Lecture 8: Plant Evolution

I. Background

A. Plants and algae share common characteristics

1. Chloroplasts with photosynthetic pigments

2. Cell walls with cellulose

3. Food reserves stored as starch in plastids

B. Characteristics differentiating plants and algae

1. Structural, chemical and reproductive adaptations for terrestrial life

a. Structural specialization to extract resources needed for photosynthesis from the terrestrial

environment

i. Stomata for gas exchange

b. Chemical adaptations include a waxy cuticle to prevent desiccation

i. Secondary product not common to all plants

C. Plants have novel modes of reproduction: Embryophytes

1. Challenges of the terrestrial world

a. Gametes need to be dispersed in a non-aquatic environment

i. Plants have gametes within gametangia

ii. Protective structures comprised on sterile cells

iii. Prevent desiccation

iv. Egg is fertilized within female reproductive organ

b. Embryos must be protected from desiccation

i. Zygote develops to the embryo stage before exiting the gametangia

D. Reproduction involves life cycles with an alternation of generations

1. Haploid gametophyte generation produces and alternates with a diploid sporophyte

2. Sporophyte produces gametophytes

3. Heteromorphic

a. Sporophytes and gametophytes differ in morphology

i. Sporophyte is larger in all plants except mosses

4. In plant evolution, the trend is toward a reduction in haploid gametophyte generation

a. Diploid state is dominant

E. Four major periods of plant evolution

1. Origin of plants from aquatic ancestors

a. Ordovician period (475 mya)

b. Cuticles and jacketed gametangia

c. Vascular tissue

2. Diversification of seedless vascular plants

a. Beginning of the Devonian (400 mya)

3. Origin of the seed

a. End of the Devonian (360 mya)

b. Seed

i. Plant embryo packaged with a store of food and covered with a resistant coat

c. Early seed plants evolved into the gymnosperms, including conifers

4. Emergence of flowering plants

a. Early Cretaceous (130 mya)

b. Seeds within flower’s protective ovary

c. Most modern plants are angiosperms

II. Classification of Plants

A. Division (Phylum)

1. Major taxonomic category of plants

a. Eleven divisions

B. Nonvascular Plants (byrophytes)

1. D. Bryophyta (mosses)

2. D. Hepatophyta (liverworts)

3. D. Anthocerophyta (hornworts)

C. Seedless Vascular Plants

1. D. Lycophyta (lyophytes)

2. D. Equisitophyta (horsetails)

3. D. Polypodiophyta (ferns)

D. Seed Plants

1. Gymnosperms

a. D. Coniferophyta (conifers)

b. D. Cyadophyta (cycads)

c. D. Ginkgophyta (ginkgo)

d. D. Gnetophyta (gnetae)

2. Angiosperms

a. D. Magnoliophyta (flowering plants)

III. Origin of Plants

A. Evidence that green algae (charophytes) and plants evolved from a common ancestor

1. Homologous chloroplasts

a. Both have the same accessory pigments

b. Both have chloroplasts with thylakoid membranes

2. Biochemically similarity

a. Cellulose in cell walls

3. Similarity in mitosis and cytokinesis

4. Similarity in sperm ultrastructure

5. Genetic relationship

a. DNA and rRNA similarities

IV. Bryophytes

A. Three divisions (phylum)

1. D. Bryophyta (mosses)

2. D. Hepatophyta (liverworts)

3. D. Anthocerophyta (hornworts)

B. Evolutionary advantage

1. Embryophyte condition

a. Gametangia protect developing gametes

b. Antheridium

i. Male gametangium

ii. Produces flagellated sperm

c. Archegonium

i. Female gametangium

C. Characteristics of Bryophyta

1. Lack of vascular tissue generally limits them to moist environments and restricts them to small size

2. Moss gametophytes are differentiated into “leaves” and “stems”

3. Bryophyta are colonial plants

4. They have rhizoids that anchor the plant to the soil

5. Mosses lack true leaves, roots, and stems

a. Some have conducting cells, which function similarly to the vascular tissue of higher plants

