Plants are considered by most biosystematists to be in a separate taxonomic group from Green Algae because all plants have, in addition to the above characteristics...
FIRST: An Overview of Alternation of Generations.
A few terms of import:
BRYOPHYTE - non-vascular plant (liverworts & mosses)
TRACHEOPHYTE - vascular plant (all higher plants that have xylem and phloem, from ferns to palm trees!)
(In general, these types of plants are classified into two major taxa:
Superphylum Bryophyta
Bryophytes are the "amphibians" of the plant world. They...
SPOROPHYTE: the diploid generation of plants, which grows mitotically from the zygote. The sporophyte produces haploid spores inside the SPORANGIUM (go back to the beginning of the vocabulary list! We have come full circle!)
(Examples:
In bryophytes, these are the short-lived generation: the little brown stalk shooting up out of the green moss during breeding season, or the little green sporophyte on the bottom of the liverwort's female "palm tree" structure that releases yellow spores.
In tracheophytes, it's the "main" generation: the grass, the pine tree, the oak tree, the rose bush, the palm tree, etc...)
SPOROPHYLL: a specialized leaf that bears sporangia.
(phyll is a Greek word meaning "leaf")
Pause and remember: microspores and megaspores!
microsporophyll: a sporophyll that bears male spores
megasporophyll: a sporophyll that bears female spores
SPORANGIUM - a container-like structure found on the SPOROPHYTE. It contains diploid cells which undergo meiosis to become spores. (Each of the spores is genetically unique, as is typical of cells produced via meiosis)
(Examples: the little, knoblike structures on the
tip of the moss sporophyte you saw in the slide show; also the
small, attached structures on the bottom of a fern frond (a
sporophyll!) which
produce spores.)
SPORE - a haploid cell produced via meiosis inside a sporangium. A spore can grow (mitotically) into a haploid plant called a gametophyte.
(Example: the brown, dusty spheres you rub off the bottom of a fern frond.)
1. No special designation is given to a spore which grows into a bisexual (monoecious) gametophyte.
2. A spore which grows into a female gametophyte (i.e., one which bears only female sex organs, archegonia) is called a MEGASPORE.
3. A spore which grows into a male gametophyte (i.e., one which bears only male sex organs, antheridia) is called a MICROSPORE.
(Example: Liverwort gametophytes are those lovely,
liver-shaped, leafy plants you saw in the slide show. Until they're
breeding, you can't tell male and female apart. However, in breeding
season, the females grow the little "palm tree" shaped
sex structures and the males grow the little "umbrella"
shaped sex structures which mark them as separate sexes. Liverworts
are dioecious!)
GAMETOPHYTE: the haploid plant generation that grows from a haploid spore. It may be bisexual (monoecious) or have separate sexed individuals (dioecious), depending on the species.
(Examples:
In bryophytes, it's the leafy, obvious generation you see out under the air conditioner: the thallus of the liverwort or the moss.
In tracheophytes, it's usually not very obvious or easy to see! In ferns, the gametophyte is a tiny, green thing that lives for only a few weeks, gives birth to a sporophyte and then is crushed out of existence by its growing "child." In pines and in flowering plants, you can see male gametophytes floating around on the wind, and they can make you sneeze miserably. They are POLLEN GRAINS!)
ANTHERIDIUM: male sex organ located on the gametophyte. This structure produces sperm via MITOSIS.
ANTHERIDIOPHORE: gametophyte structure (often a
stalk) that bears antheridia. (phor is a Greek word meaning
"to bear")
ARCHEGONIUM: female sex organ located on the gametophyte. This structure produces eggs (ova) via MITOSIS.
ARCHEGONIOPHORE: gametophyte structure (often a
stalk) that bears archegonia. (phor is a Greek word meaning
"to bear")
FERTILIZATION: the process of the fusion of two
haploid nuclei (usually of sperm and egg) to produce a single,
diploid cell known as a ZYGOTE.
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We have already seen a slide show of the bryophytes. Here are a few of their characteristics:
- no vascular tissue
- no true organs (root, stem or leaf)
- gametophyte generation dominant
- waxy cuticle very reduced
- stomates are fixed; can't be opened or closed
- tied to a moist habitat because of anatomical features that are not sufficient to prevent desiccation AND because they release sperm directly into the environment, and the sperm must swim to the stationery females.
Once again, the Bryophyte phyla are...
We now move on to the TRACHEOPHYTES (Vascular Plants).
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Some of the problems that plants had to face to make
a complete transition to land:
1. desiccation
(solution: waxy cuticle and stomates!)
2. gamete transport
(solution: spores; pollen)
3. support against gravity
(solution: cellulose and lignin)
4. gas exchange can only be done across a wet surface
(solution: recessed, wet stomates)
5. nutrient & water transport
(solution: xylem and phloem)
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A tour through the Seedless Tracheophytes
Phylum Psilophyta - The Whisk Ferns
Phylum Lycophyta - The Club "Mosses"
Phylum Sphenophyta - The Horsetails
Phylum Pterophyta - The Ferns
Recall the main differences between root and
stem:
1. root is below ground, stem is above ground (not
always!)
2. root lacks nodes and internodes; stem has them!
3. root and stem have different arrangement of vascular
tissues in the stele.
4. root has endodermis and pericycle; stem lacks
them
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(Let's have a look at stem tissue arrangement:
Note: stem stele arrangement is often just a nudge
"ahead" of the root stele arrangement. For example,
a plant with a eustele in its stem will often have a siphonostele
or protostele in its root.
Why? Who knows? Not I.
Vascular plants, having true organs, also have specialized
organs to serve specific functions. We have already seen examples
of VERY specialized root, stem and leaf.
But all vascular plants have specialized leaves whose
(almost) sole purpose is to bear the reproductive structures.
These leaves are called SPOROPHYLLS.
A leaf that bears a sporangium manufacturing spores
that will become male gametophytes (microspores) is called a MICROSPOROPHYLL.
A leaf that bears a sporangium manufacturing spores
that will become female gametophytes (megaspores) is called a
MEGASPOROPHYLL.
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Example of a sporophyll: fern frond.
(It just so happens that fern spores grow into little
gametophytes that are bisexual: they don't have microspores or
megaspores)
Another example of a sporophyll:
A woody bract of a pine cone
A cluster of sporophylls is called a STROBILUS
(colloquially, they're called "cones")
So a "pine cone" is actually a whorled
cluster of sporophylls.
The big, woody pine cones with which you are familiar
are clusters of MEGASPOROPHYLLS. They are females! (They bear
the seeds).
The male strobili appear in the springtime, release
their pollen and drop off the tree in a very short time. These
are clusters of MICROSPOROPHYLLS. They are males.
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We already know that when a spore hits the ground,
it has the potential to grow into a gametophyte (which will make
eggs or sperm or both).
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In the seedless tracheophytes, the gametophyte grows
up independently of the sporophyte in most cases, but it lives
only for a short time (one season) before "giving birth"
to the new sporophyte, which crushes the gametophyte out of existence
as it grows to maturity.
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What makes the TRACHEOPHYTES different from the bryophytes?>
