CHAPTER 29
PLANT DIVERSITY I: HOW PLANTS
COLONIZED LAND
An Overview of Land Plant Evolution
• More than 280,000 species of plants inhabit Earth
today.
• Land plants evolved from a green algae, called
charophyceans.
Evolutionary
adaptations to
terrestrial living
characterize the
four main groups of
land plants
• There are four main
groups of land
plants: bryophytes,
pteridophytes,
gymnosperms, and
angiosperms.
• Bryophytes, pteridiophytes, gymnosperms, ands
angiosperms demonstrate four great episodes in
the evolution of land plants:
• the origin of bryophytes from algal ancestors
• the origin and diversification of vascular plants
• the origin of seeds
• the evolution of flowers
Charophyceans are the green algae most
closely related to land plants
• The plasma membranes have a particular
cellulose structure of the cell wall.
• The presence of peroxisomes.
• Enzymes in peroxisomes help minimize the loss of
organic products due to photorespiration.
• Land plants that have flagellated sperm cells
which are similar to charophyceans.
Several terrestrial adaptations distinguish
land plants from charophycean algae
• apical meristems
• multicellular embryos dependent on the parent plant
• alternation of generations
• sporangia that produce walled spores
• gametangia that produce gametes
• Apical meristems, localized regions of cell
division at the tips of shoots and roots.
• Alternation of
generations
• Gametophyte- haploid
cells, produces: gametes
(egg and sperm.)
• Sporophyte- diploid cells,
produces: haploid spores.
• A spore is a reproductive
cell that can develop into a
new organism without
fusing with another cell.
• Sporangia- are found on the
sporophyte and haploid
produce spores by meiosis.
• Spores are covered by a
polymer called
sporopollenin, the most
durable organic material
known.
• Gametangia- produces gametes
• Archegonium- a female gametangium, produces
a single egg cell in a vase-shaped organ.
Antheridia- a male gametangia, produces many sperm
cells that are released to the environment.
• Cuticle- covers leaves with polyesters and
waxes.
• protects the plant from microbial attack.
• waterproofing to prevent excessive
water loss.
• Stomata- pores in the epidermis of leaves allow
the exchange of carbon dioxide and oxygen
between the outside air and the leaf interior.
• Xylem- Tube-shaped cells carry water and
minerals up from roots. Cells are dead.
• Phloem- is a living tissue in which nutrientconducting cells arranged into tubes distribute
sugars, amino acids, and other organic products.
The Origin of Land Plants
Land plants evolved from charophycean
algae over 500 million years ago
• The oldest known traces of land plants are found
in mid-Cambrian rocks from about 550 million
years ago.
Bryophytes- The three phyla of bryophytes
are mosses, liverworts, and hornworts
• Bryophytes are represented by three phyla:
• phylum Hepatophyta - liverworts
• phylum Anthocerophyta - hornworts
• phylum Bryophyta - mosses
The gametophyte is the dominant
generation in the life cycles of
bryophytes
• Sporophytes are
smaller and
present only part
of the time.
• Gametophoresgenerate
gametes
• Bryophytes are anchored by tubular cells or
filaments of cells, called rhizoids.
• not composed of tissues
• lack specialized conducting cells
• do not play a primary role in water and mineral
absorption.
• The gametophytes of hornworts and some
liverworts are flattened and grow close to the
ground.
Bryophyte sporophytes disperse
enormous numbers of spores
Bryophytes provide many ecological and
economic benefits
• Wet regions dominated by Sphagnum or peat moss are
known as peat bogs.
• Carbon reservoirs- stabilizes atmospheric carbon
dioxide levels.
• Used in the past as diapers and a natural antiseptic
material for wounds.
A diversity of vascular plants evolved over
400 million years ago
• Cooksonia, an extinct plant over 400 million
years old, is the earliest known vascular plant.
Pteridophytes: Seedless Vascular Plants
• phylum Lycophyta - lycophytes
• phylum Pterophyta - ferns, whisk ferns, and horsetails
A sporophyte-dominant life cycle
evolved in seedless vascular plants
• The leafy fern plants are sporophytes.
• The gametophytes are tiny plants that grow on or just
below the soil surface.
A homosporous sporophyte produces a single type of
spore.
• A heterosporous sporophyte produces two kinds
of spores.
• Megaspores develop into females gametophytes.
• Microspores develop into male gametophytes.
Lycophyta and Pterophyta are the two
phyla of modern seedless vascular plants
• Phylum Lycophyta - Modern lycophytes are relicts
of a far more eminent past.
• By the Carboniferous period, lycophytes existed as
either small, herbaceous plants or as giant woody trees
with diameters of over 2m and heights over 40m.
• The giant lycophytes thrived in warm, moist swamps,
but became extinct when the climate became cooler
and drier.
• The smaller lycophytes survived and are represented by
about 1,000 species today.
• The phylum Pterophyta:
• Psilophytes, the whisk ferns
• Sphenophytes are commonly
called horsetails because of
their often brushy appearance.
• Ferns first appeared in the Devonian and have
radiated extensively until there are over 12,000
species today.
• Ferns produce clusters of sporangia, called sori, on
the back of green leaves (sporophylls) or on
special, non-green leaves.
Overview of Seed Plant Evolution
(1) the evolution of seeds, which lead to the gymnosperms
and angiosperms, the plants that dominate most modern
landscapes
(2) the emergence of the importance of seed plants to
animals, specifically to humans.