6. Alternation of generation is apparent

a. The gametophyte dominates and bears the gametangia at the apex of the plant

b. Some mosses have separate sexes; others bear archegonia and antheridia on the same plant

c. Flagellated sperm are transported to the archegonium by splashing raindrops

i. Swim to the archegonium where they fertilize the egg

d. Sperm may also be transported by insects

e. Diploid zygote grows into a multicellular sporophyte by mitosis

f. Sporophyte grows and is nutritionally dependent on the gametophyte

g. Sporogenous cells divide meiotically to produce haploid spores

h. Spore germinates into a protonema

i. Protonema develops into the gametophyte

j. Haploid gametophyte is dominant

D. Characteristics of Hepaticophyta

1. Members of phylum Hepaticophyta have a dominant gametophyte generation like mosses

2. Their body form is a flattened, lobelike thallus

3. Rhizoids anchor the plant to the soil

4. Sexual reproduction involves archegonia and antheridia on gametophytes as is seen in mosses

5. Asexual reproduction involves production of gemmae that disperse via raindrops or small animals

a. Liverworts may also reproduce asexually by thallus branching

D. Characteristics of Antherocerophyta

1. Members of phylum Antherocerophyta resemble liverworts but may not be closely related to them

2. Hornworts have a single disc-shaped chloroplast in each cell, similar to many algae

3. Antheridia and archegonia are embedded in the thallus, and the sporophyte that develops after

fertilization forms a “horn”

4. Meiosis occurs within sporangia, and spores are formed

V. Seedless Vascular Plants (ferns and their allies)

A. Characteristics

1. Vascular tissue (xylem and phloem) for conduction and support

2. Most have true roots and leaves

3. Two types of leaves

a. Megaphylls and microphylls

4. Microphylls are small and have a single vascular strand

5. Megaphylls are larger and have more than one vascular strand

a. Ferns, horsetails, and seed plants have megaphylls

6. Ferns have a dominant sporophyte generation

D. Characteristics of Polypodiophyta

1. Alternation of generation includes a dominant sporophyte generation

2. Fern bodies consist of a rhizome (an underground stem), roots, and leaves called rhizomes

a. Newly emerging fronds are fiddleheads

3. All sporophyte structures contain vascular tissues

4. Spore production occurs on the undersides of the fronds in sporangia, which are typically found in

clusters called sori

5. Spores germinate and grow by mitosis into a gametophyte called the haploid prothallus

a. The prothallus is typically heart shaped and lacks vascular tissues

b. The prothallus is anchored by rhizoids

c. The prothallus bears antheridia and archegonia on the ventral surface

d. Water is still required for transportation of the flagellated sperm to the archegonium

e. The zygote grows into the free-living sporophyte plant

E. Characteristics of Equisitophyta

1. Horsetails have hollow, jointed stems

2. Horsetails have true roots, stems, and leaves

3. The stems of the horsetail have silica, which makes them abrasive

4. Strobili are the reproductive structures that bear the sporangia

F. Characteristics of Lycophyta

1. Club mosses are small plants with rhizomes and short, erect branches

2. Extant species such as Lycopodium are small plants with true roots and stems, and microphylls

3. Strobili are borne on the stems and produce the spores

VI. Spore Production

A. Types of spore production

1. Heterosporous

a. Two types

b. Heterosporous plants produce megaspores and microspores

2. Homosporous plants produce only one type of spore

B. Bryophytes, whisk ferns, horsetails, and most ferns and club mosses are homosporous

C. Some mosses and ferns are heterosporous

D. Seed plants are heterosporous

E. Spore production

1. Microsporangia produce microsporocytes (also known as microspore mother cells), which produce

microspores by meiosis

2. Microspores can develop into a male gametophyte that produces sperm in antheridia

3. Megasporangia produce megasporocytes (also known as megaspore mother cells), which produce

megaspores by meiosis

4. Microspores can develop into a female gametophyte that produces eggs in archegonia

VII. Seed Producing Vascular Plants

A. Background

1. Two major groups of seed bearing plants

a. Gymnosperms

b. Angiosperms

i. Flowering plants

2. Seeds

a. After fertilization the megasporangium and its enclosed structures develop a seed

