26
The Plant Kingdom: Seedless Plants
Lecture Outline
I.
Plants evolved from a green alga (a charophyte) and 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 (nonreproductive cells) as well as
gametes
1. The fertilized egg develops into a multicellular embryo within the female
gametangium
D. The plant life cycle alternates between haploid and diploid generations
1. The haploid portion is the gametophyte generation
a) It produces haploid gametes by mitosis in the antheridia (the male
parts) or the archegonia (the female parts)
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
3. All plants produce spores by meiosis; algae and fungi produce spores by
meiosis or mitosis
4. This is called alternation of generation
5. In the lower plants, the gametophyte generation is the dominant stage; in
higher plants, the sporophyte generation is the dominant stage
II.
Four major groups of plants have evolved
A. The bryophytes are seedless and disperse via haploid spores
B. 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
C. The seedless vascular plants (ferns and allies) disperse primarily via spores,
and diversified during the Silurian and Devonian periods
D. The gymnosperms are vascular plants that produce seeds, typically in a cone
1. The gymnosperms arose during the end of the Devonian
E. The angiosperms are vascular plants that produce seeds after flowering
1. The angiosperms arose during the early Cretaceous
III.
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, generally classified into 3 phyla
B. Moss gametophytes are differentiated into "leaves" and "stems"
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
a) Some have conducting cells, which function similarly to the vascular
tissue of higher plants
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 chemicals that attract the sperm
d) The diploid zygote grows into a multicellular sporophyte by mitosis
e) The sporophyte grows, and is nutritionally dependent, on the
gametophyte
(1) 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
i) The haploid gametophyte is dominant
5. Sphagnum is the most commercially important moss
a) Cells of the gametophyte hold much water, and Sphagnum is used as a
soil conditioner
6. The term "moss" is often misused
a) Reindeer moss is a lichen, Spanish moss is an angiosperm
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, lobe-like 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 that disperse via
raindrops or small animals
a) Liverworts may also reproduce asexually by thallus branching
D. Hornworts gametophytes 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 that develops after fertilization forms a "horn"
4. Meiosis occurs within sporangia, and spores are formed
E. Bryophytes are used for experimental studies on genetics, plant hormones,
photoperiod, etc.
1. Members of this group are small and easy to grow
IV.
The evolution of bryophytes is based on fossil evidence of ancient plants and on
structural and molecular evidence
A. The fossil record is very incomplete
B. Bryophytes may represent a side-line in evolution
1. Bryophytes may even have descended from vascular plants by becoming
simpler and losing vascular tissue
C. Hornworts may be the most ancient based on structural and molecular
evidence
1. This is based on nuclear, chloroplast, and mitochondrial genes
2. It appears that the hornworts split off early, and that all others evolved
from a common ancestor
V.
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)
a) These plants have vascular tissue (xylem and phloem) for conduction
and support
b) Most have true roots and leaves
2. 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 that possesses
microphylls
b) Megaphylls are larger and have more than one vascular strand
(1) Megaphylls probably evolved from stem branchings that filled in
with additional webbing
(2) Ferns, horsetails, and seed plants have megaphylls
(3) Megaphylls evolved over a long period of time, perhaps in
response to decreases in atmospheric carbon dioxide
(4) Plants needed a flattened blade with more stomata for gas
exchange
B.
Ferns have a dominant sporophyte generation
1. Members of phylum Polypodiophyta 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 sori
5. Spores germinate and grow by mitosis into a gametophyte called the
haploid 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
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
D. Horsetails have hollow, jointed stems
1. Members of phylum Equisitophyta 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, and they
were used by pioneers to clean pots and pans ("scouring rushes")
5. Strobili are the reproductive structures that bear the sporangia
E. Club mosses are small plants with rhizomes and short, erect branches
1. Members of phylum Lycophyta were also dominant in the 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.
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
(1) Microspores can develop into a male gametophyte that produces
sperm in antheridia
d) Megasporangia produce megasporocytes (also known as megaspore
mother cells), which produce megaspores by meiosis
(1) Microspores can develop into a female gametophyte that
produces eggs in archegonia
3. Heterospory was the forerunner of the evolution of seeds
G. Seedless vascular plants are used for experimental studies
1. Ferns and other seedless vasculars are useful in studying apical meristems,
as they have an unusual single apical cell
2. Ferns are also polyploids with many hundreds of chromosomes, useful in
studying how genes are expressed and suppressed
VI.
Seedless vascular plants arose between 420 mya and 360 mya
A. The oldest megafossils of vascular plants are from the mid-Silurian Period
B. Microfossils of small seedless vasculars are known from the early Silurian
C. The oldest vascular plants, members of phylum Rhinophyta, are known from
fossils dated at approximately 420 mya to 380 mya
1. One reconstructed plant, Rhinia major, a classic rhinophyte, superficially
resembled whisk ferns, having rhizoids, sporangia, and xylem
2. It is now reclassified based on some of the microscopic structures of some
of its morphological characters
3. It is now known as Aglaophyton major, while other Rhinia species stand as
previously classified
a) Aglaophyton lacks conducting tissues characteristic of vascular plants
4. Rhinia gwynne-vaughanii remains as one of the Earth’s first vascular plants
Research and Discussion Topics

Describe the past dominance of primitive plants like cycads and horsetails. Why do
the gymnosperms and angiosperms dominate today?

What was the mechanism by which the plants during the Carboniferous period
produced coal deposits? What plants dominated during this time? What are the
different types of coal produced? How are these types used?

Investigate the evolutionary evidence for the development of megaphylls. How is it
believed that heterospory evolved?