key point: history of land plants is the increasing...

Plants

•  Key point: History of land plants is the increasing adaptation to terrestriality.

•  Colonized land 475 mya (Ordovician), began growing taller 370 mya (Carboniferous).

•  Plants form the basis for every terrestrial ecosystem.

Challenges of terrestriality

The motile sperm of Charophyta, the bryophytes, and Pterophytes need a water medium to find ova.

Advantages of terrestriality

Important adaptations to life

on land •  Waxy cuticle •  Gametangia and protected

embryo •  Roots (below) stems (above) •  Leaves, stomata: photosynthesis

Origins: Charophyte Green Algae EVIDENCE

•  Homologous chloroplasts

•  Biochemical similarity: cellulose in cell wall

•  Similarity in mitosis, cytokinesis –  Phragmoplast:

charophytes & Plants •  Similarity in sperm

ultrastructure •  Genetic relationship:

some nuclear genes, ribosomal RNA

Origins: Charophyte Green Algae ESSENTIAL DIFFERENCES

Green Algae Plants

Medium: water (whole alga has access)

Medium: air, non-supportive (roots, stiff

stem, cuticle)

Photosynthesis in most cells (light limited)

Photosynthesis in aerial parts (leaves, stomata)

Reproduction mostly asexual

Reproduction sexual: gametangia, embryo

Major events in plant evolution

A.  Changes in life history B.  Vascular tissue C.  Seeds D.  Flowering plants

A. Changes in life history

“The changes that affect an organism’s schedule of reproduction and survival.”

1.  Alternation of generations –  Evolved independently in fungi, cellular

slime molds, brown algae, red algae, and green algae.


Generalized Alternation of Generation in Plants


2.  Generations are heteromorphic.


Gam

etophyte S

porophyte



3.  Shift of dominance from gametophyte (n) to sporophyte (2n).


Gam

etophyte S

porophyte



3.  Shift of dominance from gametophyte (n) to sporophyte (2n).

4.  Replacement of flagellated sperm by pollen.


B. Vascular tissue •  Cells joined into

conducting tubes. •  Xylem: dead, lignified

tubes conduct water, mineral from roots upward.

•  Phloem: living tubes conduct sugar, amino acids from photosynthetic parts downward.

C. Seeds

•  An embryo packed with food.

•  First seeds unencapsulated Gymnosperms.

D. Flowering Plants: Angiosperms

•  Complex structure containing seeds within protective ovary.

•  Most diverse (species) plant group.

Embryo: zygotes are retained within tissues of the female parent plant. All descendants known as “Embryophyta”

Vascular tissue: Cells joined into tubes to transport water and soil nutrients upward (xylem) and carbohydrates downward (phloem). All descendants known as “Tracheophytes”

Seeds: Embryo packed with a supply of nutrients inside a protective coat. All descendants known as “Spermatophytes”

Flowers: Complex structure containing seeds within protective ovary. All descendants known as “Angiosperms”

Increasing Terrestriality

Key point: History of land plants is the increasing adaptation to terrestriality.

BRYOPHYTES Mosses, Liverworts, Hornworts

KEY POINTS •  Gametophyte dominant •  No vascular tissue •  Ancient but persistent group.


•  Not monophyletic (???) •  Date to > 475 mya •  Very successful, 24,000

species, but never dominated landscape.

•  Non-vascular; therefore only found in moist environments.

•  Need water for sperm transport and absorption. Fig. 29.7 from textbook indicates

that bryophytes are paraphyletic, however…


•  Not monophyletic (???) •  Date to > 475 mya •  Very successful, 24,000

species, but never dominated landscape.

•  Non-vascular; therefore only found in moist environments.

•  Need water for sperm transport and absorption.

Common Characteristics •  Gametophyte generation

dominates •  Lack stiff, supporting

structures –  Therefore low-growing

•  Separate male and female gametophyte –  Male gametangium =

antheridium (flagellated sperm cells)

–  Female gametangium = archegonium (single egg)

•  Fertilization within archegonium ⇒ zygote ⇒ embryo

Bryophytes

Bryophyta: Mosses •  Best known of bryophytes

–  Note that “bryophytes” is a term of convenience, where as Bryophyta is restricted to the mosses.

•  Small individual plants in tight packages--form spongy, supporting mats.

