Stephanie Bobbitt
Chapter 29) Plant Diversity I: How Plants Colonized Land
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Daniel Oh 2014
An Overview of Land Plant Evolution
o Evolutionary adaptations to terrestrial living characterize the four main groups of land plants
 four main land plant groups: bryophytes (ex: moss), pteridophytes (ex: ferns), gymnosperms
(cone-bearing plants), and angiosperms (flowering plants)
 bryophytes differ from fungi mainly in reproduction – ex: bryophytes’ offspring develop from
multicellular embryos that stay attached to “mother” plant
 vascular tissue that evolved in an ancestor to pteridophytes, gymnosperms, and angiosperms
group these three into a vascular plant category
 vascular tissue: cells are connected to tubes that move water and nutrients through plant body
 pteridophytes are called seedless plants bec4ause there is no seed stage in life cycle;
gymnosperms and angiosperms are seed plants
 seed: plant embryo and food supply within a protective coat
 flower: reproductive structure that bears seeds in protective ovaries
 figure 29.1 shows plant evolution
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Daniel Oh 2014
Charophyceans are the green algae most closely related to land plants
 plants are multicellular, eukaryotic, photosynthetic autotrophs; have cell walls made of cellulose
and chlorophylls a and b in chloroplast  algae can have these characteristics, too
 two structures land plants only share with charophyceans: (1) plasma membrane of both have
rose-shaped array of proteins (called rosette cellulose-synthesizing complexes) that synthesize
cellulose mircofibrils of cell walls (2) both have peroxisome enzymes in organelles that minimize
loss of organic product from photorespiration
 sperm of land plants resembles sperm of charophyceans; formation of a phragmoplast during
the synthesis of cross-walls in cell division
 phragmoplast: alignment of cytoskeletal elements and Golgi-derived vesicles across midline of
dividing cell
Several terrestrial adaptations distinguish land plants from charophycean algae
 Apical Meristems, Producers of a Plant’s Tissues
• plants need complex bodies to get carbon dioxide and light aboveground and water and
minerals in the soil
• apical meristems: regions of cell division at tips of shoots and roots that sustains growth of
plant that allows the plant to get resources; meristems also produce cells that differentiate
into a plant’s different tissues (ex: surface epidermis for protection)
 Multicellular, Dependent Embryos
• embryo has placental transfer cells that help move nutrients from parent to embryo
• land plants are called embryophytes – recognizes multicellular, dependent embryos as a
land plant characteristic
 Alternation of Generations
• alternation of generations:
reproductive cycle where two
multicellular body forms
alternate
• gametophyte: haploid; produces
gametes
• sporophyte: diploid; produces
reproductive cells called spores
that can develop into a new
organism without fusing with
another
• size and complexity of
sporophyte depend on plant
group
• gametophyte is the dominant
generation in bryophytes;
sporophyte is dominant in
pteridophytes, gymnosperms,
and angiosperms
 Walled Spores Produced in Sporangia
• sporopollenin: most durable organic material; makes walls of plant spores
• sporangia: multicellular organs on sporophytes that make spores; has spore mother cells
that do meiosis and generate haploid spores
 Multicellular Gametangia
• gametangia: multicellular organ where gametes are produced
• archegonia: female gametangia that makes a single egg cell and keeps it at the organ’s base
• antheridia: male gametangia that makes many sperm cells that are released into the
environment when mature
 Other Terrestrial Adaptations Common to Many Land Plants
• Adaptations for Water Conservation: (1) epidermis of land plants are coated with a cuticle
(layer of polymers called polyesters and waxes) that protects plant from microbial attack and
waterproofs plant so little water is lost from organs; (2) pores called stomata allow
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Daniel Oh 2014
exchanging of carbon dioxide and oxygen between air and leaf interior and most water exits
leaves through stomata
• Adaptations for Water Transport: xylem and phloem are tissues in a plant’s vascular system
that carry minerals and water up from roots
• Secondary Compounds as Terrestrial Adaptations: land plants produce secondary
compounds to defend themselves, maintain structure, etc.; humans use some secondary
compounds for medical applications
The Origin of Land Plants
o Land plants evolved from charophycean algae over 500 million years ago
 lots of connections between land plants and charophycean: homologous chloroplasts,
homologous cellulose walls, homologous peroxisomes, phragmoplasts, homologous sperm, and
molecular systematics
o Alternation of generations in plants may have originated by delayed meiosis
 zygote of charophycean undergoes meiosis to make haploid spores and plant zygotes undergo
mitosis – a genetic change caused a delay in meiosis until mitotic divisions had already occurred
o Adaptations to shallow water preadapted plants for living on land
 natural selection favors algae living in shallow water that can survive when not completely in
