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Plants without Seeds:
From Sea to Land
29
Plants without Seeds: From Sea to Land
• The Plant Kingdom
• The Conquest of the Land
• The Nontracheophytes: Liverworts, Hornworts,
and Mosses
• Introducing the Tracheophytes
• The Surviving Nonseed Tracheophytes
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The Plant Kingdom
• A plant is a photosynthetic eukaryote that uses
chlorophylls a and b, stores carbohydrates, and
develops from an embryo protected by tissues of
the parent plant.
• Because of their development from embryos,
plants are sometimes referred to as
embryophytes.
• The kingdom Plantae is monophyletic, forming a
single branch of the evolutionary tree.
Figure 29.1 What Is a Plant?
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The Plant Kingdom
• The surviving members of the kingdom Plantae
fall naturally into ten phyla.
• The seven plant phyla whose members possess
well-developed vascular systems are called the
tracheophytes.
• The remaining three phyla (liverworts, hornworts,
and mosses) lack tracheids and are collectively
referred to as the nontracheophytes.
Table 29.1 Classification of Plants (Part 1)
Table 29.1 Classification of Plants (Part 2)
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The Plant Kingdom
• Alternation of generations is a universal feature
of the life cycles of plants.
• This life cycle includes both multicellular diploid
individuals and multicellular haploid individuals.
• Gametes are produced by mitosis, not meiosis.
Meiosis produces spores that develop into
multicellular haploid individuals.
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The Plant Kingdom
• The multicellular, diploid plant is called the
sporophyte.
• Cells contained in the sporangia on the sporophyte
produce haploid, unicellular spores by meiosis.
• The multicellular, haploid plant formed by mitosis
and cytokinesis of a spore is called the
gametophyte.
• The gametophyte produces haploid gametes.
• The fusion of two gametes results in the formation
of a diploid cell, the zygote, and the cycle repeats.
Figure 29.2 Alternation of Generations
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The Plant Kingdom
• The sporophyte generation extends from the
zygote through the adult, multicellular, diploid
plant.
• The gametophyte generation extends from the
spore through the adult, multicellular, haploid
plant to the gamete.
• Some protist life cycles also feature alternation of
generations, suggesting that the plants arose from
one of these protist groups.
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The Plant Kingdom
• Evidence indicates that the closest living relatives
of the plants are a group of green algae called
charophytes.
• It has not been determined which charophyte
clade (stoneworts or Coleochaete) is the true
sister group to the plants.
• The ancestral green algae lived at the margins of
ponds or marshes. From these marginal habitats,
early plants made the move onto land.
Figure 29.3 The Closest Relatives of Land Plants
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The Conquest of the Land
• Plants or their immediate ancestors pioneered
and modified the terrestrial environment, first
invading 400–500 mya.
• The availability of water is a key difference
between the aquatic and terrestrial environments.
• Land plants had to adapt to new challenges in the
terrestrial environment, such as obtaining water,
dealing with gravity, and dispersing gametes.
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The Conquest of the Land
• Some adaptations to life on land:
 The cuticle, a waxy covering that prevents drying
 Gametangia, cases that enclose gametes and
prevent drying
 Embryos, young sporophytes contained within a
protective structure
 Pigments that afford protection against
mutagenic ultraviolet radiation
 Thick spore walls to prevent drying and resist
decay
 A mutualistic association with a fungus that
promotes nutrient uptake from the soil
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The Conquest of the Land
• The first plants lacked vascular tissue.
• Some of the mosses, which are nontracheophytes,
have a small amount of simple conducting tissue.
• The true tracheophytes, which have specialized
conducting cells called tracheids, arose later.
Figure 29.4 From Green Algae to Plants
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The Conquest of the Land
• The nontracheophytes have developed ways to
obtain water and minerals in the absence of a
vascular system:
 Many grow in dense masses through which
water can move by capillary action.
 They have leaflike structures that catch and
hold water that splashes onto them.
 They are small enough that minerals can be
distributed evenly by diffusion.
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The Conquest of the Land
• The tracheophytes have a well-developed vascular
system which consists of two specialized tissues:
 Phloem conducts products of photosynthesis
from sites where they are produced to sites
where they are used or stored.
 Xylem conducts water and minerals from the soil
to the aerial parts of the plants. Xylem, stiffened
by a substance called lignin, also provides
support in the terrestrial environment.
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The Nontracheophytes:
Liverworts, Hornworts, and Mosses
• The nontracheophytes usually grow in dense
mats in moist habitats and are generally small in
size.
• Layers of maternal tissue prevent loss of water
from the embryo.
• The nontracheophytes have a thin cuticle, though
it is not highly effective in retarding water loss.
