Chapter 29
Plant Diversity I:
How Plants Colonized Land
Figure 29.2 Charophyceans: Chara (top), Coleochaete orbicularis (bottom)
Figure 3.2 Water transport in plants
Figure 29.14 Three clades that are candidates for designation as the plant kingdom
I. Overview of land plant evolution
A. Four main groups of land plants
•Bryophytes
•Pteridophytes
•Gymnosperms and
•Angiosperms
 Groups are distinguished from algae by reproduction (life
cycle) that involves the development of a multi-cellular
embryo attached to the mother plant for its protection and
nourishment.
1. Bryophytes – liverworts, hornworts, mosses
- Bryophytes have no vascular tissues.
- The rest three groups are all vascular plants.
- Vascular plants have cells that are joined to produce
tubes that transport water and nutrients throughout the
plant.
- Bryophytes live in damp/moist environments and are small
so they don’t need vascular tissue. They are sometimes
called non-vascular plants.
- Algae that we saw in last chapter live in water and don’t
need vascular tissue because nutrients come from
surrounding water.
The vascular plants are, in order of their evolution:
2. Pteridophytes – ferns, horsetails, lycophytes
a. seedless plants
3. Gymnosperms – conifers, ginkgo, cycads, gnetopsids
a. early seed plants
b. produce naked seeds
4. Angiosperms – flowering plants
a. seeds protected by growing in ovaries
b. majority of modern plants are in this group
Some highlights
of plant evolution
B. Charophyceans - green algae most closely related to
land plants.
1. Both charophyceans and land plants are
• multi-cellular,
• eukaryotic,
• photoautotrophs.
2. Both have “rosette cellulose-synthesizing complexes” –
rose-shaped arrays of proteins that synthesize the cellulose
components that make up plant cell walls.
 Because all these features are shared between the
groups, both land plants and charophyceans must have a
common ancestor.
Charophyceans,
closest algal relatives
of the plant kingdom
In order to grow on land, the land plants needed to evolve
terrestrial adaptations to survive.
C. Terrestrial adaptations can be used to distinguish land
plants from charophycean algae. These adaptations are:
1. Apical meristems
2. Multi-cellular, dependent embryos
3. Alternation of generations
4. Spore walls contain sporopollenin
5. Multi-cellular gametangia
1. Apical meristems
a. localized areas of cell division at tips of roots and shoots.
Apical meristems of plant shoots and roots
2. Multi-cellular, dependent embryos
a. Embryo develops within female tissue; female plant
provides nutrition (sugars, proteins).
b. Placental transfer cells that enhance the transfer of
nutrients from the parent to the embryo.
Figure 29.5 (p. 579) – Placental transfer cell in a liverwort
(a bryophyte)  See Text book.
3. Alternation of generations
Two multi-cellular body forms:
a. Gametophyte (haploid) that produces
gametes. Gametes fuse to form zygotes that develop
into…
b. Sporophytes (diploid) that produce spores. Spores are
haploid cells that can develop into a new organism without
fusing with another cell.
Alternation of
generations: a
generalized
scheme
4. Spore walls contain sporopollenin
a. Sporopollenin is a polymer that makes the walls of plant
spores very tough and resistant to harsh conditions.
b. Sporopollenin is the most durable organic material known.
c. Spores are produced by sporangia (cells in the
sporophyte) through the process of meiosis.
d. Durable spores are an adaptation for surviving on land.
•Can withstand long periods of adverse conditions.
•Easily transported by wind and water.
A fern spore
5. Multi-cellular gametangia
a. Gametangia are the gametophyte forms of bryophytes,
pteridophytes, and gymnosperms. Gametes are produced
within these organs.
b. Female gametangia are called archegonia  (produce
and retain egg cells)
c. Male gametangia are called antheridia  (produce
sperm)
Gametangia
6. Other terrestrial adaptations common to many land plants
a. Epidermis covered by a waxy cuticle to prevent excess
loss of water. Pores (stomata) in cell layer can be opened
and closed to allow O2 out and CO2 in.
Cuticle of a stem from Psilotum (a pteridophyte).
b. Except for bryophytes, land plants have vascular tissue in
roots, stems, and leaves.
- Xylem consists of dead cells that carry water and nutrients
from roots to the rest of the plant.
- Phloem consists of living cells that distribute sugars and
amino acids throughout the plant.
Xylem and
phloem in
the stem of
Polypodium,
a fern (a
pteridophyte)
II. Origin of land plants
A. Theory is that land plants evolved from charophycean
algae over 500 million years ago.
Evidence:
1. Homologous chloroplasts
2. Homologous cell walls made of cellulose
3. Homologous peroxisomes
4. Similar DNA sequences
B. Alternation of generations in plants may have originated
by delayed meiosis
Zygote  Sporophyte  Many, many spores
1. Occurs on land because it’s more difficult to produce
zygotes. (No water for swimming sperm)
2. By producing sporophyte, many gametophytes can be
produced from one zygote because many, many spores are
produced. This maximizes output of sexual reproduction.
C. Adaptations to shallow water pre-adapted plants for living
on land
1. Charophycean algae inhabit shallow waters and need to
survive when water levels drop. Lead to increasing ability to
survive entirely on dry land.
III. Bryophytes
A. Gametophyte is the dominant generation in the life
cycles of bryophytes
Bryophytes
B. Life cycle of bryophytes
1. Bryophyte sporophytes produce and disperse huge
numbers of spores.
The life cycle of
Polytrichum, a moss
C. Ecological and economic benefits of bryophytes
1. Bryophytes were the world’s only plants for 100 million
years.
2. Peat bogs are made mostly of moss called
sphagnum. They contain 400 billion tons of carbon and cut
down the amount of greenhouse gases. Peat is harvested,
dried, and used as a fuel.
3. Sphagnum is harvested for use as a soil conditioner and
plant packing material.
Sphagnum, or peat moss
IV. Origin of vascular plants
- Pteridophytes = ferns
- Gymnosperms = fir trees
- Angiosperms = flowering plants
A. Vascular plants evolved additional terrestrial adaptations
1. Xylem and phloem
2. Dominant sporophyte generation independent of the
gametophyte  Different from the bryophytes
B. Cooksonia evolved over 400 million years ago  oldest
known vascular plant
Cooksonia, a vascular plant of the Silurian
V. Pteridophytes: seedless vascular plants
Examples of pteridophytes (seedless vascular plants) – next
page…………….
A. Pteridophytes provide clues to evolution of roots and
leaves
1. There is evidence that roots evolved from subterranean
portions of stems.
2. There are two types of leaves:
a. Leaves of lycophytes are microphylls. Microphylls are
small leaves with a single, unbranched vein.
b. Leaves of other modern vascular plants are
megaphylls. Megaphylls are typically larger and have a
branched vascular system.
Hypotheses for the evolution of leaves
B. Sporophyte-dominant life cycle evolved in seedless
vascular plants (Pteridophytes)
1. Alternation of generations
2. Dominant sporophyte versus dominant gametophyte in
bryophytes.
3. Plants are dispersed to new environments as spores; no
seeds present
The life
cycle of a
fern
Figure 29.24b Fern sporophyll, a leaf specialized for spore production
C. Importance of Pteridophytes
1. Dominant plants in Carboniferous period
2. Extensive beds of coal from these plants
Artist’s conception of a Carboniferous forest based on
fossil evidence