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26 The Plant Kingdom: Seedless Plants
Outline
I.
Plants have adapted to life on land
A.
A cuticle prevents desiccation of plant tissues
B.
Stomata allow diffusion of carbon dioxide into the intercellular spaces of
leaf tissue
C.
Multicellular gametangia have sterile (non-reproductive cells) as well as
gametes
The fertilized egg develops within the female gametangium
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II.
There are four major groups of plants
A.
The bryophytes are seedless and disperse via haploid spores
B.
C.
D.
E.
The ferns, gymnosperms, and flowering plants have vascular tissue
1.
Xylem conducts water and minerals
2.
Phloem conducts dissolved sugars and other materials
3.
Lignin strengthens cell walls
The seedless vascular plants disperse primarily via spores and diversified
during the Silurian and Devonian periods
The gymnosperms are vascular plants that produce seeds, typically in a
cone
The angiosperms are vascular plants which produce seeds after flowering
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III.
The plant life cycle alternates between haploid and diploid generations
A.
Plants exhibit alternation of generations
1.
The haploid portion is the gametophyte generation
a.
It produces haploid gametes by mitosis in the antheridia or
the archegonia
b.
The gametes fuse to form the diploid zygote, the first stage of
the sporophyte generation
2.
The diploid portion is the sporophyte generation
a.
The zygote develops within the archegonium
b.
It produces haploid spores by meiosis
c.
The spores divide by mitosis and develop into the
gametophyte generation
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IV.
Mosses and other bryophytes are nonvascular plants
A.
There are over 15,000 species of mosses, liverworts, and hornworts
1.
The lack of vascular tissue generally limits them to moist
environments and restricts them to small size
2.
This may be a polyphyletic group
B.
Moss gametophytes are small leafy shoots
1.
Members of phylum Bryophyta are colonial plants
2.
They have rhizoids that anchor the plant to the soil
3.
Mosses lack true leaves, roots and stems
Some have conducting cells which function similar to the vascular
tissue of higher plants
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4.Alternation of generation is apparent
a. The gametophyte dominates and bears the gametangia
at the apex of the plant
b. Some mosses have separate sexes, others bear archegonia
and antheridia on the same plant
c. Flagellated sperm are transported to the archegonium by
splashing raindrops and swim to the archegonium where
they fertilize the egg
1).
Sperm may also be transported by insects
2).
The archegonium secretes sucrose which attracts the
sperm
d. The diploid zygote grows into a multi-cellular sporophyte by
mitosis
e. The sporophyte grows on, and is nutritionally dependent on
the gametophyte
The gametophyte has a foot, a seta (stalk) and a
capsule, which contains sporogenous or spore mother cells
f.
The sporogenous cells divide meiotically to produce haploid
spores
g. The spore germinates into a protonema
h. The protonema develops into the gametophyte
5. Sphagnum is the most commercially important moss
a. Cells of the gametophyte hold much water and Sphagnum is
used as a soil conditioner
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C. Liverwort gametophytes are either thalloid or leafy
1.Members of phylum Hepaticophyta have a dominant gametophyte
generation like mosses, but their body form is a flattened, lobelike thallus
2.Rhizoids anchor the plant to the soil
3.Sexual reproduction involves archegonia and antheridia on
gametophytes as is seen in mosses
4. Asexual reproduction involves production of gemmae which
disperse
via raindrops or small animals
a. Hornworts may reproduce asexually by thallus branching
D. Hornworts are inconspicuous thalloid plants
1. Members of phylum Antherocerophyta resemble liverworts but
may not be closely related to them
2. Hornworts have a single disc-shaped chloroplast in each cell similar
to many algae
3. Antheridia and archegonia are embedded in the thallus, and the
sporophyte which develops after fertilization forms a “horn”
4. Meiosis occurs within sporangia, and spores are formed
V.
E. Bryophytes are used for experimental studies
Members of this group are small and easy to grow
The evolution of bryophytes is obscure
A.
Bryophytes may represent a side line in evolution
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B.
VI.