i. The ovule is surrounded by layers of sporophyte tissue

ii. Layers are referred to as integument

b. Gymnosperms bear exposed seeds on cones in almost all species

c. Angiosperms produce seeds within a fruit

3. Both types of seed bearing plants are vascular

a. Have xylem and phloem

4. Both exhibit alternation of generations

a. Sporophyte is the dominant generation

b. Gametophyte is reduced and lives within the sporophyte plant

i. It is nutritionally dependant

5. Both are heterosporous producing microspores and megaspores

VIII. Gymnosperms—naked seed plants

A. Phylogeny

1. Four polyphyletic phyla

a. D. Coniferophyta (conifers)

b. D. Cyadophyta (cycads)

c. D. Ginkgophyta (ginkgo)

d. D. Gnetophyta (gnetae)

B. Pinophyta—Conifers

1. Woody plants that produce their seeds in cones

2. Produce 2 tissues

a. Wood

i. Tracheids have long, slender cells with pits

b. Bark

3. Produce resins and do not lose all their needles (leaves) at once

4. Most conifers are monoecious, bearing both male and female cones on the same tree

5. Conifer life cycle (e.g., Pine)

a. The pine tree is the sporophyte and produces microspores and megaspores in male and

female cones

b. Male cones have microsporophylls with microsporangia that contain microsporocytes

(microspore mother cells)

i. Microsporocytes undergo meiosis to produce haploid microspores

ii. Microspoere develop into the reduced male gametophyte—pollen grain

c. Pollen grains are typically carried by wind currents to the female cones

d. Female cones are the seed cones

i. Female cones have megasporangia (ovules) that contain megasporocytes (megaspore

mother cells)

e. Megasporocytes undergo meiosis to produce 4 haploid megaspores

i. One develops into the female gametophyte

ii. Remaining megaspores degenerate

f. The female gametophyte produces an egg in each of the archegonia

i. Gametophyte is reduced

ii. Microscopic structure found within the cone

g. The female cone opens at the time of pollen release

i. Pollen grains fall between the scales and stick to a drop of fluid produced by the cone

h. As the fluid evaporates, pollen is drawn closer to the megasporangium, and then the scales

close

i. Pollen grains grow to the egg within the archegonium via a pollen tube that grows into the

female gametophyte

i. Pollen tubes grow slowly so that fertilization occurs several years later

j. The generative cell divides and forms a stalk cell and a body cell

i. Body cell divides and forms 2 nonmotile sperm cells

k. Sperm cells are discharged by the egg

l. One sperm fuses with the egg to form a zygote

i. The germinated pollen grain with the new pine tube is the mature male gametophyte

ii. The other sperm degenerates

m. The tissue of the female gametophyte becomes nutritive tissue in the mature seed

n. A seed coat surrounds the embryo and nutritive tissue

i. Seed development takes several months and may remain within the cone for several

