Chapter 29 Plant Diversity I: How Plants Colonized Land
Overview: The Greening of Earth
• For the first 3 billion years of Earth’s history, the land was lifeless.
• Thin coatings of cyanobacteria existed on land about 1.2 billion years
• About 500 million years ago, plants, fungi, and animals joined them.
• More than 290,000 species of plants inhabit Earth today.
• Most plants live in terrestrial environments, including deserts,
grasslands, and forests.
• Some species, such as sea grasses, have returned to aquatic
• The presence of plants has enabled other organisms to survive on
• Plant roots have created habitats for other organisms by
• Plants are the source of oxygen and the ultimate provider of
food for land animals.
Concept 29.1 Land plants evolved from green algae
• Researchers have identified a lineage of green algae called
charophyceans as the closest relatives of land plants.
• Many key characteristics of land plants also appear in a variety of
• Plants are multicellular, eukaryotic, photosynthetic autotrophs.
• But red, brown, and some green algae also fit this description.
• Plants have cell walls made of cellulose.
• So do green algae, dinoflagellates, and brown algae.
• Plants have chloroplasts with chlorophyll a and b. • So do green algae, euglenids, and a few dinoflagellates.
• Land plants share four key features only with the charophyceans.
1. The plasma membranes of land plants and charophyceans possess
rosette cellulose-synthesizing complexes that synthesize the cellulose
microfibrils of the cell wall.
• These complexes contrast with the linear arrays of
cellulose-producing proteins in noncharophycean algae.
• Also, the cell walls of plants and charophyceans contain a
higher percentage of cellulose than the cell walls of
2. A second feature that unites charophyceans and land plants is the
presence of peroxisome enzymes to help minimize the loss of organic
products as a result of photorespiration.
• Peroxisomes of other algae lack these enzymes.
3. In those land plants that have flagellated sperm cells, the structure of
the sperm resembles the sperm of charophyceans.
4. Finally, certain details of cell division are common only to land plants
and the most complex charophycean algae.
• These include the formation of a phragmoplast, an
alignment of cytoskeletal elements and Golgi-derived
vesicles, during the synthesis of new cross-walls during
• Over the past decade, researchers involved in an
international initiative called “Deep Green” have
conducted a large-scale study of the major transitions in
• These researchers have analyzed genes from a
wide range of plant and algal species.
• Comparisons of nuclear and chloroplast genes
support the hypothesis that the charophyceans are
the closest living relatives of land plants. • Many charophycean algae inhabit shallow waters at the edges of
ponds and lakes, where they experience occasional drying.
• In such environments, natural selection favors individuals that can
survive periods when they are not submerged in water.
• A layer of a durable polymer called sporopollenin prevents
exposed charophycean zygotes from drying out until they are in
• This chemical adaptation may have been the precursor to the
tough sporopollenin walls that encase plant spores.
• The accumulation of such traits by at least one population of
ancestral charophyceans enabled their descendents—the first land
plants—to live permanently above the waterline.
• The evolutionary novelties of the first land plants opened an expanse
of terrestrial habitat previously occupied only by films of bacteria.
• The new frontier was spacious.
• The bright sunlight was unfiltered by water and plankton.
• The atmosphere had an abundance of carbon dioxide.
• The soil was rich in mineral nutrients.
• At least at first, there were relatively few herbivores or
Concept 29.2 Land plants possess a set of derived terrestrial
• A number of adaptations evolved in plants that allowed them to
survive and reproduce on land.
• What exactly is the line that divides land plants from algae?
• We will adopt the traditional scheme, which equates the kingdom
Plantae with embryophytes (plants with embryos).
• Some botanists now propose that the plant kingdom should be
renamed the kingdom Streptophyta and expanded to include
the charophyceans and a few related groups. • Others suggest the kingdom Viridiplantae, which includes
chlorophytes as well as plants.
• Five key traits appear in nearly all land plants but are absent in the
• We infer that these traits evolved as derived traits of land
• The five traits are:
1. Apical meristems.
2. Alternation of generations.
3. Multicellular embryo that is dependent on the parent plant.
4. Sporangia that produce walled spores.
5. Gametangia that produce gametes.
• In terrestrial habitats, the resources that a photosynthetic organism
requires are found in two different places.
• Light and carbon dioxide are mainly aboveground.
• Water and mineral resources are found mainly in the soil.
• Therefore, plants show varying degrees of structural specialization for
subterranean and aerial organs—roots and shoots in most plants.
• The elongation and branching of the shoots and roots maximize their
exposure to environmental resources.
• This growth is sustained by apical meristems, localized regions of cell
division at the tips of shoots and roots.
• Cells produced by meristems differentiate into various tissues,
including surface epidermis and internal tissues.
Alternation of generations
• All land plants show alternation of generations in which two
multicellular body forms alternate. • This life cycle also occurs in various algae.
• However, alternation of generations does not occur in the
charophyceans, the algae most closely related to land plants.
• In alternation of generations, one of the multicellular bodies is called
the gametophyte and has haploid cells.
• Gametophytes produce gametes, egg and sperm, by mitosis.
• Fusion of egg and sperm during fertilization form a diploid
• Mitotic division of the diploid zygote produces the other multicellular
body, the sporophyte.
• Meiosis in a mature sporophyte produces haploid reproductive
cells called spores.
