Chapter 22-Gymnosperms

Gymnosperms

Key innovations in the evolution of land plants

were pollen and seeds.

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 is an immature

microgametophyte (male

gametophyte).

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.

The seed replaces spores as the mechanism of

dispersal.

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, the micropyle, that allows

the entrance of the pollen.

Within the megasporangium, meiosis produces a

megaspore which gives rise to the female gametophyte

and ultimately the egg cell is produced in the archegonium.

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 some fern relatives and also

present in progymnosperms

Life cycle of Selaginella

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 from

four (typical number of products of meiosis) to one.

Key steps in the evolution of seeds.

•Integument

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.

Wood enables gymnosperms and angiosperms to

grow much taller than other plants.

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 was a substantial tree, reaching

perhaps 75 m tall.

Archeopteris was a substantial tree, reaching

perhaps 75 m tall.

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).

Another intermediate

group were the “seed

ferns” (also extinct).

These plants were

originally thought to

be tree ferns, but

fossils were found

with seeds attached

to the fronds.

Botanists now recognize

five living lineages of

seed plants.

Four of these

lineages are what

we refer to as

gymnosperms.

The gymnosperms with

the most ancestral

features are the Cycads.

Cycads were a dominant part of the earth’s

vegetation about 200 million years ago.

Cycas revoluta

Cycads were a dominant part of the earth’s

vegetation about 200 million years ago.

Only 11 genera and about 125 species survive.

Cycads were a dominant part of the earth’s

vegetation about 200 million years ago.

Only 11 genera and about 125 species survive.

These species are almost all found in tropical

regions.

Cycas revoluta

Cycads are dioecious, meaning that they have two

types of sporophytes.

Cycads are dioecious, meaning that they have two

types of sporophytes.

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)

are large and often protected by woody plates or

prickles.

In cycads, the seed producing strobuli (or cones)

are large and often protected by woody plates or

prickles.

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.

Ginkgo, unlike cycads, is a large tree that

superficially resembles angiosperms more than

most gymnosperms.

The leaves are deciduous (fall off in winter) and

broad, unlike the leaves of most modern

gymnosperm trees.

Like cycads, Ginkgo is dioecious.

Pollen is produced in small strobuli and

transported to ovulate trees by the wind.

Like cycads, Ginkgo is dioecious.

Pollen is produced in small strobuli and

transported to ovulate trees by the wind.

The pollen germinates and, like cycads,

produces sperm that swim to the egg cells in

ovules.

Hirase Sakugorō

Motorized sperm of Ginkgo biloba

Conifers

The conifers

Many of these plants

possess a distinctive

woody seed cone.

The name conifer,

means “cone-bearer”

The conifers

Some conifers do not

have the woody cone.

The conifers

The most familiar of these

is the juniper, which produces

a fleshy “berry” that could

easily be confused with an

angiosperm fruit.

The conifers

The Pinaceae includes the Pines, Firs, and

Spruces.

These are among the most important forest

trees for wood production.

The Pinaceae includes the Pines, Firs, and

Spruces.

These are among the most important forest

trees for wood production.

Leaves are needle-like

and single or in groups

called fascicles, which

are borne on special

short shoots.

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.

http://sonic.net/bristlecone/

Pinus, the pines

Pines almost always have needles clustered in

groups of 2 to 5 per fascicle.

Pinus, the pines

The needles are oval to nearly triangular in

cross section.

Pinus, the pines

The woody cones bear the seeds.

The Firs, genus Abies.

Symmetrical trees that are generally smaller than

pines.

http://www.cnr.vt.edu/dendro/dendrology/syllabus/factsheet.cfm?ID=94

The Firs, genus Abies.

The cones are upright and woody.

The cones shatter to release

seeds.

The Firs, genus Abies.

Needles are borne singly, instead of in fascicles.

The Firs, genus Abies.

Each years growth produces a new whorl of

branches.

The Firs, genus Abies.

The age of tree can be

determined by the

number of whorls.

The Firs, genus Abies.