•  Anchor to substrate with rhizoids –  “roots”, like “leaves” not

homologous with vascular plants

•  Life cycle good example of alternating generations

Bryophytes

Fig. 29-8-3

Key Haploid (n) Diploid (2n) Protonemata

(n)

“Bud”

“Bud”

Male gametophyte (n)

Female gametophyte (n)

Gametophore

Rhizoid

Spores

Spore dispersal

Peristome

Sporangium MEIOSIS Seta

Capsule (sporangium)

Foot

Mature sporophytes

Capsule with peristome (SEM)

Female gametophytes

2 m

m

Raindrop

Sperm

Antheridia

Egg

Archegonia

FERTILIZATION (within archegonium)

Zygote (2n)

Embryo

Archegonium

Young sporophyte (2n)

Bryophytes

Hepatophyta: Liverworts

•  Very inconspicuous, lobed bodies hugging ground

•  Life cycle like mosses

•  Thalloid form & Leafy form

•  Likely sister-group of remaining (all other) Plantae

Bryophytes

Anthocerophyta: Hornworts

•  Resemble liverworts •  Name derives from

sporophytes in horn-like capsules of matlike gametophyte

•  Cells have a single large chloroplast

Bryophytes

Vascular tissue: Cells joined into tubes to transport water and soil nutrients upward (xylem) and carbohydrates downward (phloem). All descendants known as “Tracheophytes”

Seedless Vascular Plants Lycophytes & Pterophytes

KEY POINTS •  Plants with xylem and phloem •  Branched sporophyte dominates •  Ancestral state retains flagellated sperm

and thus inhabit moist environments •  Paraphyletic •  “Vascular plant” monophyletic:

Tracheophyta.

General Macroevolution

•  Earliest are found in mid-Paleozoic, 425 mya (e.g. Cooksonia)

•  Dominated landscape by end of paleozoic

•  Branched sporophyte dominant generation

•  All with flagellated sperm

Seedless Vascular Plants


•  Earliest are found in mid-Paleozoic, 425 mya (e.g. Cooksonia)

•  Dominated landscape by end of paleozoic

•  Branched sporophyte dominant generation

•  All with flagellated sperm

General Macroevolution

Lycopod with microphylls shown

Fern with characteristic macrophylls

Fern sperm with multiple

flagellae

General Structure

•  Roots & shoots •  Lignified vascular

tissue: Xylem, Phloem •  Some heterosporous

–  Megaspores (female) –  Microspores (male) –  As in seed plants


General Structure

•  Roots & shoots •  Lignified vascular

tissue: Xylem, Phloem •  Some heterosporous

–  Megaspores (female) –  Microspores (male) –  As in seed plants


Megaspores Microspores

Lycopod Selaginella

Lycophyta: Lycopods Club mosses, quillworts, ground pines

•  Relicts of flourishing past, two lineages: –  Giant, tree-like, woody –  Small herbaceous

•  Only small forms extant –  E.g. Lycopodium,

Selaginella •  Many are tropical epiphytes;

temperate forms grow at ground level

•  Sporangia borne on sporophylls: leaves specialized for reproduction


Lycopodium Club moss or ground pine

Isoetes Quillwort

Lycophyta: Lycopods Club mosses, quillworts, ground pines

•  Relicts of flourishing past, two lineages:

–  Giant, tree-like, woody –  Small herbaceous

•  Only small forms extant –  E.g. Lycopodium, Selaginella

•  Many are tropical epiphytes; temperate forms grow at ground level

•  Sporangia borne on sporophylls:

–  leaves specialized for reproduction


Note Pterophyta are sister to Spermatophyta: Megaphyll leaves, roots that can branch

Pterophyta I: Horsetails

•  Previously considered own phylum, Sphenophyta, now placed within Pterophyta.

•  Late Paleozoic forms grew to 15m

•  Today only the genus Equisetum –  15 species mostly northern

hemisphere •  Conspicuous horsetail

sporophyte •  Homosporous…bisexual

gametophyte


An exception: Equisetum giganteum from Chile

Pterophyta I: Horsetails

•  Underground rhizome from which stems arise

•  Stems: hollow, jointed with whorls of small branches

•  Cone-like sporangia at tip of stem.


Pterophyta II: True ferns •  Most diverse seedless

vascular plants •  Today ~12,000 species •  Most diversity in tropics, also

common in temperate regions.

•  Fronds are large leaves with branched veins: megaphyll

•  Compound leaflets grow from fiddlehead tip

•  Leaves may sprout directly from prostrate stems (rhizomes); or as upright treeferns


Pterophyta II: True ferns


•  Most diverse seedless vascular plants

•  Today ~12,000 species •  Most diversity in tropics, also

common in temperate regions.

