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 O

2

out and CO

2

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
pteridophyt
e)

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