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In
the LOTR The Two Towers we find the tree herders
that
can move on their own despite being plants. Today’s
exception
is a version of this, if on a much smaller scale.
There
are a few types of trees that have motile parts; they
don’t
rely on wind, gravity, insects or anything else to
move
from one place to another.
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Plants are most often called by their binomial system names
(genus, species), but this is just naming, not categorizing in a large way.
Animals, protists, fungi, etc. are all divided into different phyla based on
their similarities ad differences. But for some reason (I hope there’s a reason)
botany uses divisions instead of
phyla.
There are 10-12 divisions of plants (we’ll use 10), covering
everything from the mosses to the flowers. Compare that to 21 phyla of animals
and it seems like the plants will be easier to classify – but not so fast.
Group plants according to various characteristics and you
start to muddy the waters. You might divide them up according to whether they
have vascular tissue or not. Non-vascular plants are short because they don’t
have vessels to move water very high – these are the Anthocerotophyta (hornworts), the Bryophyta (mosses), and the Marchantiophyta
(liverworts). Together they are called the Bryophytes.
The vascular plants are all seven of the other divisions – the
Lycopodiophyta (the spikemosses and clubmosses), Pteridophyta (ferns and
horsetails), Coniferophyta (conifers), Cycadophyta (cycads), Ginkgophyta (just
one species, Ginkgo biloba), the Gnetophyta (a weird group), and Angiospermae
(flowering plants). The conifers,
cycads, ginkgo and gnetophytes are often grouped together as the gymnosperms –
you’ve probably heard of them.
On the other hand, you might divide the plants up into the
seed plants and non-seed plants. In that case, you lump the club mosses and the
ferns with the bryophytes, since they all reproduce using spores, not seeds.
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Every
plant alternates between versions of itself that are
diploid
(sporophyte) and haploid (gametophyte). Different
types
of plants spend different amounts of time as one or
the
other. Mosses and other bryophytes are almost always
in
their gametophyte state, while trees are sporophytes with
microgametophytes
(antheridium and archegonium) located
in
their flowers only.
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Here’s one classification method you may not have heard
before – gametophyte dominant vs. sporophyte dominant plants. This has to do
with the cycle of life of plants.
Every plant has two lives. Part of its life is spent as a
haploid gametophyte (produces haploid gametes) while another plant of the
species is a diploid sporophyte.
The sporophyte produces spores that grow into the gametophyte, then the
gametophyte produces gametes that join together during fertilization to become
a new sporophyte.
Some plant types (like bryophytes) exist mostly in the gametophyte
stage and are therefore called gametophyte dominant. Other plants (like trees
and flowers) spend all there time as sporophytes and only small parts of them
become gametophytes (like pollen or cones).
One way you shouldn’t classify plants is based on their
movement. Sure, some plants can grow in a certain direction, toward or away
from some stimulus (tropisms, see this post), but plants aren’t motile. They don’t pick up and move themselves
from one place to another under their own power.
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The
Himalayan Balsam is an invasive viny flower that has
become
a problem in Europe and is invading the US as well.
Their
seed pods contain fins under great strain when fill
with
water. The slightest provocation will cause them to
explode,
sending seeds more than 25 ft (7.5 m).
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Plants disperse seeds as new plants, but they also disperse
their male gametes in order to find the egg on another plant of their species. You
have to get the pollen of seed plants (containing male gametes) or the male
gametes themselves to the egg. Like seeds, pollen grains can be moved by
insect, by wind, by rain, etc. These are the ways most plants get their male
gametes to the egg in order to create a new plant, either in a seed or without
a seed.
But there are exceptions, and this is weird exception. Some
plants have male gametes that are motile. They swim to the egg! No big deal for
animals, they pretty much all have motile male gametes (we’ll look at the
exceptions to that), but it’s quite the stunner in plants.
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Who
knew SpongeBob cartoons were science lesson. Plankton
is
green, a phytoplankton – an algae to be precise. Those two
antennae?
Probably his two flagella, the way he would swim
around.
The feet and the single eye-not so scientific.
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Bryophyte males gametes are swimmers. The mosses, liverworts
and hornworts live close to the
ground and must have standing water for the make gamete to reach the female
gametophyte and egg. The haploid gametophytes are the moss that we usually see.
The antheridium grow on the top of the male plants to produce male gametes,
while the archegonium on the female plant tip produces the ovule with the egg.
When the water is high enough the antheridium releases the male gametes and
they swim to the egg using two flagella. The sporophyte (diploid) plant grows
from the top of the female gametophyte.
Ferns, horsetails, and club mosses are taller than mosses
because they are vascular, but they still require water for their male gametes
to swim to the egg. The gametpophyte is a heart shaped leaf that lies near the
ground. At one spot the archegonium grows the egg, while the male gametes in
the antheridium grow nearby. Water resting on the leaf allows the male gametes
to swim across the leaf to the egg (or from leaf to leaf). The sporophyte grows
from the heart-shaped gametophyte and is the fern we usually think of.
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Cycads
(left) are gymnosperms whose trunk are formed from
the
bases of the leaves as they grow and are lost. There are
about
300 species of cycads known, with several added every
year.
The Ginkgo biloba is the only extant species of the
division.
Because its wood is insect resistant, some trees may
be
over 2500 years old.
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Cycads (about 300 species) and the lone extant ginkgo, Ginkgo biloba, do have pollen grains
that represent the male gametophyte plant. They get blow or carried to the
female gametophyte cone and then it gets weird.
The ovule produces a drop of
liquid that sticks into the air. The pollen gets caught in this drop and then the
drop and the pollen are pulled back into the ovule. The pollen tube grows into
the female reproductive organ, but not right to the egg. When the pollen tube
reaches the entrance of the archegonium, it ruptures and the gametes are
released into a watery fluid that surrounds the eggs.
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Cycad
and ginkgo male gametes move on their own, the only
exceptions
in the seed plants. They have hundreds to thousands
of
cilia, as opposed to flagella in bryophyte gametes, which
pull
the cell forward. Ginkgo male gametes are huge (0.3 mm),
larger
than an entire Wolffia globosa plant.
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The male
gametes of cycads and ferns are very different, from where they swim, to their
relative sizes – ginkgo male gametes are HUGE compared to those of ferns, to
the use of thousands of cilia as opposed to a couple of flagella. However, research shows that they are
remarkable similar in structure and function.
A 2006 study
looked at the proteins involved in gamete movement in ferns and ginkgo. Their
results indicated that most of the proteins in both were homologous enough that
it suggested a direct descent from bryophyte to gymnosperm, not a case of
parallel evolution.
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In
Guam, there is a neurologic disease that looks a lot like
Alzheimer’s.
Research in 2004 found that it was actually
coming
from cycad trees. Here’s how it happens. Cyanobacteria
live
in the tree roots and put the toxin BMAA into the tree
tissues.
Bats eat the fruits and people eat the bats. Than BMAA in
the
brain causes disease.
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This is
believed to attract more insects as pollen distributors, and the researchers
did find that more insects visited the plant when the temperature was
increased. The mechanism may involve volatilizing more attracting chemicals
though the added heat, which would then attract more pollinators (usually
weevils). Pretty advanced for a
plant with a so-called primitive reproduction mechanism.
Next week – the base of the undulipodia has a special story
all its own. Is it another instance of bacteria evolving into one of our
organelles? And it has two very different jobs – which came first?
For
more information or classroom activities, see:
Alternation
of generations –
Seed
dispersal –
Pollen
–
Cycads
–
Ginkgo
biloba –