•  Fronds are large leaves with branched veins: megaphyll

•  Compound leaflets grow from fiddlehead tip

•  Leaves may sprout directly from prostrate stems (rhizomes); as upright treeferns; or as epiphytes

LIFE CYCLE: sporophyte with specialized leaves, each with clustered sporangia below (sori) which launch spores, grow

into gametophyte

Pterophyta II: Whiskferns

•  Previously considered own phylum, Psilophyta, now placed within Pterophyta.

•  Simple plants. •  Diploid sporophyte has

dichotomous branches (like primitive Cooksonia)

•  True roots and leaves are absent –  Like Lycopods! However,

these have been secondarily lost.

The CARBONIFEROUS •  360-300 mya •  Height of seedless vascular

diversity and ecological dominance.

•  Formed first forests. •  Eventually become COAL:

–  Dead plants did not completely decay.

–  Became peat bogs. –  Eventually covered by sea. –  Heat and pressure from

sediments converted peat to coal.

4 square miles of Carboniferous coal forest in Pennsylvania (see http://www.mnh.si.edu/highlight/riola/ for highlights)

Gymnosperms and the Evolution of Seed Plants

Seed Plants

Key points •  Seed replaces spore •  Fertilization by pollen instead of sperm •  Two groups: gymnosperms

–  (naked seeds) •  Angiosperms

–  (protected seeds)

Gymnosperms: Conifers & Allies

1.  Success marked by change in life cycle

2.  Evolution 3.  Four divisions

Changes in Life Cycle

1.  Gametophyte highly reduced –  Retained within

reproductive tissue of sporophyte and not cast out as independent generation.

–  Shift toward diploidy.


2.  Fertilization by pollen rather than swimming sperm. –  There is no longer a

reliance on water!


3.  Seed. –  Zygote not independent –  Zygote to embryo

packaged with food in a seed coat.

–  Seed is “naked” born on scales of cones. •  Protects from

desiccation. •  Increases dispersal

capabilities: replaces spore as dispersal agent.

Fig. 30-6-4

Microsporangium (2n)

Microsporocytes (2n)

Pollen grains (n)

Pollen cone

Microsporangia

MEIOSIS

Mature sporophyte (2n)

Haploid (n) Diploid (2n)

Key

MEIOSIS

Surviving megaspore (n)

Pollen grain

Megasporocyte (2n)

Ovule

Integument Ovulate cone

FERTILIZATION

Pollen tube

Female gametophyte

Sperm nucleus (n)

Egg nucleus (n)

Archegonium

Seedling

Seeds

Seed coat (2n)

Food reserves (n)

Embryo (2n)

Megasporangium (2n)

Evolution •  Appear much earlier than

Angiosperms, in Devonian. “Modern” gymnosperms by early-mid Mesozoic

•  Permian marks end of Paleozoic.

•  Mesozoic: Age of Dinosaurs (zoologists); Age of Gymnosperms (botanists)

Permian harshness (formation of Pangaea)

Interior aridity

Demise of Carboniferous forests

Mass Extinctions

Rise of Gymnosperms

Four Divisions

•  Cycadophyta –  Cycads

•  Ginkgophyta –  Ginkgo

•  Gnetophyta –  Gnetales

•  Coniferophyta –  Conifers

Cycadophyta: Cycads •  Cycads or “Sago” palm •  Currently ~130 species

–  Slow-growing, tropical & subtropical

•  Flourished in Mesozoic •  Massive cone-shaped

structures bearing pollen or ovules

•  Primarily insect pollinated, some wind pollinated.

•  Seeds eaten in Asia after removing Alkaloids







Until 1990s, pollination was assumed to be by wind, with insect pollination limited to Angiosperms. Now, insect pollination is known for most species.

Hot males, nice females

Ginkgophyta: Gingko •  Diverse in Mesozoic, single

species today •  Known only from fossils until

discovered growing in Chinese Buddhist temples

•  Fan-shaped, deciduous leaves

•  Male trees widely planted landscape tree –  Resistant to drought,

pollution, pests •  Seeds of female produce

stench when crushed

Maiden-hair tree

Ginkgophyta: Gingko •  Diverse in Mesozoic, single

species today •  Known only from fossils until

discovered growing in Chinese Buddhist temples

•  Fan-shaped, deciduous leaves

•  Male trees widely planted landscape tree –  Resistant to drought,

pollution, pests •  Seeds of female produce

stench when crushed

Cycads & Ginkgos •  Have flagellated sperm:

Differ from all other Gymnosperms, and Angiosperms.

