Evolution of Plants
18.1 Adapting to Terrestrial Living
~ 288,700 species of plants are now in existence
These are terrestrial
However, green algae, the likely ancestors of plants,
are aquatic and not well adapted to living on land
Three challenges had to be overcome
1. Mineral absorption
2. Water conservation
3. Reproduction on land
Mineral Absorption
Plants require relatively large amounts of six
inorganic minerals
Nitrogen, potassium, calcium, phosphorus,
magnesium, sulfur
Plants absorb these materials through their roots
The first plants developed symbiotic associations
with fungi
These mycorrhizae enabled plants to extract
minerals from rocky soil
Water Conservation
To avoid drying
out, plants have a
watertight outer
covering, termed
the cuticle
Stomata
(singular, stoma)
are pores in the
cuticle that allow
gas and vapor
exchange
Fig. 18.2
Regulate opening
and closing of the
stoma
Reproduction on Land
Spores developed
as a means to
protect gametes
from drying out on
land
In a plant life cycle,
there is alternation
of generations
Diploid with
haploid
Fig. 18.3 Generalized plant life cycle
The diploid generation is called the sporophyte
The haploid generation is called the gametophyte
As plants evolved, the sporophyte tissue dominated
Fig. 18.4 Two types of gametophytes
Pine
Moss
Primitive plant
Mostly
gametophyte
Vascular plant
Gametophyte
barely visible
18.2 Plant Evolution
Four key evolutionary innovations serve to trace the
evolution of the plant kingdom
1. Alternation of generations
Plants developed a more dominant diploid
phase of the life cycle
2. Vascular tissue
Transports water and nutrients throughout the
plant body
Thus plants were able to grow larger and in
drier conditions
18.2 Plant Evolution
Four key evolutionary innovations serve to trace the
evolution of the plant kingdom
3. Seeds
Protected the embryo, thus allowing plants to
dominate their terrestrial environments
4. Flowers and fruits
Flowers protected the egg and improved the
odds of its fertilization
Fruits surrounded the seeds and aided in their
dispersal
Fig. 18.5 The evolution of plants
18.3 Nonvascular Plants
Only two phyla of living plants lack a vascular system
The simplest of all
living plants
Liverworts (Phylum
Hepaticophyta)
Hornworts (Phylum
Anthocerophyta)
Mosses (Phylum Bryophyta) were the first
plants to evolve strands of specialized
conduction cells
The conducting cells do not have
specialized wall thickenings
Thus, a primitive vascular system, at the
most
Hair-cup moss, Polytrichum
18.4 The Evolution of Vascular Tissue
The first vascular plant appeared approximately
430 million years ago (mya)
Early plants became
successful colonizers
of land through the
development of
vascular tissue
Efficient water- and
food-conducting
systems
Fig. 18.8 The vascular system of a leaf
18.5 Seedless Vascular Plants
Two phyla of modern-day vascular
plants lack seeds
Ferns (Phylum Pterophyta)
Club mosses (Phylum Lycophyta)
Both have free-swimming sperm that
require free water for fertilization
By far, the largest group are ferns
~ 12,000 living species
18.6 Evolution of Seed Plants
Seeds are embryo covers that protect the embryonic
plant at its most vulnerable stage
Seed plants produce two kinds of gametophytes
Male gametophytes
Called pollen grains
Arise from microspores
Female gametophytes
Contains the egg
Develops from a megaspore
produced within an ovule
Pollination is the transfer of pollen by insects, winds
Thus, there is no need for free water for fertilization
A seed has three parts
1. A sporophyte plant embryo
2. A source of food for the embryo called endosperm
3. A drought-resistant protective cover
Fig. 18.12
Used for food storage
Seeds have greatly improved the adaptations of
plants to living on land
1. Dispersal
Facilitate migration dispersal
2. Dormancy
Postpone development until conditions are favorable
3. Germination
Permit embryonic development to be synchronized
with habitat
4. Nourishment
Offer an energy source of young plants
All seed plants are derived from a single common
ancestor
There are five living phyla
Four are gymnosperms
Ovules not completely
enclosed by sporophyte
at time of pollination
Fifth is angiosperms
Ovules completely
enclosed by a vessel of
sporophyte tissue, the
carpel, at time of pollination
Cycad
Fig. 18.11
18.7 Gymnosperms
Gymnosperms are nonflowering seed plants
They include four phyla
Conifers (Coniferophyta)
Cycads (Cycadophyta)
Gnetophytes (Gnetophyta)
Ginkgo (Ginkgophyta)
Conifers
The most common (and familiar) of the
gymnosperms
Include
Pine, spruce, cedar,
redwood and fir trees
Conifers are trees that
produce their seeds in
cones
Seeds (ovules) develop
on scales within cones
and are exposed at the
Fig. 18.14
time of pollination
Cycads
Fig. 18.15a
Have short stems and
palmlike leaves
The predominant land plant
in the Jurassic Period
Acts like a plant
standing on its head!
Welwitschia
mirabilis
Gnetophytes
The most closely related to
angiosperms
Only three types of plants;
all unusual
Have flagellated sperm
Fig. 18.15b
Ginkgo
Only one living species exists
The maidenhair tree, Ginkgo biloba
Resistant to air
pollution
Reproductive
structures found on
different trees
Have flagellated
sperm
Fig. 18.15c
18.8 Rise of the Angiosperms
Angiosperms comprise 90% of all living plants
> 300,000 species
Virtually all our food is derived, directly or indirectly
from them
In gymnosperm reproduction, pollen grains are
carried passively by the wind
Angiosperms have evolved a more direct way of
transferring pollen
Induce animals to carry it for them
How?
Flowers!
18.8 Rise of the Angiosperms
Flowers are the reproductive organs of angiosperms
A flower employs bright colors to attract insects and
nectar, to induce the insects to enter the flower
There they are coated with pollen grains, which
they carry with them to other flowers
Fig. 18.17 An
angiosperm flower
Petals
Fused
carpel
Stamens
Geranium
A flower consists of four concentric circles, or whorls,
connected to a base called the recepatcle
Outermost whorl (Sepals)
Protects flower from physical damage
Second whorl (Petals)
Attracts pollinators
Third whorl (Stamens)
Produces pollen grains in the anther
Innermost whorl (Carpel)
Produces eggs in the ovary
Rising from the ovary, is a slender stalk, the style, with
a sticky tip, the stigma
18.9 Why Are There
Different Kinds of Flowers?
Different insect pollinators are attracted to specific
types of flowers
The most numerous insect pollinators are bees
Bees are first attracted by the odor of nectar
They then focus on the flower’s color and shape
Bee-pollinated flowers are usually yellow or
blue
Fig. 18.18 How a bee sees a flower
Ludwigia peruviana
Normal light
Bee covered
in pollen
Light emphasizing
UV reflection
Other pollinators include
Moths
Attracted to scented, white or pale-colored flowers
Flies
Pollen
on beak
Attracted to foulsmelling brown flowers
Hummingbirds
Attracted to red flowers
These are not
typically visited by
insect pollinators
Fig. 18.19
18.11 Improving Seed Dispersal:
Fruits
A fruit is a mature ripened ovary containing fertilized
seeds
Fruits aid in the dispersal of seeds to new habitats
By animals
By water
By wind
Fig. 18.22 Different ways of dispersing fruit
Maples
Berries
Coconuts
By animals
By water
By wind