Plant Diversity - Lab One
This is the first lab in a two part lab to give you an overview of the plant kingdom.
Biologists generally refer to the plant kingdom in an artificial way as "lower plants"
and "higher plants". This corresponds to the broad group of plants that reproduce by
spores (lower plants) and the groups of plants that reproduce by seeds
(higher plants).
The term "lower plants" or
"higher plants" also refers
when these groups of plants
first evolved, with lower
plants representing the
ancient plant groups and
higher plants representing
the modern plant groups.
Where did terrestrial plants come from?........
Around 500 million years ago, the interconnected land masses of the planet were
unoccupied by plants (or animals).
Then, sometime around 450 million years ago, plants first emerged from bodies of
freshwater such as lakes and ponds, to form the group of plants we refer to as the
Bryophytes. These are the mosses and liverworts.
DNA evidence points to the group of fresh water green algae, the Charophytes, as the
ancestors to bryophytes and thus all plants that dwell on land. Hence, all land plants
living today, from the tiny duckweed to the mighty redwood have as their great, great,
great, grandmother,a single species of Chara. Chara is also called pondweed and can
be easily found in local lakes.
Chara (stonewort) .....the ancestor group to
terrestrial plants....found in local lakes
Bryophytes are represented by several distinct groups, including the
mosses and liverworts.
Moss
Liverwort
As the first plants to emerge from the water and inhabit the terrestrial
biosphere, they are very primitive in structure. They resemble their alga
ancestor in many ways such as reproducing via spores and not seeds. It will
take the plant kingdom some 150 million years to evolve the kind of structures
and seed reproduction that modern plants now have
Liverwort growing on soil at seep
bog (MSUM Science Center)
Clump of moss. Inset: two
different moss species
The next group of plants to evolve after the Bryophytes are referred to collectively
as the "seedless vascular plants"
They are a diverse group that dominated the earths flora circa 350-250 million years
ago. In essence, these were the first true land plants since they evolved the kind of
"equipment" required to to occupy dry ground, namely, roots and a
vascular system.
Living representatives include:
Ferns
Clubmosses
Horsetails
I. Bryophytes - Greek for moss
As plants with similar evolutionary rank, mosses and liverworts share
many characteristics with regard to structure and reproduction, although they
appear quite different from one another
Liverwort is named for its
lobed thallus that resembles
a human liver
Thallus - Botanical term for
the body of the plant
Moss
Since they represent the first plants to colonize the land, bryophytes are considered
the most primitive of all terrestrial plants.........here's why:
(i)
they lack roots.....instead they have tiny single-celled hair like rhizoids that grow
from the underside of the plant into the soil.
Liverwort thallus cross section
Rhizoids cannot function
as roots since they are only
a single cell
QuickTi me™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
soil
Instead, the rhizoid functions
to anchor the plant to the ground
to keep it from washing away
in a rain storm
(ii) they lack a vascular system - these are the xylem and phloem pipes that serve
to transport water and minerals (xylem) and sugars (phloem) throughout the plant body
(iii) they reproduce by spores and not seeds: these are single-celled reproductive cells that
are released to the wind to establish a new plant
moss spores from a spore capsule
Because of these primitive qualities, we should really consider these plants
as the "beach head" of the plant kingdom, rather than true terrestrial plants.
...without roots and a vascular system, these plants are incapable of dwelling on dry
ground. They require more or less permanent wet ground, and must grow close
to the ground so as to absorb water with their entire body like a sponge.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
I. Seedless Vascular Plants
For lack of a better name, this group is referred by botanists as the seedless
vascular plants. Not highly related to each other, except by evolutionary status, these
plants legitimately can be thought of as the first true land plants. In there hey-day,
they formed a vast and luxuriant green landscape consisting of dense forests of
now extinct 60 foot tall tree species of horsetails, clubmosses, and ferns.
Today, they make up only a small fraction of the earths flora since they were
unable to compete with seed plants that evolved later.
scene from when the
seedless vascular plants
ruled the world
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Dragon fly with
2 ft wingspan!
Ferns are the most familiar members to us and are the most widespread
in today's landscape
Their mode of reproduction is typical of all primitive, spore bearing plants in how
they utilize a distinct "alternation-of-generation" method of sexual reproduction.
Alternation-of-generations: All plants, unlike animals, are actually comprised of
two organisms. One is called the gametophyte plant and the other the sporophyte plant. How
this most radically differs to how animals reproduce is that plants include a reproductive
cell called a spore, in addition to the other two kinds of reproductive cells, sperm and egg
during sexual reproduction.
The fern alternation-of-generation method of sexual reproduction
The function of the sporophyte plant is to produce spores....spores then germinate and
grow into the gametophyte plant.
The function of the gametophyte plant is to produce the gametes, either egg (function of the female gaemtophyte
plant) or sperm (the function of the male gametophyte plant).
Compare the fern sporophyte plant to its gametophyte plants:
sperm bearing
structures
rhizoids
rhizoids
egg bearing
structures
female gametophyte
......actual size ~1/2 inch
male gametophyte
....actual size ~1/4 inch
sporophyte
........actual size ~ 1.5 ft
The plants in this group vary in how and where the sporophyte plant bears
the spores:
Ferns bear spores underneath their leaves in structures called sori (singular= sorus)
Rows of sori on fern frond
a single sorus
with clusters of
sporangia
Clubmosses and Horsetails bear their spores in primitive cones called strobili
(singular = strobilus)
QuickTi me™ and a
TIFF (LZW) decompressor
are needed to see this picture.
The scales of the cone that
bear the spores are called
sporophylls
Horsetail strobilus
The End