•  What are the implications if Gymnosperms are monophyletic?

•  What are the implications if sperm with no flagellae is homologous?

Gnetophyta •  Gnetum: tropical tree/

vine. •  Ephedra: Mormon or

Mexican tea of American deserts, jointed stem.

•  Welwitschia: Largest known leaves, deserts of SW Africa, deep root, exposed leaves.

•  Fossils from Permian, but peak diversity in Cretaceous.

Gnetophyta •  Have vessel elements:

Differ from all other Gymnosperms, share this with Angiosperms.

•  What are the implications if Gymnosperms are monophyletic?

•  What are the implications if vessel elements are homologous?

Coniferophyta (or Pinophyta): Conifers

•  Pines, firs, spruce, larches, cedars, hemlocks, (all Pinaceae), yews, cypress, redwoods, hoop pines, umbrella pines, yellow-woods, plum-yews.

•  ~600 species, dominate vast regions of taiga: northern and southern evergreen forests.

Coniferophyta (or Pinophyta): Conifers

•  Evergreen, even perform limited photosynthesis year round.

•  Needles are leaves •  Commercially important as

timber •  Tallest plant: coastal redwood

(>110m) •  Heaviest plant: giant sequoia

(2500 metric tons) •  Oldest plant: Bristlecone pine

>4600 years old

Summary: Gymnosperms

•  Monophyletic sister group to angiosperms.

•  Peak diversity in the Mesozoic, still ecologically dominant in some ecosystems.

•  Evolution of seed and pollen key transitions (shared with Angiosperms)

Angiosperms

Structure & Classification

Flowers: Complex structure containing seeds within protective ovary. All descendants known as “Angiosperms”

KEY POINT

•  Flower and fruit as defining reproductive strategy

Lecture Outline

•  General •  Morphology

– Three Tissues – Two Systems – Lots of terminology…

•  Monocots & Dicots •  Life Forms

Terminology

•  Sets the stage for understanding the next 4 lectures! – Angiosperm life cycle – Plant tissues and growth – Transport in plants – Plant control systems

Angiosperms

•  Anthophyta: “flower plant”

•  270,000 described species. –  Compare this to

~900 species of Gymnosperms!

General Aspects

Angiosperms •  Refined vascular tissue •  Vessel elements

present in xylem –  shorter, wider cells

placed end to end –  perforation plates at

end of each cell –  line up end-to-end to

create vessels •  Specialized for

transport, less for support

General Aspects

Angiosperms •  Vessel elements •  Also present in

Gnetales and absent from many basal angiosperm groups.

•  ??? Possibilities ???

General Aspects

Angiosperms •  Of course…

•  Characterized by flowers

•  Characterized by fleshy ovary protecting seed

General Aspects

Two Systems •  Roots: below-ground

non-photosynthetic –  Anchor –  Absorption –  Storage

•  Aerial shoots: above-ground photosynthetic and/or transport –  Leaves –  Stems

•  These are interdependent

Morphology

Roots Anchor Shoot System

•  Tap root –  Single large vertical root

with secondary rootlets –  Firm anchor –  Food storage used by

plant when producing flower, fruit (harvest before flowering)

•  Fibrous root –  Mat-like and spreading,

shallow, wide coverage –  Grasses (good erosion

control)

Morphology

•  Root hairs: For absorption at root tip

•  Adventitious roots: above ground roots help support stem –  Adventitious = Name for

any plant part growing in abnormal place

–  Prop roots –  Aerial roots

Morphology

Roots Anchor Shoot System

•  Nodes •  Internodes •  Axillary buds •  Apex = terminal bud •  Apical dominance

Morphology

Shoots Stems, leaves, flowers

Morphology

•  Modified stems •  Stolons

–  Horizontal above-ground runners (e.g. strawberries)

•  Rhizomes –  Horizontal below-ground

stems (potatoes, iris) •  Bulbs

–  Vertical below ground with leaves modified for storage.