Agriculture, the cultivation and harvest of plants (primarily
seed plants), began approximately 10,000 years ago in
Asia, Europe, and the Americas.
• Seed plants are vascular plants that produce seeds.
• Important reproductive adaptations:
• continued reduction of the gametophyte
• the advent of the seed
• the evolution of pollen.
1. Reduction of the gametophyte continued
with the evolution of seed plants
• The gametophytes of seed plants are even more
reduced than those of seedless vascular plants.
2. Seeds became an important means of
dispersing offspring
• A seed consists of a sporophyte embryo packaged
along with a food supply within a protective coat.
• All seed plants are heterosporous, producing two
different types of sporangia that produce two types
of spores (megaspores and microspores).
• Layers of sporophyte tissues, integuments,
envelop and protect the megasporangium.
• An ovule consists of integuments, megaspore, and
megasporangium.
3. Pollen eliminated the liquid-water
requirement for fertilization
• They are carried away by wind or animals until
pollination occurs when they land in the vicinity of
an ovule.
Gymnosperms
• The four phyla of
gymnosperms
are ginko,
cycads,
gnetophytes, and
conifers
• Phylum Ginkgophyta consists of only a single
extant species, Ginkgo biloba.
• Ornamental species has fanlike leaves that turn gold
before they fall off in the autumn.
• Landscapers usually only plant male trees because the
seed coats on female plants decay, they produce a
repulsive odor.
• Cycads (phylum Cycadophyta) superficially
resemble palms.
• Palms are actually flowering plants.
• Phylum Gnetophyta consists of three very
different genera.
• Weltwitschia plants, from deserts in southwestern
Africa, have straplike leaves.
• Gentum species are tropical trees or vines.
• Ephedra (Mormon tea) is a shrub of the American
deserts.
The life cycle of a pine demonstrates
the key reproductive adaptations of
seed plants
• increasing dominance of the sporophyte
• seeds as a resistant, dispersal stage
• pollen as an airborne agent bringing gametes together.
• Conifers, are heterosporous, developing male and
female gametophytes from different types of spores
produced by separate cones.
• Small pollen cones produce microspores that develop into
male gametophytes, or pollen grains.
• Larger ovulate cones make megaspores that develop into
female gametophytes.
• The conifers, phylum Coniferophyta, is the
largest gymnosperm phylum. Conifers include
pines, firs, spruces, larches, yews, junipers, cedars,
cypresses, and redwoods.
• Much of our lumber and paper comes from the
wood (actually xylem tissue) of conifers.
• Coniferous trees are amongst the largest and oldest
organisms of Earth.
• Redwoods from northern California can grow to heights
of over 100m.
• One bristlecone pine, also from California, is more than
4,600 years old.
Angiosperms (Flowering Plants)
• There are abut 250,000 known species of
angiosperms.
• All angiosperms are placed in a single phylum, the
phylum Anthophyta.
Systematists are identifying the angiosperm
clades
• As late as the 1990s, most plant taxonomists divided
the angiosperms into two main classes, the monocots
and the dicots.
• Recent systematic analyses have upheld the
monocots as a monophyletic group.
• However, molecular systematics has indicated that
plants with the dicot anatomy do not form a
monophyletic group.
• One clade, the eudicots, does include the majority of
dicots.
One
cotyledon
Two
cotyledon
The flower is the defining reproductive
adaptation of angiosperms
• The flower is an
angiosperm structure
specialized for
reproduction.
• A flower is a
specialized shoot with
four circles of
modified leaves:
sepals, petals, stamens,
and carpals.
• The sepals at the base of the flower are modified
leaves that enclose the flower before it opens.
• The petals lie inside the ring of sepals.
• Stamens, the male reproductive organs, are the
sporophylls that produce microspores that will give
rise to gametophytes.
• A stamen consists of a stalk (the filament) and a terminal sac (the anther) where
pollen is produced.
• Carpals are female sporophylls that produce
megaspores and their products, female
gametophytes.
• At the tip of the carpal is a sticky stigma that receives pollen. A style
leads to the ovary at the base of the carpal. Ovules and, later, seeds are
protected within the ovary.
Fruits help disperse the seeds of
angiosperms
• A fruit is a mature ovary.
• As seeds develop from ovules after fertilization, the wall
of the ovary thickens to form the fruit.
• Fruits protect dormant seeds and aid in their dispersal.
The life cycle of an angiosperm is a highly
refined version of the alternation of
generations common in plants
• All angiosperms are heterosporous, producing
microspores that form male gametophytes and
megaspores that form female gametophytes.
• The immature male gametophytes are contained within
pollen grains and develop within the anthers of stamens.
• Each pollen grain has two haploid cells.
• Ovules, which develop in the ovary, contain the female
gametophyte, the embryo sac.
• It consists of only a few cells, one of which is the egg.
Angiosperms and animals have shaped one
another’s evolution
• Ever since they colonized the land, animals have
influenced the evolution of terrestrial plants and vice
versa.
• This type of mutual evolutionary influence between
two species is termed coevolution.
Plants and Human Welfare
Agriculture is based almost entirely on
angiosperms
Plant diversity is a nonrenewable resource
• Almost all of our food is based on cultivation of only
about two dozen species.
• More than 25% of
prescription drugs
are extracted from
plants, and many
more medicinal
compounds were
first discovered in
plants and then
synthesized
artificially.