• A spore is a reproductive cell that can develop into a new
organism without fusing with another cell.
• Mitotic division of a plant spore produces a new multicellular
• Unlike the life cycles of other sexually producing organisms,
alternation of generations in land plants (and some algae) results in
both haploid and diploid stages that exist as multicellular bodies.
• For example, humans do not have alternation of generations
because the only haploid stage in the life cycle is the gamete,
which is single-celled.
Walled spores produced by sporangia
• Plant spores are haploid reproductive cells that grow into
gametophytes by mitosis.
• Sporopollenin makes the walls of spores very tough and
resistant to harsh environments.
• Multicellular organs called sporangia are found on the sporophyte
and produce spores. • Within sporangia, diploid cells called sporocytes undergo meiosis and
generate haploid spores.
• The outer tissues of the sporangium protect the developing spores
until they are ready to be released into the air.
• Plant gametophytes produce gametes within multicellular organs
• A female gametangium, called an archegonium, produces a
single egg cell in a vase-shaped organ.
• The egg is retained within the base.
• Male gametangia, called antheridia, produce and release sperm into
• In many major groups of living plants, the sperm have flagella and
swim to the eggs though a water film.
• Each egg is fertilized within an archegonium, where the zygote
develops into the embryo.
• The gametophytes of seed plants are so reduced in size that
archegonia and antheridia have been lost in some lineages.
Multicellular, dependent embryos
• Multicellular plant embryos develop from zygotes that are retained
within tissues of the female parent.
• The multicellular, dependent embryo of land plants is such a
significant derived trait that land plants are also known as
• The parent provides nutrients, such as sugars and amino acids, to
• The embryo has specialized placental transfer cells that
enhance the transfer of nutrients from parent to embryo.
• These are sometimes present in the adjacent maternal tissues
as well. • This interface is analogous to the nutrient-transferring embryo-
mother interface of placental mammals.
• Additional derived traits have evolved in many plant species.
• The epidermis of many plants has a cuticle consisting of polymers
called polyesters and waxes.
• The cuticle waterproofs the epidermis, preventing excessive
water loss, and offers protection from microbial attack.
• Many land plants produce secondary compounds, so named because
they are the products of secondary metabolic pathways that branch
from primary metabolic pathways.
• Alkaloids, terpenes, and tannins defend against herbivores and
• Flavonoids absorb harmful UV radiation and may act as signals
in symbiotic relationships with beneficial soil microbes.
• Phenolics deter attack by pathogenic microbes.
Land plants have diversified since their origin from algal ancestors.
• Fossils of plant spores have been extracted from 475-million-year-old
rocks in Oman.
• These spores were embedded in plant cuticle material that is similar
to spore-bearing tissue in living plants.
• These fossils clearly belong to plants.
• A 2001 study of the “molecular clock” of plants suggests that the
common ancestor of living plants existed 700 million years ago.
• A 2003 study suggests a new date of 490 to 425 million years,
roughly the same age as the spores found in Oman.
• Land plants can be informally grouped based on the presence or
absence of an extensive system of vascular tissue, cells joined into
tubes that transport water and nutrients throughout the plant body. • Plants that do not have an extensive transport system are
described as “nonvascular plants,” although some mosses do
have simple vascular tissue.
• Nonvascular plants are informally called bryophytes.
• There is some uncertainty about whether or not bryophytes are
monophyletic and represent a clade.
• Vascular plants form a clade consisting of 93% of all land plants.
• Three smaller clades are found within the vascular plants.
• Lycophytes include club mosses and their relatives.
• Pterophytes include the ferns and their relatives.
• These two clades are called the seedless vascular plants.
• A third clade of vascular plants includes the seed plants, the
vast majority of living plants.
• A seed is an embryo packaged with a supply of nutrients within a
• Seed plants can be divided into two groups: gymnosperms and
• Gymnosperms are called “naked seed plants” because their seeds
are not enclosed in chambers.
• Angiosperms are a huge clade including all flowering plants.
Concept 29.3 The life cycles of mosses and other bryophytes are
dominated by the gametophyte stage
• Bryophytes are represented by three phyla:
• Phylum Hepatophyta—liverworts
• Phylum Anthocerophyta—hornworts
• Phylum Bryophyta—mosses
• Note that the name Bryophyta refers only to one phylum, but the
informal term bryophyte refers to all nonvascular plants. • It has not been established whether the diverse bryophytes form a
• Systematists continue to debate the sequence in which the three
phyla of bryophytes evolved.
• Bryophytes acquired many unique adaptations after their evolutionary
split from the ancestors of modern vascular plants.
• They also possess some ancestral traits characteristic of the
• In bryophytes, gametophytes are the largest and most conspicuous
phase of the life cycle.
• Sporophytes are smaller and are present only part of the time.
• Bryophyte spores germinate in favorable habitats and grow into
gametophytes by mitosis.
• The gametophyte is a mass of green, branched, filaments that are
one cell thick, called a protonema.
• A protonema has a large surface area that enhances absorption of
water and minerals.
• In favorable conditions, protonema generate gamete-producing
structures, the gametophores.
• Bryophytes are anchored by tubular cells or filaments of cells, called
• Unlike roots, rhizoids are not composed of tissues, lack
specialized conducting cells, and do not play a primary role in
water and mineral absorption.
• Bryophyte gametophytes