Transcript The Origin of Land Plants

Chap 29: Plant Diversity I:
How Plants Colonized Land
An Overview of Land Plant Evolution
The key to this chapter is that the ability to live on land or begin to inhabit a
drier environment, certain adaptations had to be present or evolve. So one of
your potential essays would be “What adaptations did aquatic plants have to
make to occupy a terrestrial environment?”
Also recall that the land plants evolved from a green algae called
charophyceans.
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Figure 29.1 Some highlights of plant evolution
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Evolutionary adaptations to terrestrial living characterize the four main groups
of land plants
1.
Bryophytes: mosses
a)
Reproduction Adaptation: Offspring develop from embryos that
are attached to the “mother” plant.
i.
Protection
ii.
Nutrition
iii. This is simply exemplified by seeds being produced
“within” the fertilized part of a plant. This occurs in other
land plants as well.
iv. Dominate generation is the gametophyte
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Specialized tissue to transport water and sugar was another adaptation to
land. This vascular tissue is comprised of cells that form tubes for the
transport of these substances.
Most bryophytes lack vascular tissue and many times are called
nonvascular plants. Some bryophytes do have vascular tissue.
2.
Pteriodophyes or ferns
a)
Seedless plants
b) Reproduce by spores requiring a moist environment
The adaptation of seeds was another important feature of “being able to
inhabit land” and lead to two other groups of land plants, the
gymnosperms and angiosperms.
A seed is the embryo surrounded by a food supply.
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3.
Gymnosperms or conifers
a)
4.
Possess seeds that are not enclosed in “chambers” or are naked.
Angiosperms or flowering plants
a)
Seeds are in protected chambers, like at the base of the ovary and
are protected by the developed ovary that surrounds it (like fruit)
Dominate generation in ferns, gymnosperms and angiosperms is the
sporophyte.
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Figure 29.2 Charophyceans: Chara (top), Coleochaete orbicularis (bottom)
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Charophyceans are the green algae most closely related to land plants
•
Features that are shared with algae
•
cell walls made of cellulose
•
presence of chlorophylls a and b
So these characteristics DO NOT define land plants
•
Distinguishing land plant features
• Rosette cellulose-synthesizing complexes: the presence of
specialized proteins in the p. membrane of both land plants and
charophyceans. These proteins synthesize the cellulose in the cell
walls.
•
Other algae have a different (linear) arrangement of these
proteins.
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•
Distinguishing land plant features (cont’d)
• Peroxisomes and contained enzymes: help to break down the
two-carbon units used in photorespiration to carbon dioxide.
• Sperm structure of land plants resembles sperm of
charophyceans.
• Formation of cell plates during cell division are common
between some Charophyceans and land plants.
So what was the essay question you just answered?
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Several terrestrial adaptations distinguish land plants from charophyceans
1.
Apical Meristems
a)
Produce new plant tissue.
b) An apical meristem is a localized area of growth.
c)
2.
Cells produced in these areas will eventually become specialized
for various functions: vascular tissue, roots, leaves.
Multicellular, Dependent Embryos
a)
The protected embryos are in the parental tissue
b) Placental transfer cells: help to deliver nutrients to the embryo
c)
This dependence of the embryo on the parental plant is a common
characteristic of land plants.
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Figure 29.3 Apical meristems of plant shoots and roots
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Figure 29.4 Embryos of land plants: Marchantia (left), Shepherd's purse (right)
Maternal tissue
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3.
Alternation of Generations
a)
Does not occur in the Charophyceans therefore this characteristic
evolved independently in the land plants.
b)
Gametophyte
c)
i.
1n
ii.
Produces the gametes
Sporophyte
i.
2n
ii.
Results from fertilization and mitosis
iii. Meiosis in the sporophyte produces the spores that can
produce another complete organism, usually smaller, the
gametophyte and the gametophyte produces the gametes
and around and around we go.
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Figure 29.6 Alternation of generations: a generalized scheme
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4.
Walled Spores Produced in Sporangia
a)
Sporangia are organs that produce the spores from spore mother
cells via meiosis.
b)
The spores are 1n and will grow into gametophytes
c)
These spore are protected by a substance called sporopollenin.
i.
This substance keeps these spores viable after drought
conditions or long distance travel.
ii.
This is a key adaptation to the land environment.
iii. Charophyceans do not have sporangia and spores do not
have sporopollenin
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5.
Multicellular Gametangia
a)
The gametes produced by bryophytes, ferns and gymnosperms
all are formed in gametangia.
i.
Female gametangia: archegonia
•
ii.
Each archegonia produces a single egg cell
Male gametangia: antheridia
•
Each antheridia produces many sperm
•
These sperm contain flagella to move through the moist
environment and find the archegonia and its egg.
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Figure 29.9 Gametangia: Archegonium of Marchantia (left), Antheridium of a
hornwort (right)
Egg
Sperm cells
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6.
Other Terrestrial Adaptations Common to Many Land Plants
a)
Adaptations for Water Conservation
i.
Cuticle
•
ii.
Protection from bacteria and water loss
Stomata
b) Adaptations for Water Transport
•
c)
Xylem and Phloem
Secondary Compounds as Terrestrial Adaptations
•
Toxic substances for predator defense: alkaloids, tannins.
All taste or smell bad.
•
Flavonoids: protect against UV radiation
•
Lignin
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The Origin of Land Plants
Land plants evolved from charophycean algae over 500 million years ago
•
What evidence do we have that relates Charophyceans, a green algae,
to land plants?
• chloroplasts have chlorophyll b and carotenes as accessory
pigments
•
thylakoids are stacked in disks
• Chloroplastic DNA sequence comparison between Charophycean
and land plants
• Cellulose cell walls are most plant like in terms of the amount of
cellulose and the cell plate making features
•
Presence of peroxisomes and anti-photorespiration enzymes
• Sperm of plants and Charophyceans are closely related in their
structure.
•
rRNA gene sequencing shows similarities.
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Alternation of generations in plants may have originated by delayed meiosis
•
The sporophyte generation undergoes mitosis to produced a new
plant body, etc. Yet it then undergoes meiosis to produce 1n spores.
•
So perhaps a mutation occurred causing meiosis to be delayed thus
producing the 2n sporophyte.
•
This would increase the number of cells undergoing meiosis and
producing 1n spores and thus maximizing the sexual reproductive
opportunities on land.
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Adaptations to shallow water preadapted plants for living on land
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The occasional drying out that accompanied living in shallow water
favored Charophyceans that could survive some drying out.
•
Tougher cell walls of the zygotes were required and the chemical
adaptation of sporopollenin was selected for.
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