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.

The Spruces, genus Picea.

Seed cones are pendant and

light and somewhat papery.

The Spruces, genus Picea.

There are about 40

species of Spruce, also

restricted to the

Northern Hemisphere.

The hemlocks, genus Tsuga.

Pyramidal trees with slender, horizontal branches

and drooping tops.

http://www.ces.ncsu.edu/depts/hort/consumer/factsheets/trees-

new/tsuga_canadensis.html http://www.urbanext.uiuc.edu/treeselector/index_tree.cfm

?id=117

The hemlocks, genus Tsuga.

Pyramidal trees with slender, horizontal branches

and drooping tops.

Cones are small and pendant, like spruce.

http://www.ces.ncsu.edu/depts/hort/consumer/factsheets/trees-

new/tsuga_canadensis.html

The douglas firs, Pseudotsuga

Douglas firs are recognized by their pendant cones

that have distinctive 3-lobed bracts between the

scales.

Larches and tamaracks, genus Larix.

These are deciduous conifers and they stand out

in the autumn when their needles turn brilliant

yellow.

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 Cupressaceae includes the junipers,

cypresses and redwoods.

The members of this family are highly variable.

The cypresses (genus Cupressus) have woody cones,

whereas junipers (genus Juniperus) have fleshy cones.

juniper cypress

The Cupressaceae includes the junipers,

cypresses and redwoods.

Redwoods and relatives

are large trees.

The redwoods are the tallest

existing trees, with many

specimens over 300 feet.

Redwood

Taxaceae includes the yews.

Taxaceae includes the yews (genus Taxus)

A single seed is covered by a fleshy aril, which is

formed from the ovule.

Podocarpaceae and Araucariaceae are different

from other gymnosperms in that they have a

single ovule per cone scale.

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.

Podocarpaceae and Araucariaceae

The so-called Norfolk Island

pine and monkey-puzzle tree

are widely planted ornamentals.

Pine Life Cycle

Pines, like all seed

plants, are

heterosporous.

The vegetative plant

is a diploid

sporophyte that

produces a male

gametophyte in one

structure and a

female gametophyte

in another.

Pine Life Cycle

Pollen is produced in

a male strobilus, a

series of densely

packed sporophylls

that are borne on the

lower branches of

the trees.

Pine Life Cycle

Each strobilus has a large

number of small

microphylls attached

spirally.

Microsporocytes undergo

meiosis and produce

haploid microspores.

Pine Life Cycle

Each strobilus has a large

number of small

microphylls attached

spirally.

Microsporocytes undergo

meiosis and produce

haploid microspores.

The microspore nuclei

divide several times inside

the spore wall to produce

the pollen grain

Pine Life Cycle

The ovulate cones are

produced from the top

branches of the tree.

Pine Life Cycle

Two ovules develop on the

upper surface of each

scale.

The megaspore inside the

ovule develops very slowly

into the female

gametophyte, a process

that takes just over a year

in pines.

Pine Life Cycle

Immature and mature

female cones are often

seen on the same tree.

Pine Life Cycle

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.

Pine Life Cycle

The pollen germinates and

slowly generates a pollen

tube that grows toward the

egg.

The nuclei of the pollen

divide and produce 2

sperm.

Pine Life Cycle

One sperm nucleus

enters the egg cell and

eventually fuses with

the egg nucleus.

The fertilization

produces the diploid

zygote that eventually

becomes the embryo.

Other Gymnosperms –

the Gnetophytes

Gnetophytes have some characteristics similar

to flowering plants.

Gnetophytes are represented by three living

genera: Ephedra, Gnetum, and Welwitschia.

Ephedra is a shrub found in desert areas of the

western U.S.

Stem node with

scale-like leaves.

Seed inside aril.

Gnetum is tropical, with about 30 species of trees,

shrubs, and vines.

Welwitschia is a plant of the very dry Namib Desert

of southern Africa.

It produces long, leather leaves that trail along the

ground, with a short stem and a very long tap root.