Morphology

•  Primary photosynthetic part of plant (usually)

•  Part 1: Blade •  Part 2: Petiole

–  Absent in many grasses and relatives (monocots)



Morphology

•  Highly variable –  Within individuals –  Between species –  Between deeper

clades •  Shape •  Arrangement •  Margins •  Venation •  Simple vs

Compound


Morphology

•  Some can be highly modified: –  Tendrils –  Spines –  Storage –  Asexual

reproduction –  Bracts –  Insectivory


Morphology

•  Some can be highly modified: –  Tendrils –  Spines –  Storage –  Asexual

reproduction –  Bracts –  Insectivory

Venus flytrap

Sundew

Pitcher plant


Morphology

•  Angiosperm structure specialized for sexual reproduction.

•  Specialized shoot made up of (usually) four rings of modified leaves (floral organs): –  Sepals –  Petals –  Carpels: Female –  Anthers: Male


Morphology

•  Carpels are the female organs –  Ovules contain

megasporangium –  Stigma receives

pollen –  Style leads from

stigma to ovary –  Ovary contains

ovules


Morphology

•  Stamens are the male organs –  Filament is the

stalk –  Anther houses

microsporangia and produces pollen.

–  Pollen will contain male gametophyte


Morphology

•  Perfect flowers contain both carpels and stamens

•  Imperfect flowers contain either carpels or stamens –  Monoecious: having

separate male and female flowers on the same plant.

–  Dioecious: having separate male and female plants.


Morphology

Evolutionary Trends 1.  Reduction in number

of floral parts 2.  Floral parts fused,

compound carpels to single and large

3.  From radial to bilateral symmetry

4.  Ovary drops below petals and sepals (“inferior” ovary)

Water lily, Nymphaceae, showing plesiomorphic state

Orchid, showing derived

state


Morphology







Morning glory,

showing derived

state


Morphology







Pea flower, showing derived

state


Morphology






Major Angiosperm Dichotomy

•  Monocots and Dicots

•  Refers to numbers of “seed leaves” or cotyledons.

•  Monocots are monophyletic.

•  Dicots are NOT.

Monocots: Grasses, lilies, palms, etc.

Dicots: Everything else


•  Monocots and Dicots

•  Refers to numbers of “seed leaves” or cotyledons.


•  Dicots are NOT.


•  Monocots and Dicots •  Refers to numbers of “seed leaves” or cotyledons.


•  Dicots are NOT. •  “Dicot is a term of

convenience.

Dicots & Monocots

•  But it is a useful distinction…

Dicots & Monocots

•  Discussion question: •  For these traits, which

are plesiomorphic and which are apomorphic?

Life Forms •  Plant forms have evolved to

fill numerous ecological roles or “niches”.

•  Developmental plasticity and Indeterminate growth allow individuals to fit particular ecological conditions.

•  Why is this so important to a plant?

Life Forms •  Grasses

–  Monocots –  No petiole –  Limited branching –  No woody tissue

(herbaceous) •  Forbs •  Shrubs •  Trees •  Epiphytes •  Aquatic •  Herbaceous •  Woody

Life Forms •  Grasses •  Forbs

–  Dicots –  Generally wildflowers –  Herbaceous

•  Shrubs •  Trees •  Epiphytes •  Aquatic •  Herbaceous •  Woody

Life Forms •  Grasses •  Forbs •  Shrubs

–  Woody tissue –  No distinct single trunk

•  Trees •  Epiphytes •  Aquatic •  Herbaceous •  Woody

Life Forms •  Grasses •  Forbs •  Shrubs •  Trees

–  Woody tissue –  Single main trunk –  Apical dominance

pronounced •  Epiphytes •  Aquatic •  Herbaceous •  Woody

Life Forms •  Grasses •  Forbs •  Shrubs •  Trees •  Epiphytes

–  Plant that grows on another plant

•  Aquatic •  Herbaceous •  Woody

Life Forms •  Grasses •  Forbs •  Shrubs •  Trees •  Epiphytes •  Aquatic

–  Numerous adaptations for living in water

•  Herbaceous •  Woody

Life Forms •  Grasses •  Forbs •  Shrubs •  Trees •  Epiphytes •  Aquatic •  Herbaceous

–  Die down each year. –  Can be annual to

perennial. –  No structural lignin

•  Woody –  Perennial and persistent –  Structural lignin (wood)

•  Dichotomy used in above distinctions.

Summary

•  Flowering plants have been very successful: innovation of flower & fruit.

•  Diverse morphology structured around the root system and the shoot system.

•  Developmental plasticity and evolution of different life forms important in colonization of novel ecological niches.

key point: history of land plants is the increasing...

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