Chapter 29
Plant Diversity I:
How Plants Colonized Land
I. Overview of land plant evolution
A. Four main groups of land plants
 Groups are distinguished from algae by reproduction (life
cycle) that involves the development of a multicellular
embryo attached to the mother plant for its protection and
nourishment.
B. Charophyceans are the green algae most closely related
to land plants.
1. Both charophyceans and land plants are multicellular,
eukaryotic, photoautotrophs.
2. Both have “rosette cellulose-synthesizing
complexes”. Rosette cellulose-synthesizing complexes
are rose-shaped arrays of proteins that synthesize the
cellulose components that make up plant cell walls.
3. Presence of peroxisome enzymes.
4. Both have similarly flagellated sperm
5. Both use phragmoplasts when creating cell plates.
Because all these features are shared between the
groups, both land plants and charophyceans must have a
common ancestor.
Figure 29.2 (p. 578) – Charophyceans, closest algal
relatives of the plant kingdom. (Fig. 29.3, Ed. 7)
1. Bryophytes – liverworts, hornworts, mosses
Bryophytes do not have vascular tissues.
- The next 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
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
Figure 29.1 (p. 577, Ed. 6; Fig. 29.7, Ed. 7) – Some highlights
of plant evolution.
In order to grow on land, the land plants needed to evolve
terrestrial adaptations to survive (pp. 576-577, Ed. 7).
C. Terrestrial adaptations can be used to distinguish land
plants from charophycean algae. These adaptations are:
1. Apical meristems
a. Apical meristems are localized areas of cell division at tips
of roots and shoots.
Figure 29.3 (p. 579, Ed. 6) – Apical meristems of plant shoots
and roots.
2. Multicellular, dependent embryos
a. Embryo develops within female tissue; female plant
provides nutrition (sugars, proteins).
Figure 29.4 (p. 579, ed. 6; p. 577, ed. 7)
b. Placental transfer cells that enhance the transfer of
nutrients from the parent to the embryo.
Figure 29.5 (p. 579, ed. 6; p. 577, ed. 7)
3. Alternation of generations
Two multicellular 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.
Figure 29.6 (p. 580, ed. 6) – 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.
Figure 29.7 (p. 580, ed. 6) – A fern spore.
5. Multicellular 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)
Figure 29.9 (p. 581) – 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.
Figure 29.10 (p. 581, ed. 6) – 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.
Figure 29.11 (p. 582, ed. 6) – 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.
Charophycean algae inhabit shallow waters and need to
survive when water levels drop. Lead to increasing ability to
survive entirely on dry land.
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.
III. Bryophytes
A. Gametophyte is the dominant generation in the life cycles
of bryophytes
Figure 29.15 (p. 585, ed. 6)
Figure 29.9 (p. 582, ed. 7)
B. Life cycle of bryophytes
Figure 29.16 (p. 586, ed.6) – The life cycle of Polytrichum, a
moss. (Fig. 29.8, p. 581, ed. 7)
1. Bryophyte sporophytes produce and disperse huge
numbers of spores.
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.
Figure 29.19 (p. 588) – 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
Figure 29.20 (p. 589) – Cooksonia, a vascular plant of the
Silurian.
V. Pteridophytes: seedless vascular plants
Figure 29.21 (p. 590, ed. 6) – Examples of
pteridophytes (seedless vascular plants).
Called Pterophytes (Fig. 29.14, p. 587, ed. 7)
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.
Figure 29.22 (p. 591, ed. 6; Fig. 29.13, p. 586, ed. 7) –
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
Figure 29.23 (p. 592, ed. 6; Fig. 29.12, p. 585, ed. 7) – The life
cycle of a fern.
C. Importance of Pteridophytes
1. Dominant plants in Carboniferous period
Figure 29.25 (p. 594, ed. 6; Fig. 29.15, p. 588, ed. 7) – Artist’s
conception of a Carboniferous forest based on fossil
evidence.
2. Extensive beds of coal from these plants