Meet the Plants
Plant features:
Multicellular eukaryotes
Photosynthetic autotrophs (a few are parasitic heterotrophs)
Mostly terrestrial (some aquatic)
- requires new modes of nutrition, support, protection & reproduction
Multicellular gametangia, stomates
Characteristic cellular features:
Chloroplasts with thylakoid membranes and chlorophyll
(other eukaryotic organelles, including mitochondria, also present)
Starch as a storage compound (in plastids)
Cellulose walls (structural strength)
Cuticle (waxy coating) on the outside
Bryophytes & Seedless Vascular Plants
Plants share several key features with their likely sister group,
the charophyceans (green algae)
A common ancestor of modern charophyceans and plants moved
from an aquatic habitat onto land
- this involves changes in size, gas exchange, protection for fragile
embryos, protection from desication, ways for sperm to reach egg,
mechanisms of dispersal (getting around)... etc
Kingdom Plantae 1st appeared about 475 million years ago,
along with terrestrial fungi
most plants retain a symbiotic association with fungi
Stages in Land Plant Evolution:
1) Early land plants (~400 MYA)  bryophytes and seedless vascular plants
2) Diversification (~390 MYA)  abundance of seedless vascular plants
1) Seeds (~ 300 MYA)  Gymnosperms (naked-seeded plants)
2) Flowers and fruits (~ 130 MYA)  Angiosperms (flowering plants)
3 contrasting opinions
of what constitutes
the “plant kingdom”
Standard definition: true plants have
multicellular embryos
3 contrasting opinions
of what constitutes
the “plant kingdom”
should we include their
closest algal relatives, the
charophyceans, who share
so many features with land
plants?
3 contrasting opinions
of what constitutes
the “plant kingdom”
should we include green algae (chlorophytes),
who share the same photosynthetic machinery?
Unlike green algae, terrestrial plants have different cell and tissue types
Coleochaete orbicularis, a disc-shaped
charophycean showing simple structure
Polypodium sp., a terrestrial plant with
greater cell & tissue complexity
Evidence for common ancestor
of green algae & plants
Features shared by plants and charophyceans:
Primary endosymbiosis (double membrane around chloroplast)
Photosynthetic pigments (chlorophyll a, b and carotenoids)
Starch storage in plastids
Production of cell walls from cellulose
Cell plate formation during cell division
Sporic life cycle
New features in terrestrial plants:
Specialized tissues + organs (roots, stems, leaves) derived from the
apical meristem, the growing tip of a shoot
Alternation of multicellular generations
Multicellular gametangia
Walled spores produced in sporangia
Multicellular, dependent embryos
Plants organs derived from apical meristem
Above ground
Below ground
Alternation of Generations
gametophyte (N)
sporophyte (2N)
All plants have multicellular haploid stages, and multicellular diploid
stages, which produce each other
Walled spores
spores = haploid cells that can
grow into the gametophyte by
mitosis
protective outer layer
allows spores to resist
dry conditions
Sporophyte (2N) stage has multicellular structures called sporangia
that produce the haploid spores through meiosis
Multicellular Gametangia
Archegonium: vase-shaped female
organ holds single egg cell
Antheridium: male organ
releases sperm
Gametophyte (N) stage has multicellular structures called
gametangia that produce the haploid gametes
Multicellular, Dependent Embryos
Placental transfer cells,
analogous to mammalian
placenta
Multicellular embryos develop from zygote inside tissue of female
parent, which provide embryo with nutrients during development
Non-vascular plants (7%)
mosses, liverworts, hornworts
- no roots or true leaves
Vascular plants (93%)
have a system of tubes that
transport water + nutrients
throughout the plant
Seedless vascular plants
ferns + horsetails
Seed plants
gymnosperms + angiosperms
Trends in Plant Evolution:
Increasing adaptation to a
terrestrial environment
Progressive reduction of
gametophyte, increasing
dominance of sporophyte
Increased protection
(esp. of reproductive parts)
Increased height, structural
strength, and more complex
transport tissues
Campbell & Reece 2002
# of living species in different plant groups
6500
1000
12,000
100
1
250,000
550
Key innovations are special features that allow one group to
speciate
1) Early land plants (~438-408 MYA, Paleozoic Era)
Bryophytes and 1st vascular plants appeared at same time
Bryophytes features (e.g. moss)
Short
Lack elaborate vascular tissue
No true leaves, roots, or stems.
Dominant gametophyte
Flagellated (=swimming) sperm
Bryophytes
(plants lacking vascular tissues)
They are all short -- why?
Typically associated with moist
habitats
Most parts only 1 cell thick
Bryophyte life cycle
gametophytes are
anchored into soil
by threadlike
protonema and
single-celled
rhizoids
sporophytes =
brown stalks
growing out of
the female
gametophyte
Bryophyte life cycle
1) spores can disperse by wind – why is dispersal important?
- allows colonization of new (maybe better) habitats
- allows “escape” for offspring if local conditions turn bad
- prevents inbreeding
2) sperm have to swim to reach egg – this requires environmental water
- limits where bryophytes can grow
- think about how this limited the ability of bryophytes to take
over the world of dry land, compared to plant groups that evolved later
- on the plus side: no need for pollinators, just rain or splashes
3) what features of the bryophyte body also limit:
- the size of these plants?
- the environment in which they can live?
Sphagnum sp.
Forms extensive peat bogs in northern
latitudes (arctic regions) around world
Seedless vascular plants appeared ~ 420 MYA
Seedless vascular plant features (e.g., ferns)
- Dominant sporophyte
- Most lack true roots, stems, and leaves
- Primitive vascular tissues
Diversification culminated in Carboniferous Period (~300-350 MYA),
when our modern-day coal deposits were first formed
damp conditions
favored the growth
of huge forests of
seedless vascular
plants, limiting
opportunities for
seed plants until a
global drying out
Primitive vascular plants
had branching sporophytes
that were independent of
the gametophyte for growth
Enable multiple sporangia
per individual
Fossil Cooksonia, a seedless plant with
water-conducting tissue in stems, but
no true roots or leaves
Seedless Vascular Plants – have primitive vascular tissues
Phylum Pterophyta
Lycopodium sp.
Psilotum sp.
Phylum Lycophyta
Equisetum sp.
(horsetail)
Polypodium sp. (fern)
Fern Life cycle
Complex tissues and organs 1st appeared in seedless vascular plants
xylem - tubes of dead cells w/ lignin-reinforced cell walls, used for
transporting water + dissolved minerals
phloem - living cells that transport sugars + other organic nutrients
roots - organ for uptake of nutrients from soil...
leaves - increase surface area for photosynthesis
Microphylls, single vein
- found in Lycophytes, the
club mosses
Megaphylls, w/ complex branching
vascular system
Major evolutionary development:
modified leaves called sporophylls
on which sporangia grow
Fern sporophylls bear many sori,
small clusters of sporangia on the
underside of the sporophyll
(look for these in lab)
In groups we will see later, sporophylls
diversified in shape and structure;
stopped looking like leaves
in gymnosperms 
in angiosperms 