years before being shed

o. A papery wing develops to allow dispersal by wind

C. Cycadophyta

1. Seed cones and compound leaves

2. Only about 140 species of cycads remain today

3. Cycads are restricted to the tropics and subtropics

4. The leaves of cycads resemble the leaves of ferns or palms

5. Reproduction is similar to that of pines except that cycads are dioecious

a. Seed cones are on female plants and pollen cones are on male plants

b. Flagellated sperm but cycads do develop a pollen tube

i. Water not needed for fertilization

ii. They are pollinated by air or insects

c. Pollen grain germinates and grows a pollen tube

i. Sperm cells are released at the top of the tube and then swim to the egg

D. Ginkgophyta

1. Ginkgoes are extinct in the wild

a. All trees today come from cultivated specimens from China

2. Ginkgo has deciduous leaves

3. Reproduction

a. Dioecious

b. Produces flagellated sperms, but does develop a pollen tube

E. Gnetophytes

1. Include three unusual genera

a. Gnetum

b. Ephedra

c. Welwitschia

2. Gnetophytes have cells in their xylem that serve as the vessel elements

3. The cone clusters of some gnetophytes resemble flowers

3. Gnetum is a genus of tropical vines and trees with leaves and other characteristics similar to

flowering plants

4. Ephedra is a shrub found in dry regions that resembles horsetails

5. Welwitschia is a monospecific genus found in southwestern African deserts with only two leaves,

which shed over time

Flowering Vascular Plants that Produce Seeds: Angiosperms

I. Background

A. Flowers

1. Flowers are the organ of sexual reproduction

a. Seeds within a fruit

b. Require double fertilization

2. Angiosperms have vessel elements in their xylem and sieve-tube members in their phloem

B. Classes of flowering plants

1. Monocots

a. Typically herbaceous

b. Examples: Lilies, grasses, corn, palms, and orchids

c. Parallel leaf venation

d. Floral parts in 3s or multiples of 3

e. Fibrous root system

f. Scattered vascular bundles

g. One cotyledon in the seed

h. Persistent endosperm in the mature seed

2. Dicots

a. May be herbaceous or woody

b. Examples: Roses, oak trees, potatoes, and daisies

c. Netted leaf venation

d. Floral parts in multiples of 4 or 5

e. Taproot system

f. Vascular bundles arranged in a ring

g. Two cotyledons in the seed

h. Endosperm absent in the mature seed

II. Morphology of Flowering Plants

1. Flowers consist of sepals, petals, stamens, and carpels arranged in whorls around a stalk (peduncle)

a. Sepals and petals are sterile modified leaves

b. Stamens and carpels are fertile modified leaves

c. A flower with all four parts is complete

d. A flower missing any of the four parts is incomplete

e. A flower with both stamens and carpels is perfect

f. A flower having stamens or carpels (but not both) is imperfect

2. Sepals

a. Sepals function in protecting the other flower parts while it is a bud

b. The sepals are collectively referred to as the calyx

3. Petals

a. The petals are typically brightly colored to attract animal pollinators

b. The petals are collectively referred to as the corolla

4. Stamen

a. The stamens each have a filament (stalk) and an anther

b. In the anther microspores develop into pollen grains

i. Each pollen grain has two sperm

5. Carpel

a. Female reproductive organs located in the center of the flower

b. Produce ovules which ultimately may develop into seeds

c. The female portion of the flower is sometimes referred to as a pistil

i. The pistil may consist of a single carpel (simple pistil) or several fused carpels

(compound pistil)

d. Each pistil has a stigma at the distal end

i. Pollen grain lands

e. Each pistil has a style (neck) and an ovary

i. Contains the ovule

f. Each ovule contains a female gametophyte that produces one egg and two polar nuclei

g. After fertilization the ovule develops into a seed

III. Life Cycle of Flowering Plants

A. Double fertilization

1. Flowering plants have a dominant sporophyte generation

a. Gametophyte is reduced to just a few cells

2. Ovules contain a megasporocyte

a. Megasporocyte undergoes meiosis to produce 4 haploid megaspores

3. Only one megaspore develops

a. Megaspore divides mitotically

b. Develops into the female gametophyte (embryo sac)

4. The female gametophyte consists of only 8 cells

a. Three of these cells participate in fertilization

i. One egg

ii. Two haploid polar bodies

5. The anther contains microsporocytes

a. Microsporocytes undergo meiosis to produce 4 haploid microspores

6. Each of the microspores develops into the male gametophyte

a. The male gametophyte is the pollen grain

7. Pollination involves transfer of the pollen grain to the style of the flower

a. Pollen grain lands on a style of the same species

b. Pollen grain grows a pollen tube down through the stigma to the ovaries

8. Both sperm enter the embryo sac

9. Fertilization involves both sperm

a. Referred to as double fertilization

i. One sperm fertilizes the egg resulting in a diploid zygote

ii. The other sperm fuses with the two haploid polar nuclei forming a triploid cell

10. The triploid cell develops into endosperm

a. Endosperm is a nutritive material for the embryo

b. Endosperm is persistent in monocot seeds

c. Endosperm is absorbed into the cotyledons in dicot seeds

IV. Development of Seeds and Fruit

A. Seeds and fruits develop after fertilization

1. Fruits protect the seeds from desiccation

2. Fruits may aid in dispersal of seeds by wind, water, or animals