Plant Diversity: How Plants Colonized Land
 More than 280,000 species of plants inhabit Earth
today
 Most plants live in terrestrial environments
 Referred to as “land” plants
 Evolved from green algae called charophyceans
 There are four main types of land plants
 Bryophytes
 Most common are mosses
 Pteridophytes
 ferns
 Gymnosperms
 Pines and conifers (cone-bearing plants)
 Angiosperms
 Flowering plants
 Bryophytes are distinguished from algae by several
features derived during evolutionary adaptations to
living on land
 Many are reproductive: offspring develop from a
multicellular embryo that remains attached to the
“mother” plants, which protects and nourishes it
 Bryophytes have not evolved to have vascular tissue
 Pteridophytes, angiosperms, and gymnosperms are
called vascular plants
 Cells are joined into tubes that transport water and
nutrients throughout the plant body
 Pteridophytes are called “seedless” plants because
there is no seed stage in their life cycles
 Gymnosperms and angiosperms are seed plants
 Seed consists of a plant embryo packaged along with a
food supply within a protective coat
 First evolved 360 million years ago
 Early seeds were not enclosed in any specialized
chambers
 Gave rise to the present day gymnosperms
 Gymno = naked; sperma = seed
 Flower evolution led to further diversification of
plants beginning about 130 million years ago
 Flowers are complex reproductive structures that bear
seeds within protective chambers called ovaries
 Gave rise to modern day angiosperms
 Angion = container
Origin of bryophytes from algae
2. Origin of vascular plants
3. Origin of seeds
4. Origin of flowers
1.
 All plants are multicellular, eukaryotic,
photosynthetic autotrophs
 Cell walls made of cellulose, and presence of
chlorophyll a and b
 Plasma membranes are equipped with rose shaped
arrays of proteins that synthesize the cellulose of cell
walls
 Rosette-cellulose-synthesizing complexes
 Several characteristics are present in all land plants
that are absent in their closest algal relatives
 Apical meristems
 Multicellular, dependent embryos
 Alternation of generation
 Walled spores
 Multicellular gametangia
 Because plants cannot move from place to place, the
elongation and branching of their shoots and roots
maximize their exposure to environmental resources
 This growth in length occurs because of apical
meristems
 Localized regions of cellular division at the tips of shoots
and roots
 Cells produced differentiate into the plants’ various
tissues
 Develop within tissues of the female parent
 Parental tissues provide the developing embryo with
nutrients, such as sugars and amino acids
 Have specialized placental transfer cells which
enhance the transfer of nutrients from parent to
embryo
 Known as embryophytes because of this
characteristic
 During the life cycle of all land plants, two
multicellular body forms alternate, each form
producing the other
 Gametophytes are haploid and produce gametes
 Fertilization of gametes forms diploid zygotes that
divide and create sporophytes
 Diploid, and produce haploid cells called spores
 Reproductive cell that can develop into a new organism
without fusing with another cell
 Plant spores are haploid reproductive cells that have
the potential to grow into multicellular, haploid
gametophytes by mitosis
 A polymer called sporopollenin, the most durable
organic matter known, makes the walls of spores very
tough an resistant to harsh environments
 Makes it possible for wind-carried spores to disperse
through dry air without harm
 Multicellular organs called sporangia produce spores
 Within it, spore mother cells undergo meiosis and
generate the spores
 The gametophyte forms within multicellular organs
called gametangia
 Females are called archegonia that is vase shaped and
produces a single egg cell and retains the egg within the
base of the organ
 Males are called antheridia produce sperm cells that
are released in the environment when mature
 Sperm bear flagella and swim through water droplets or water
film to eggs
 Eggs are fertilized within the archegonia, where the
zygote begins to develop into an embryo
 Water conservation
 Water transport
 Secondary compounds
 The epidermis of leaves and other aerial parts of most
