-like tissue that surrounded the

megasporangium

Key steps in the evolution of seeds.

• Retention of the megaspore in the megasporangium

Another innovation in gymnosperms is wood.

Reduction in the number of functional megaspores

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.

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.

Archeopteris could achieve this

height because of the presence

of true wood, with lignified

secondary growth very much like

modern gymnosperms.

The gymnosperms with the most ancestral

features are the Cycads.

Only 11 genera and about 125 species survive.

One sporophyte produces the females structures (ultimately

ovules and egg cells) and the other sporophyte produces the

male structures (ultimately producing pollen).

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.

Like cycads, Ginkgo is dioecious.

The pollen germinates and, like cycads, produces sperm that swim

to the egg cells in ovules.

The leaves are deciduous (fall off in winter) and broad,

unlike the leaves of most modern gymnosperm trees.

These species are almost all found in tropical regions.

Cycads

In cycads, the seed producing strobuli (or cones)

Hirase Sakugorō

Motorized sperm of Ginkgo biloba

Some conifers do not have the woody cone.

Conifers

Many of these plants possess a distinctive woody seed cone.

The name conifer, means “cone-bearer”

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.

The needles are oval to nearly triangular in cross section

The woody cones bear the seeds.

Pines almost always have needles clustered in groups of

2 to 5 per fascicle.

The Firs, genus Abies.

Symmetrical trees that are generally smaller than pines.

Firs are restricted to cool regions of the Northern Hemisphere,

where there are about 40 species.

Each years growth produces a new whorl of branches.

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.

Taxaceae includes the yews.

Podocarpaceae and Araucariaceae are different from

other gymnosperms in that they have a single ovule per

cone scale.

Pine Life Cycle

Pines, like all seed plants, are heterosporous.

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.

These are mainly Southern Hemisphere conifers.

Some are major lumber trees.

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.

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.

The megaspore inside the ovule

develops very slowly into the female

gametophyte, a process that takes

just over a year in pines

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

It produces long, leather leaves that trail along the ground, with

a short stem and a very long tap root.

Welwitschia

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

What are the 3 lineages included in the gnetophytes?

List characteristics of Gnetum.

List characteristics of Welwitschia.

List characteristics of Ephedra.

Matching-Gymnosperms ___ Pinaceae ___ Cupressaceae ___ Conifers ___ gnetophytes ___ gymnosperms ___ Taxaceae ___ dioecious ___ cycads ___ pollen ___ ginkgophytes

A. Gymnosperm lineage with single living species, Ginkgo biloba

B. Family that includes junipers, cypresses, and redwoods

C. Key reproductive innovation for life on land

D. Family that includes the yews

E. “Naked seed” plants

F. Family that includes pines, firs, and spruces

G. Mostly widely known and economically important gymnosperms

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