water – sporopollenin prevents drying out
o Plant taxonomists are reevaluating the boundaries of the plant kingdom
 “deep green”: focuses on deepest phylogenetic branching in plant kingdom to identify and name
major plant clades
 three versions of plant kingdom – (1) kingdom Plantae is made of embryophytes; (2) a new
kingdom Streptophyta should be made to include charophyceans and other related groups; (3) a
new kingdom Viridiplantae should be made to include charophyceans and chlorophytes
o The plant kingdom is monophyletic
 monophyletic: derived from a common ancestor
Bryophytes
o The three phyla of bryophytes are mosses, liverworts, and hornworts
 phylum Hepatophyta: aka liverworts
 phylum Anthocerophyta: aka hornworts
 phylum Bryophyta: aka mosses
 these three are not monophyletic
o The gametophyte is the
dominant generation in
the life cycles of
bryophytes
 gametophytes are the
most dominant phase
of life history
 protonema: one-cellthick filaments made
by germinating moss
spores; have big
surface area
 gametophore:
gamete-producing
structure
 most bryophytes do
not have specialized
tissues to distribute
water and compounds
 rhizoids: long,
tubular, single cells
that are not made of
tissue and do not
have specialized
conducting cells
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some moss are described as “leafy” because they have stem-like structures with leaf-like
appendages, but they are not leaves because they do not have lignin-coated vascular cells (ex:
Polytrichum shown in figure)
 reproduction of bryophytes: sperm swims towards eggs, which stay in the bases of archegonia 
gametes fuse to make zygotes
o Bryophyte sporophytes disperse enormous numbers of spores
 bryophyte sporophytes have photosynthetic plastids, but s till depend on maternal
gametophytes for sugars, amino acids, minerals, and water
 bryophytes have the simplest sporophytes
 moss sporophytes have a foot (elongated stalk called a seta) and a spore-making organ
(sporangium or capsule); seta moves sugars, amino acids, etc. to the capsule to make spores
 moss capsule is the meiosis and spore production site
 immature capsules have a protective cap of gametophyte tissue called the calyptra
 peristome: upper part of capsule used for gradual spore discharge
o Bryophytes provide many ecological and economic benefits
 mosses can live in a variety of habitats because they can lose body water without dying and
replenish it later and can absorb UV radiation
 Sphagnum: moss that makes undecayed organic material called peat (does not decay because of
resistant phenolic compounds in cell walls and because it secretes acidic and phenolic
compounds that reduce bacterial activity)
 Sphagnum is used for soil conditioning and plant roots packing
The Origin of Vascular Plants
o vascular plants have branched sporophytes
o pteridophytes are called seedless vascular plants because they do not make seeds
o Additional terrestrial adaptations evolved as vascular plants descended from moss-like ancestors
 protracheophyte polysporangiophytes: no lignified vascular tissue like bryophytes, but have
branched sporophytes that were not dependent on gametophytes
o A diversity of vascular plants evolved over 400 million years ago
 Cooksonia: earliest known vascular plant that had small lignified cells and bulbous sporangia
Pteridophytes: Seedless Vascular Plants
o two phyla of seedless vascular plants = phylum Lycophyta and phylum Pterophyta
o Pteridophytes provide clues to the evolution of roots and leaves
 pteridophyte roots probably evolved from lowermost, subterranean portions of ancient vascular
plants’ stems
 leaves of lycophytes probably evolved from tissue flaps on stem surface where vascular tissue
grew (these leaves are microphylls)
 megaphylls: leaves of other modern vascular plants (can be big because of good vascular system)
o A sporophyte-dominant life cycle evolved in seedless vascular plants
 homosporous: type of plant that makes one type of spore that develops into a bisexual
gametophyte with male and female sex organs
 hetersporous: type of plant that makes two kinds of spores – megaspores: develop into female
gametophytes with archegonia; microspores: develop into male gametophytes with antheridia
o Lycophyta and Pterophyta are the two phyla of modern seedless vascular plants
 Phylum Lycophyta (Lycophytes)
• most lycophytes are tropical plants and use trees as substratums
• sporophylls: specialized leaves with sporangia
 Phylum Pterophyta (Ferns and Their Relatives)
• Psilophytes: survivors of early lineage of vascular plants
• Sphenophytes (Horsetails): have a brushy appearance, found in marshy places, have upright
and horizontal stems (rhizomes)
• Ferns: most widespread and diverse pteridophytes, have horizontal rhizomes, produce sori
(clusters of sporangia); life cycle of fern shown in figure on next page
o Seedless vascular plants formed vast “coal forests” during the Carboniferous period
 seedless vascular plants left fossil fuel as coal
 peat accumulated because dead plants did not completely decay  heat and pressure converted
the peat into coal

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Daniel Oh 2014
Monday, December 17, 2012 4:38:35 PM Central Standard Time
Daniel Oh 2014
Monday, December 17, 2012 4:38:35 PM Central Standard Time