• The nontracheophytes are widespread across six
continents.
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The Nontracheophytes:
Liverworts, Hornworts, and Mosses
• In nontracheophytes, the familiar green structure
visible to the naked eye is the gametophyte.
• A nontracheophyte sporophyte produces
unicellular, haploid spores as products of meiosis
within a sporangium or capsule.
• The spore germinates and gives rise to a
multicellular, haploid gametophyte whose cells
contain chloroplasts.
Figure 29.5 A Nontracheophyte Life Cycle
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The Nontracheophytes:
Liverworts, Hornworts, and Mosses
• Gametangia are specialized sex organs where
gametes are formed.
• The archegonium is a multicellular, flask-shaped
female sex organ with a long neck and a swollen
base that contains a single egg.
• The antheridium is a male sex organ in which
sperm are produced in large numbers.
• The sporophyte produces a sporangium, or
capsule, within which meiotic divisions produce
spores and thus the next gametophyte
generation.
Figure 29.6 Sex Organs in Plants
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The Nontracheophytes:
Liverworts, Hornworts, and Mosses
• Liverworts, phylum Hepatophyta, may be the
most ancient surviving plant clade.
• Rhizoids are water-absorbing filaments that are
found on the lower surfaces of the simplest
liverwort gametophytes.
• Liverwort sporophytes have a stalk that connects
the capsule and the foot. This capsule can
elongate to raise the capsule above ground level,
aiding in the dispersion of spores.
• Other liverworts utilize springlike structures to
disseminate their spores.
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The Nontracheophytes:
Liverworts, Hornworts, and Mosses
• The hornworts, phylum Anthocerophyta, along
with the mosses and the tracheophytes, all have
an adaptation to life on land not found in the
liverworts.
• These groups all possess stomata that allow the
uptake of CO2 and the release of O2, but they can
be closed to prevent excessive water loss.
Figure 29.8 A Hornwort
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The Nontracheophytes:
Liverworts, Hornworts, and Mosses
• Two characteristics distinguish hornworts from
liverworts and mosses:
• The cells of hornworts contain a single large,
platelike chloroplast, whereas liverworts and
mosses contain numerous small, lens-shaped
chloroplasts.
• Unlike the moss or liverwort sporophyte, whose
stalk stops growing as the capsule matures, the
hornwort sporophyte has no stalk.
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The Nontracheophytes:
Liverworts, Hornworts, and Mosses
• Cyanobacteria often populate internal, mucilagefilled cavities within hornworts.
• These cyanobacteria are able to fix atmospheric
nitrogen gas into a form that can be used by the
hornwort.
• The exact evolutionary status of hornworts is still
unresolved.
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The Nontracheophytes:
Liverworts, Hornworts, and Mosses
• The phylum Bryophyta (mosses) are probably
sister to the tracheophytes.
• Hydroid cells, found in many mosses, are a likely
progenitor of the water-conducting cells of the
tracheophytes.
• When hydroid cells die, they leave a tiny channel
through which water can flow.
• The sporophytes of mosses and tracheophytes
grow by apical cell division, whereby a region at
the growing tip provides an organized pattern of
cell division, elongation, and differentiation.
Figure 29.5 A Nontracheophyte Life Cycle
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The Nontracheophytes:
Liverworts, Hornworts, and Mosses
• Sporophyte development results in the formation of
an absorptive foot anchored to the gametophyte, a
stalk, and a capsule.
• After meiosis and spore development are complete,
the top of the capsule is shed.
• A series of toothlike structures surrounds the
opening of the capsule and digs into the mass of
spores when the atmosphere is dry.
• When the atmosphere becomes moist, they are
flung out. Thus the spores are dispersed when
conditions favor germination.
Figure 29.9b The Mosses
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Introducing the Tracheophytes
• Although the tracheophytes are a large and
diverse group, their appearance can be attributed
to a single evolutionary event.
• The sporophyte generation of a now-extinct
organism produced a new cell type, called the
tracheid.
• The tracheid is the principal water-conducting
element in the xylem in all tracheophytes except
the angiosperms.
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Introducing the Tracheophytes
• There are seven distinct phyla that are presentday evolutionary descendants of the early
tracheophytes.
• These seven phyla can be sorted into two groups:
those that produce seeds and those that do not.
Figure 29.10 The Evolution of Today’s Plants
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Introducing the Tracheophytes
• The plant kingdom invaded land between 400 and
500 million years ago.
• During the Devonian period the appearance and
proliferation of the club mosses (lycopods),
horsetails, and ferns made the environment more
hospitable to animals.
• Trees became dominant during the Carboniferous
period. The tropical swamp forests would become
coal deposits.