land plants is coated in a cuticle, which consists of
waxes
 Helps protect the plant from microbial attack and acts
as waterproofing that helps prevent excessive water
loss from the aboveground plant parts
 Has pores called stomata that support photosynthesis
by allowing the exchange of carbon dioxide and oxygen
 Also the main avenues that water exits by evaporation
 Changes in the shapes of the cells that border the
stomata can close the pores to minimize water loss
 Except for bryophytes, land plants have true roots,
stems and leaves which are defined by the presence of
vascular tissue
 Two types of vascular tissue are:
 Xylem carry water and minerals up from the roots
 These conducting cells are actually dead, with only their walls
remaining to provide a system of microscopic water pipes
 Phloem is living with nutrient-conducting cells
arranged in tubes that distribute sugars, amino acids,
and other organic products throughout the plant
 Land plants produce many unique molecules called
secondary compounds
 They are named because they are products of
“secondary” metabolic pathways that produce lipids,
carbohydrates, and other compounds
 Alkaloids, tannins, and phenolics such as flavonoids
 Have bitter tastes, strong odors, or toxic effects that help
defend agains herbivores
 Flavonoids absorb UV radiation
 Phenolics deter attack by pathogenic microbes
 Lignin hardens the cell walls of “woody” tissues
 Lignin Stone: a threat to security?
 Homologous chloroplasts
 Homologous cell walls
 Homologous peroxisomes
 Homologous sperm
 Molecular systematics
 Compared key nuclear genes
 Fossils place plants on land over 500 million years ago
 Oldest known are encased in rocks about 550 million
years old
 Petrified wood:
 There are three separate phlya of bryophytes:
 Liverworts
 Hornworts
 Mosses
 “Wort” means herb, named so because they are small
nonwoody plants named for their shapes
 Spores are dispersed to a favorable habitat, such as
moist soil or tree bark, they may germinate and grow
into gametophytes by mitosis
 Germinating spores produce a mass of green,
branched, one-cell thick filaments known as
protonema
 Sometimes mistaken as algae
 Have a large surface area that enhances absorption
 Produce buds that generate the gametophore which
is the mature, gamete producing structure
 Tissues are only a couple cells thick because they lack
the vessels to aid in water and nutrient transport
 Most grow close to the ground, anchored by rhizoids,
delicate and colorless strands
 Mosses are able to exist in very harsh environments,
and because of this, they are crucial to many
ecosystems
 One moss genus creates deposits of undecayed
material known as peat; also called “peat moss”
 Reduce bacterial activity
 Once used as wound packing
 Now used as soil conditioner
 World’s peatlands store an estimated 400 billion tons of
organic carbon
 Stabilize atmospheric CO2 concentrations and climate
 Evolved over 408 million years ago; only was 50
centimeters tall
 Diploid version of plant is larger, and remains for most
of the time
 Homosporous plants produce a single type of spore
 Produces bisexual gametophytes
 Heterosporous plants produce two kinds of spores:
 Megaspores develop female gametophytes
 Microspores develop male gametophytes
 Many are tropical epiphytes- plants that live on other
plants but are not parasites
 Others grow close to the ground on forest floors in
temperate regions
 Groups of specialized leaves called sporophylls are
clustered to form club-shaped cones
 When they are mature, the spores are released in
clouds
 Early photographers loved lycophytes back in the day!
 Their spores were oil-rich and flammable
 Photographers would ignite the spores to provide a
flash of light
 12,000 species
 Most widespread and diverse pteridophytes today
 Leaves are known as fronds
 Produce clusters of sori on the underside of the leaves
 Groups of spore producers
 Arrangement is useful in identification
 Equipped with spring-like devices that catapult spores
several meters
 Most of the coal used today was from a time period
where there were no known animals, only early plants
and the earth was flooded by shallow swamps
 Swamps were later covered by the sea and sediment
piled on top of it
 Heat and pressure converted this peat to coal, a fossil
fuel