Introduction to the Plant Kingdom - 1
The Plant Kingdom comprises a large and varied group of organisms that have the
following characteristics in common. All plants are:
• Eukaryotic
• Photosynthetic
• Multicellular with tissue development
• Sexually reproducing
Who are the Plants?
The general classification of plants uses the following features to divide plants into
four broad groups and a number of phyla:
• Presence or absence of vascular tissue
• Dispersal by spores or by seeds
• Seed plants are distinguished further by the protection of the seed.
Introduction to the Plant Kingdom - 2
The Plant Phyla
Introduction to the Plant Kingdom - 3
The Life History of all plants involves an alternation of a haploid phase (the
Gametophyte) that produces gametes in protective structures called gametangia,
with a diploid phase (the SSporophyte) that produces sporangia. To start our
discussion of plants, we will review the alternation of generations, first presented in
Biology 201. Variations in life history are essential to classification in the Plant
Kingdom.
Plant Life History - The Alternation of Generations
Sperm (n) + Egg (n)
|
Zygote (2n)
Growth by mitosis produces:
|
Embryo (2n)
|
Sporophyte (2n)
|
Sporangium (2n)**
Meiosis in Sporangium produces:
|
Spore (n)
Growth by mitosis produces:
|
Gametophyte (n)
| |
Antheridium (n) Archegonium (n)
| |
Sperm (n) Egg (n
* Variations in Life History Patterns
• Sporangia: Homosporous or Heterosporous
• Prevalence (predominance) of sporophyte or gametophyte. More complex
plants = more sporophyte dominance
• Independence or dependence of Sporophyte or Gametophyte on alternate
generation. More advanced plants have a dependent gametophyte.
Introduction to the Plant Kingdom - 4
Evolutionary Trends in the Plant Kingdom
Most plants are terrestrial. Plants evolved from green algae ancestors (Charophyceae)
that successfully inhabited land. As we have discussed, plants must obtain their
nutrients from both air and soil, and much of their structure relates to the need to
maximize obtaining nutrients and water, minimizing water loss and protecting the
plant from damage.
To this end, to survive on land, plants evolved:
• Protective surfaces to minimize desiccation - cuticle
• Gas exchange to circumvent the cuticle –stomata
• Increase in photosynthetic surface area - leaves
• Increase in dimension with conducting and support tissues
• Reproductive strategies that work without water transport of gametes
All plants disperse to new locations via single-celled spores or by seeds. Spore walls
contain ssporopollenin, a protective coating that provides protection against decay
and dehydration. Seeds are protected by a seed coat, a multicellular layer of thick-
walled resistant cells.
Introduction to the Plant Kingdom - 5
Non-Vascular Plants
The non-vascular plants include the mosses, hornworts and liverworts. Because they
lack vascular tissue they are small in vertical stature, and typically grow in clumps or
masses. Most lack a cuticle and many are capable of withstanding long periods of
desiccation. They attach to their substrate with “rhizoids”, but absorb water and
minerals through all surface cells. Some have air pores for diffusion of gases.
Some Bryophytes have a thallus (flattened-shaped) structure and others are tiny,
three-dimensional with an axis and “leaves”, called phyllodes, attached. They have
apical meristems. All Bryophytes disperse by spores and water is required to
transport the motile sperm for fertilization. The gametophyte generation is dominant.
The three non-vascular plant phyla are:
Hepatophyta
Anthocerophyta
Bryophyta
Vascular Plants
Vascular plants exhibit a number of advances that promote survival on land.
• Protection of above-ground body surfaces with a cuticle or surfaces tissues with
cell walls with impermeable layers minimizes water loss, often mentioned in
Biology as a significant problem for plants.
• The evolution of conducting tissue provided a means for plants to become large
in dimension and volume. Vascular tissue provides support for the plant as well
as conducting water and solutes throughout the plant body. Dependence on
diffusion of water and nutrients affects plant size, as seen in the Bryophytes.
• The vascular plants all have a dominant sporophyte generation. They produce
multiple numbers of sporangia on each sporophyte and the spore-dispersing
vascular plants can disperse many airborne spores which means plants can
occupy more locations rapidly.
• Transport of sperm in many plants requires water. Motile sperm requiring water
for transport limits plants ability to sexually reproduce.
• Gametophytes are greatly reduced in size as well as duration relative to the
sporophyte in the vascular plants. As vascular plants become even more
complex, sperm are retained in the male gametophyte, the pollen grain, freeing
sexual reproduction from water dependence.
• In the seed plants, the seed (embryo sporophyte surrounded by nutrients and
seed coat) replaces the haploid single-celled spore as the dispersal unit.
These adaptations to the terrestrial environment have made vascular plants the
predominant vegetation in most parts of the world. While abundant, Bryophytes are
rarely predominant.
