plants challenges of terrestriality - university of san...
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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
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Figure 29.2
ANCESTRAL ALGA
Red algae
Chlorophytes
Charophytes
Embryophytes
Viridiplantae Streptophyta
Plantae
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
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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.
Major events in plant evolution Generalized Alternation of
Generation in Plants
A. Changes in life history
2. Generations are heteromorphic.
Major events in plant evolution
Gam
etophyte S
porophyte
A. Changes in life history
2. Generations are heteromorphic.
3. Shift of dominance from gametophyte (n) to sporophyte (2n).
Major events in plant evolution
Gam
etophyte S
porophyte
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A. Changes in life history
2. Generations are heteromorphic.
3. Shift of dominance from gametophyte (n) to sporophyte (2n).
4. Replacement of flagellated sperm by pollen.
Major events in plant evolution
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.
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‘Bryo
phytes’
Pterophytes
Gymnosperm
s
Angiosperms
Lycophyte
s
Charophytes
Embryo: zygotes are retained within tissues of the female parent plant. All descendants known as “Embryophyta”
Embryo Vascular tissue: Cells joined into tubes to transport water and soil nutrients upward (xylem) and carbohydrates downward (phloem). All descendants known as “Tracheophytes”
‘Bryo
phytes’
Pterophytes
Gymnosperm
s
Angiosperms
Lycophyte
s
Charophytes
Embryo
Seeds: Embryo packed with a supply of nutrients inside a protective coat. All descendants known as “Spermatophytes”
Vascular tissue
‘Bryo
phytes’
Pterophytes
Gymnosperm
s
Angiosperms
Lycophyte
s
Charophytes
Flowers: Complex structure containing seeds within protective ovary. All descendants known as “Angiosperms”
Seeds
Embryo
Vascular tissue
‘Bryo
phytes’
Pterophytes
Gymnosperm
s
Angiosperms
Lycophyte
s
Charophytes
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Flowers
Increasing Terrestriality
Key point: History of land plants is the increasing adaptation to terrestriality.
Seeds
Embryo
Vascular tissue
‘Bryo
phytes’
Pterophytes
Gymnosperm
s
Angiosperms
Lycophyte
s
Charophytes
BRYOPHYTES Mosses, Liverworts, Hornworts
KEY POINTS • Gametophyte dominant • No vascular tissue • Ancient but persistent group.
BRYOPHYTES Mosses, Liverworts,
Hornworts
• 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. 29.5 from textbook indicates that bryophytes are paraphyletic, however…
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
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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
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Embryo Vascular tissue: Cells joined into tubes to transport water and soil nutrients upward (xylem) and carbohydrates downward (phloem). All descendants known as “Tracheophytes”
‘Bryo
phytes’
Pterophytes
Gymnosperm
s
Angiosperms
Lycophyte
s
Charophytes
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.
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
Seedless Vascular Plants
Megaspores Microspores
Lycopod Selaginella
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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
Seedless Vascular Plants
Lycopodium Club moss or ground pine
Isoetes Quillwort
Embryo
Note Pterophyta are sister to Spermatophyta: Megaphyll leaves, roots that can branch
‘Bryo
phytes’
Pterophytes
Gymnosperm
s
Angiosperms
Lycophyte
s
Charophytes
Vascular tissue
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
Seedless Vascular Plants
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.
Seedless Vascular Plants
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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
Seedless Vascular Plants 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)
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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.
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Changes in Life Cycle
2. Fertilization by pollen rather than swimming sperm. – There is no longer a
reliance on water!
Changes in Life Cycle
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
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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
Cycadophyta: Cycads
• 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
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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.
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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”
Seeds
Embryo
Vascular tissue
‘Bryo
phytes’
Pterophytes
Gymnosperm
s
Angiosperms
Lycophyte
s
Charophytes
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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
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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
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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)
Shoots Stems, leaves, flowers
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Morphology
• Modified stems • Rhizomes
– Horizontal below-ground stems (potatoes, iris)
• Bulbs – Vertical below ground
with leaves modified for storage.
Shoots Stems, leaves, flowers
Morphology
• Primary photosynthetic part of plant (usually)
• Part 1: Blade • Part 2: Petiole
– Absent in many grasses and relatives (monocots)
Shoots Stems, leaves, flowers
Shoots Stems, leaves, flowers
Morphology
• Highly variable – Within individuals – Between species – Between deeper
clades • Shape • Arrangement • Margins • Venation • Simple vs
Compound
Shoots Stems, leaves, flowers
Morphology
• Highly variable – Within individuals – Between species – Between deeper
clades • Shape • Arrangement • Margins • Venation • Simple vs
Compound
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Shoots Stems, leaves, flowers
Morphology
• Highly variable – Within individuals – Between species – Between deeper
clades • Shape • Arrangement • Margins • Venation • Simple vs
Compound
Shoots Stems, leaves, flowers
Morphology
• Highly variable – Within individuals – Between species – Between deeper
clades • Shape • Arrangement • Margins • Venation • Simple vs
Compound
Shoots Stems, leaves, flowers
Morphology
• Highly variable – Within individuals – Between species – Between deeper
clades • Shape • Arrangement • Margins • Venation • Simple vs
Compound
Shoots Stems, leaves, flowers
Morphology
• Some can be highly modified: – Tendrils – Spines – Storage – Asexual
reproduction – Bracts – Insectivory
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Shoots Stems, leaves, flowers
Morphology
• Some can be highly modified: – Tendrils – Spines – Storage – Asexual
reproduction – Bracts – Insectivory
Shoots Stems, leaves, flowers
Morphology
• Some can be highly modified: – Tendrils – Spines – Storage – Asexual
reproduction – Bracts – Insectivory
Venus flytrap
Sundew
Pitcher plant
Shoots Stems, leaves, flowers
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
Shoots Stems, leaves, flowers
Morphology
• Carpels are the female organs – Ovules contain
megasporangium – Stigma receives
pollen – Style leads from
stigma to ovary – Ovary contains
ovules
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Shoots Stems, leaves, flowers
Morphology
• Stamens are the male organs – Filament is the
stalk – Anther houses
microsporangia and produces pollen.
– Pollen will contain male gametophyte
Shoots Stems, leaves, flowers
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.
Shoots Stems, leaves, flowers
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
Shoots Stems, leaves, flowers
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
Morning glory,
showing derived
state
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Shoots Stems, leaves, flowers
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
Pea flower, showing derived
state
Shoots Stems, leaves, flowers
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)
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
Major Angiosperm Dichotomy
• Monocots and Dicots
• Refers to numbers of “seed leaves” or cotyledons.
• Monocots are monophyletic.
• Dicots are NOT.
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Major Angiosperm Dichotomy
• Monocots and Dicots • Refers to numbers of “seed leaves” or cotyledons.
• Monocots are monophyletic.
• Dicots are NOT. • “Dicot is a term of
convenience.
Dicots & Monocots
• But it is a useful distinction…
Dicots & Monocots
• But it is a useful distinction…
Dicots & Monocots
• Discussion question: • For these traits, which
are plesiomorphic and which are apomorphic?
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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
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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.