K. Plantae and the Transition to Land
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True Multicellularity: amalgamation
and specialization
Aquatic vs. Terrestrial Ecosystems
Evolution of Plantae: Charophytes
υ Plant Life Cycles: Analogy not
Homology
υ Preadaptations to Terrestrial Life
υ The Bryophytes
υ Evolution of Vascular Tissues
Φ Xylem and water movement
Φ Phloem and sugar movement
The evolution of true multicellularity in the K.
Plantae correlates with the transition to land:
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Challenges of Aquatic versus
Terrestrial Habitats
υ Constancy vs variation
υ Homogeneous vs dispersed
resources
υ Buoyancy vs gravity
υ Gamete and species dispersal
υ Desiccation
Advantages of the terrestrial
environment
υ Abundant unfiltered light
υ Mineral rich soil and gaseous
atmosphere
υ Stable substrate
υ Absence of herbivores
Features and Origin of Plant Life Cycles
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All plants exhibit Alt. of
Gen. (sporic,
diplohaplontic) life cycle
Life cycles are
heteromorphic
Early on gg-phyte
predominant w/ rapid shift
in dominance
Independence among
algae vs. dependence????
So, can we assume that
plant and algae life cycles
are also homologous?
Evolution of Multicellularity: K. Plantae
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Two evolutionary events:
υ Aggregation into colonies
υ Cellular specialization and division of labor
Φ Loss of flagella
Φ Differentiation of somatic and gametic cells
The Monophyletic Ancestry of K. Plantae:
A phylogenetic relationship with Chlorophyta
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Comparing Charophytes with terrestrial plants:
υ 1. Chloroplast homologies: Chl a, access. pig.,
primary endosymbiosis
υ 2. Biochemical similarities: Cellulose, starch
and peroxisomes
υ 3. Mitotic similarities
υ 4. Gametic ultrastructural similarities
υ 5. Genomic homology
What about their life cycles? ……Are
……Are they
homologous?
A Separate Origin: The algal Alt. of
Gen. cycle is only analogous to that
of plants
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Coleochaete: no multicellular
diploid stage, no Alt. of Gen.
n gametes
mitosis
Gametophyte
w/ haploid
thallus
mitosis
Zygote
meiosis
n spores (4)
Zygotic life cycle
*
What if…:
zygote
Gametophyte
w/ haploid
thallus
n spores
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mitosis
mitosis
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An Unintentional Transition (Preadaptation) to Land: a new adaptive zone
n gametes
mitosis
meiosis
Multicellular
2n
Ancestral charophyte delayed zygotic meiosis
Mitosis yields dependent multicellular
sporophyte
Protected embryophyte
Adaptive advantage to alga: maximizes spore
production
υ Also preadaptive!
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A. Preadaptations to Terrestrial Life
υ Waxy Cuticle and sporopollenin
υ Gametangia: protection for gametes
υ Protected Embryophyte
B. The Bryophytes: Mosses, Liverworts,
Hornworts
υ Waxy cuticle
υ Gametangia
υ Still semisemi-aquatic
Φ Flag. sperm
Φ No vasc. tiss.
υ Dependent sporo.
Some highlights of plant evolution
The life cycle of Polytrichum, a
surviving moss
Note shift in
dominance
Vascular Tissue: Tracheophytes embody
an evolutionary breakthrough!
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A. Regional Specialization of Plant Body
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υ
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Roots, stems and leaves: a response to
dispersed resources
Protoderm -->dermal;
-->dermal; Ground meristem -->
-->
parenchyma of root and leaf; Procambium -->
-->
conducting tissues (X&P)
B. Structural Support: lignin
C. Vascular System:
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Xylem: dead tubetube-like cells for the conduction
of water and minerals mined from the soil
Phloem: living cells that enable the cytoplasmic
transfer of sugars and other metabolites
The three tissue systems: Dermal,
Ground and Vascular Tissues
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Apical meristematic tissue
υ Pluripotent cells
υ Stem and root
Protoderm: covering
υ Epidermis and guard cells
Ground meristem: functional &
support
υ Parenchyma, collenchyma,
sclerenchyma
Procambium: vascular & support
υ Xylem, phloem, and lateral
meristem
Xylem: water-conducting cells
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Water potential drives the movement of water
in the xylem
Tracheids and vessel
elements
Lignified cell walls
Mechanisms of water
movement
υ Root pressure
(positive)
υ TranspirationTranspirationcohesioncohesion-tension
mechanism
Φ Water potential
(negative)
Phloem: Foodconducting cells
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Distribution of the products
of PS is “source to sink”
sink”
Sieve cells or sievesieve-tube
members and companion
cells
Pressure Flow Mechanism
of Translocation
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Create hydrostatic pressure at
the source
Sap flows away from source
Solute removal at sink lowers
the pressure
Water recirculates w/ xylem
Tapping phloem sap with the help of
aphids
Pressure flow drives the movement of
phloem sap in a sieve tubes