Bryophytes may even have descended from vascular plants by becoming
simpler and losing vascular tissue
Seedless vascular plants include ferns and their allies
A.
There are about 11,000 species of ferns
1.
Extant ferns are representatives of a once larger group (in size)
2.
The fern allies are the whisk ferns, club mosses and horsetails
a.
These plants have vascular tissue (xylem and phloem) for
conduction and support
b.
Most have true roots and leaves
3.
Megaphylls and microphylls are the two types of leaves
a.
Microphylls are small and have a single vascular strand
1). Microphylls probably evolved as extensions of stems
2). The club mosses are the only extant group which
possess microphylls
b.
Megaphylls are larger and have more than one vascular
strand
1). Megaphylls probably evolved from stem branchings
2). Ferns, horsetails, and seed plants have megaphylls
B.
Ferns have a dominant sporophyte generation
1.
Members of phylum Pterophyta are primarily found in moist,
tropical habitats; a few are aquatic
2.
Alternation of generation includes a dominant sporophyte
generation
3.
Fern bodies consist of a rhizome (an underground stem), roots, and
leaves called rhizomes
a.
Newly emerging fronds are fiddleheads
b.
All sporophyte structures contain vascular tissues
4.
Spore production occurs on the undersides of the fronds in
sporangia, which are typically found in clusters called son
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5.
Spores germinate and grow by mitosis into a gametophyte called
the prothallus
a.
The prothallus is typically heart shaped and lacks vascular
tissues
b.
The prothallus is anchored by rhizoids
c.
The prothallus bears antheridia and archegonia on the ventral
surface
d.
Water is still required for transportation of the flagellated
sperm to the archegonium
e.
The zygote grows into the free-living sporophyte plant
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C.
Whisk ferns are the simplest vascular plants
1.
Members of phylum Psilotophyta lack true roots and leaves but
have vascular stems
2.
Whisk ferns show dichotomous branching of the stem (a primitive
condition)
3.
The sporangia are born on the stems and produce spores
a.
The spores germinate to form underground, haploid prothalli
b.
The prothalli have a symbiotic relationship with mycorrhizal
fungi
4. Most species are extinct; the extant species are limited mostly to
warmer climates
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D.
Horsetails have hollow, jointed stems
1. Members of phylum Sphenophyta were once among the dominant
plants and grew as tall as modern trees
a. Horsetails contributed to the coal deposits
2. Genus Equisetum is the only extant genus and grows in wet habitats
3. Horsetails have true roots, stems, and leaves
4. The stems of the horsetail have silica which makes them abrasive.
Pioneers used them to clean pots and pans. “scouring rushes”
5. Strobili are the reproductive structures which bear sporangia
E. Club mosses are small plants with rhizomes and short, erect branches
1. Members of phylum Lycophyta were also dominant in the
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Carboniferous period
2. Extant species such as Lycopodium are small plants with true roots
and stems and microphylls
3. Strobili are borne on the stems and produce the spores
F. More advanced plants are less dependent on water as a transport medium
for reproductive cells
1. Algae typically have motile spores and sperm
2. Primitive plants have nonmotile spores but motile sperm
3. Advanced plants have nonmotile spores and sperm
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G. Some ferns and club mosses are heterosporous
1. Homosporous plants produce only one type of spore
a.
Bryophytes, whisk ferns, horsetails, and most ferns and club
mosses are homosporous
2. Heterosporous plants produce megaspores and microspores
a.
Some mosses and ferns are heterosporous
b.
Seed plants are heterosporous
c.
Microsporangia produce microsporocytes (also known as
microspore mother cells) which produce microspores by
meiosis
Microspores can develop into a male gametophyte
which produces sperm in the antheridia
d.
Megasporangia produce megasporocytes (also known as
megaspore mother cells) which produce megaspores by
meiosis
Microspores can develop into a female gametophyte
which produces eggs in archegonia
3. Heterospory was the forerunner of the evolution of seeds
H. Seedless vascular plants are used for experimental studies
Megasporangium
Microsporangium
VIII. Seedless vascular plants arose more than 420 million years ago
The oldest megafossils of vascular plants are from the mid-Silurian period
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