Introduction to the Plant Kingdom - 6
To summarize, Vascular Plants have:
• Body plan along a supporting axis (the stem)
• Vascular tissue
• Xylem promotes (allows) increase in size
• Sporophyte generation independent and assimilative
• Gametophyte often dependent on the sporophyte
• Cutin and/or suberin produced on aerial parts to minimize dehydration
• Generally large surface area/volume ratio
• Allows land survival with less surface H2O
Classification of the Vascular Plants
There are seven (or nine) extant vascular plant phyla and at several phyla comprised
of extinct vascular plants. We use the fossil record of the extinct vascular plants to
trace our plant ancestors. Moreover, long-extinct precursors of today’s vascular
plants are the source of most of the world’s coal deposits.
Evolutionary Features of Vascular Plants
Introduction to the Plant Kingdom - 7
Review of Features used in Classifying Vascular Plants
Vascularization
• Protostele, siphonostele and eustele vascular patterns in stems
• The protostele is a solid core of stem vascular tissue from which leaves
diverge without leaving a stem vascular gap.
• The siphonostele has a central pith surrounded by a cylinder of vascular
tissue. In some plants that have siphonosteles the vascular strands that go
to leaves (the leaf traces) result in a leaf gap of parenchyma cells.
• The eustele, found in most seed plants, is comprised of discreet vascular
bundles surrounding the pith. Leaf trace gaps are found in the eustele.
• Microphylls vs. Megaphylls for leaves
• Microphylls contain one vascular strand, probably evolving from small
projections, or enations located along stem tissue.
• Megaphylls have a branching system of veins, and probably evolved from
side branches of stems, in a webbing process, in which tissue containing
chlorophyll filled in spaces between nearby branches.
Microphylls
Megaphylls
Introduction to the Plant Kingdom - 8
Leaf trace patterns and nodes in protostele and siphonstele
Dispersal Method
Spore (single cell) vs. seed
Sperm transport
H2O vs. pollen grain
Types of Sporangia
Plants that produce one type of sporangium are called hhomosporous. Other plants
produce two different sporangia, megasporangia and microsporangia and are
heterosporous.
A plant that is hhomosporous has one type of sporangium. However homosporous
plants can produce one type of spore that grows into a gametophyte that contains
both male and female reproductive structures (the antheridium and archegonium,
respectively) or a homosporous plant can produce two types of spores in its
sporangia. When two types of spores are produced, one grows into male
gametophytes and the second grows into female gametophytes.
Heterosporous plants always have separate sporangia. The microsporangia produce
only microspores that grow into male gametophytes, producing only antheridia.
Megasporangia produce megaspores that grow into female gametophytes, producing
only archegonia. The megasporangia and microsporangia may be found on the same
sporophyte individual, or on separate sporophyte individuals.
Introduction to the Plant Kingdom - 9
The Vascular Plant Phyla
Spore-dispersing Vascular Plant Phyla
Rhyniophyta – Extinct
Zosterophyllophyta - Extinct
Trimerophytophyta - Extinct
Lycophyta (Lycopodiophyta)
Lycopodiae
Selaginellae
Isoetae
Pteridophyta
Equisetales (Equisetales)
Psilotales (Psilotales)
Ferns (Pterophyta)
Ophioglossales – Eusporangiate
Marattiales – Eusporangiate
Filicales – Homosporous and Leptosporangiate
Salviniales – Heterosporous and Leptosporangiate
Marsileales – Heterosporous and Leptosporangiate
Seed-dispersing Vascular Plant Phyla
Progymnospermophyta - Extinct
Pteridospermales
Cordaitales
Bennettitales
The Gymnosperm Phyla
Cycadophyta
Ginkgophyta
Coniferophyta
Gnetophyta
The Angiosperms
Anthophyta
Monocotyledones
Eudicotyledones
Plus
Magnoliidae, including several orders
Nymphaeales – Water Lilies
Illiciales – Star Anise
Amborellales
Archaefructales – Earliest Anthophyte Fossil
Introduction to the Plant Kingdom - 10
Generalized Life History of the Vascular Plants
Spore (n)
|
Gametophyte (n)
|
| |
Antheridium (n) Archegonium (n)
| |
Sperm (n) Egg (n)
(motile) |
| |
|
Zygote (2n)
Growth by mitosis produces a multicellular
|
Sporophyte (2n)
|
Sporangia (2n)
|
| |
If Homosporous If Heterosporous
| |
| | |
Sporangium Megasporangium Microsporangium
| | |
Meiosis Meiosis Meiosis
| | |
Spores (n) Megaspores (n) Microspores (n)
Growth by mitosis from single-celled spore produces a multicellular
| | |
| | Megagametophyte Microgametophyte
| | (Often retained in
| | the Sporophyte)
| |
| |
Two One
Separate Gametophyte
Gametophytes with both Antheridia
(Male and Female) and Archegonia present