• At the end of the Permian period, the 200-millionyear reign of the lycopod–fern forests came to an
end as they were replaced by forests of seed
plants.
Figure 29.11 An Ancient Forest
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Introducing the Tracheophytes
• The first tracheophytes belonged to the nowextinct phylum Rhyniophyta.
• The rhyniophytes had early versions of the
structural features found in all other tracheophyte
phyla.
Figure 29.12 An Ancient Tracheophyte Relative
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Introducing the Tracheophytes
• The first rhyniophytes were found in Devonian
rocks near Rhynie, Scotland.
• The fossil plants had a simple vascular system of
xylem and phloem. They lacked leaves and roots
but were anchored to the soil by horizontal
portions of stem called rhizomes.
• Inspection of fossil sporangia showed that the
spores were in groups of four.
• Most living nonseed tracheophytes show an
arrangement of spores such as this only in the
sporophyte immediately after meiosis. It was
concluded that the Rhynie fossils must be
sporophytes.
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Introducing the Tracheophytes
• The phylum Lycophyta, the club mosses,
appeared in the Silurian period.
• The phylum Pteridophyta, the ferns, horsetails,
and whisk ferns, appeared in the Devonian
period.
• These new groups had true roots, true leaves,
and a differentiation between two types of spores.
Figure 29.14a Homospory and Heterospory
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Introducing the Tracheophytes
• Plants that bear two distinct types of spores
evolved later, and are said to be heterosporous.
• In heterosporous plants, the megaspore
develops into a larger, specifically female
gametophyte (megagametophyte).
• The microspore develops into the smaller, male
gametophyte (microgametophyte).
• Heterospory evolved independently and
repeatedly, suggesting that it affords selective
advantages.
Figure 29.14b Homospory and Heterospory
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The Surviving Nonseed Tracheophytes
• The nonseed tracheophytes have a large,
independent sporophyte and a small,
independent, short-lived gametophyte.
• The single-celled spore is the the most prominent
resting stage of the life cycle.
• Nonseed tracheophytes must have an aqueous
environment in order to be fertilized by the motile,
flagellated sperm.
• Today, the ferns are the most abundant and
diverse phylum of the nonseed tracheophytes.
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The Surviving Nonseed Tracheophytes
• The club mosses (phylum Lycophyta) have
microphylls, exhibit apical growth, and have roots
that branch dichotomously.
• Sporangia in many club mosses are contained
within conelike structures called strobili, clusters
of spore-bearing leaves inserted between a
specialized leaf and the stem.
• There are both homosporous and heterosporous
species.
• The Lycophyta and the Pteridophyta were the
dominant phyla during the Carboniferous period.
Figure 29.15 Club Mosses
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The Surviving Nonseed Tracheophytes
• The horsetails, whisk ferns, and ferns form a
clade, the phylum Pteridophyta.
• The horsetails (all are genus Equisetum) have
true roots that branch irregularly, and sporangia
on short stalks called sporangiophores.
• The leaves are reduced megaphylls and grow in
whorls.
• Stem growth is from the base of the stem
segments.
Figure 29.16 Horsetails
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The Surviving Nonseed Tracheophytes
• The whisk ferns are two genera of rootless, sporebearing plants, Psilotum and Tmesipteris.
• Psilotum has only minute scales instead of true
leaves.
• Although whisk ferns resemble the most ancient
tracheophytes, they are now considered to be
highly specialized plants that evolved fairly
recently.
Figure 29.17 A Whisk Fern
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The Surviving Nonseed Tracheophytes
• The sporophytes of the ferns typically have true
roots, stems, and leaves.
• The ferns first appeared during the Devonian period.
• About 97 percent of fern species belong to one
clade, the leptosporangiate ferns. These ferns have
sporangia with walls only one cell thick, borne on a
stalk.
• Ferns are characterized by fronds, large leaves with
complex vasculature.
• Sporangia are found on the undersurfaces of the
fronds; in most species, they are clustered in groups
called sori.
Figure 29.19 Fern Sori Are Clusters of Sporangia
Figure 29.18 Fern Fronds Take Many Forms
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The Surviving Nonseed Tracheophytes
• The sporophyte generation dominates the fern life
cycle.
• A spore germinates and forms a heart-shaped
gametophyte, bearing antheridia or archegonia (or
both) on its underside.
• The antheridia release sperm that swim to a
nearby archegonium and fertilize an egg.
• The sperm are guided by chemical attractants
released from the archegonia.
• The resulting diploid embryo forms roots and
fronds, and grows into the familiar sporophyte life
stage.
Figure 29.20 The Life Cycle of a Fern