1
Green Plant Diversity
(Chapter 28)
Evolutionary origins of green plants and land plants
While a group comprising all plants is comprised by the glaucophyte algae, red
algae, green algae, and land plants, a less inclusive group referred to as the green
plants is comprised of the green algae and “land” plants.
Of course, most green plants convert light energy into chemical-bond energy
through the process of photosynthesis (that is to say that plants are autotrophs).
In doing so they produce oxygen, some of which is used by other aerobic
organisms, i.e., organisms that use oxygen.
It would be impossible to overestimate the importance of green plants regarding
life on Earth.
Together, plants serve as critical providers of oxygen and nutrients in almost all
ecosystems and they literally have helped to shape the biosphere.
Green algae and land plants each have chloroplasts containing the photosynthetic
pigments chlorophyll a and chlorophyll b, as well as the accessory pigment β-
carotene.
They also “share” features such as thylakoids (stacks of membrane bound sacs
within their chloroplasts) and similar sperm and cell wall structures, and, they
store starch in their chloroplasts.
Green algae arose about 725 million years ago (mya) and today they comprise about
7,000 unicellular, colonial, and multicellular species.
, 2
They are important primary producers in nearshore marine and all freshwater
environments.
Green algae can also be found in some non-aquatic environments… such as in snow
fields at high elevation, in pack ice, and in ice floes.
In fact, green algae can be so abundant in these icy environments that their
pigments can lend a pink hue to ice and snow.
Green algae can also be found living in close associations (i.e., symbiotic
associations) with other organisms.
For example, lichens which commonly encrust rocks and trees in some habitats are
stable associations between green algae and fungi or cyanobacteria and fungi.
In addition, unicellular green algae are common endosymbionts (associates that live
in their partner organism) within freshwater planktonic protists.
DNA evidence suggests that all land plants were derived from freshwater species
of green algae.
The fossil evidence indicates that this important evolutionary step occurred about
475 mya during the Middle Ordovician period.
Up until the evolution of land plants, photosynthetic green algae had been confined
to aquatic habitats for about 250 million years (my).
The restriction of green algae to watery habitats was caused by their need for
water to:
a. reproduce,
b. support themselves,
c. prevent desiccation, and
d. provide protection from UV light.
Today there are about 300,000 species of extant (i.e., living) land plants and most
of them carry out photosynthesis to sustain themselves.
, 3
Land plant life cycles
Unlike all green algae, all land plants undergo mitosis after both gamete fusion and
meiosis.
The result is that plants display a life cycle within which there is a multicellular
haploid individual (the gametophyte) and a multicellular diploid individual (the
sporophyte).
This process is very different from the life cycles of non-plants (such as
ourselves), wherein gamete fusion directly follows meiosis without any mitosis (i.e.,
the multicellular haploid form does not exist).
Thus, animals and land plants possess two different types of life cycles:
1) Diplontic— only the diploid stage is multicellular (animals)
2) Haplodiplontic— multicellular haploid and diploid stages (some algae, all
land plants.
In a generalized land plant life cyclethe diploid generation (sporophyte generation)
alternates with the haploid generation (gametophyte generation).
The diploid (2n) sporophyte produces haploid (n) spores (not gametes) by meiosis.
The process of meiosis (reduction division) takes place in structures called
sporangia.
Within sporangia, diploid spore mother cells (sporocytes) undergo meiosis to each
produce 4 haploid spores.
Spores are the first cells of the gametophyte generation, and they divide by
mitosis to produce a multicellular haploid gametophyte.
The haploid gametophyte is the source of gametes (i.e., eggs and sperm) which will
be produced via mitosis (not meiosis).
Green Plant Diversity
(Chapter 28)
Evolutionary origins of green plants and land plants
While a group comprising all plants is comprised by the glaucophyte algae, red
algae, green algae, and land plants, a less inclusive group referred to as the green
plants is comprised of the green algae and “land” plants.
Of course, most green plants convert light energy into chemical-bond energy
through the process of photosynthesis (that is to say that plants are autotrophs).
In doing so they produce oxygen, some of which is used by other aerobic
organisms, i.e., organisms that use oxygen.
It would be impossible to overestimate the importance of green plants regarding
life on Earth.
Together, plants serve as critical providers of oxygen and nutrients in almost all
ecosystems and they literally have helped to shape the biosphere.
Green algae and land plants each have chloroplasts containing the photosynthetic
pigments chlorophyll a and chlorophyll b, as well as the accessory pigment β-
carotene.
They also “share” features such as thylakoids (stacks of membrane bound sacs
within their chloroplasts) and similar sperm and cell wall structures, and, they
store starch in their chloroplasts.
Green algae arose about 725 million years ago (mya) and today they comprise about
7,000 unicellular, colonial, and multicellular species.
, 2
They are important primary producers in nearshore marine and all freshwater
environments.
Green algae can also be found in some non-aquatic environments… such as in snow
fields at high elevation, in pack ice, and in ice floes.
In fact, green algae can be so abundant in these icy environments that their
pigments can lend a pink hue to ice and snow.
Green algae can also be found living in close associations (i.e., symbiotic
associations) with other organisms.
For example, lichens which commonly encrust rocks and trees in some habitats are
stable associations between green algae and fungi or cyanobacteria and fungi.
In addition, unicellular green algae are common endosymbionts (associates that live
in their partner organism) within freshwater planktonic protists.
DNA evidence suggests that all land plants were derived from freshwater species
of green algae.
The fossil evidence indicates that this important evolutionary step occurred about
475 mya during the Middle Ordovician period.
Up until the evolution of land plants, photosynthetic green algae had been confined
to aquatic habitats for about 250 million years (my).
The restriction of green algae to watery habitats was caused by their need for
water to:
a. reproduce,
b. support themselves,
c. prevent desiccation, and
d. provide protection from UV light.
Today there are about 300,000 species of extant (i.e., living) land plants and most
of them carry out photosynthesis to sustain themselves.
, 3
Land plant life cycles
Unlike all green algae, all land plants undergo mitosis after both gamete fusion and
meiosis.
The result is that plants display a life cycle within which there is a multicellular
haploid individual (the gametophyte) and a multicellular diploid individual (the
sporophyte).
This process is very different from the life cycles of non-plants (such as
ourselves), wherein gamete fusion directly follows meiosis without any mitosis (i.e.,
the multicellular haploid form does not exist).
Thus, animals and land plants possess two different types of life cycles:
1) Diplontic— only the diploid stage is multicellular (animals)
2) Haplodiplontic— multicellular haploid and diploid stages (some algae, all
land plants.
In a generalized land plant life cyclethe diploid generation (sporophyte generation)
alternates with the haploid generation (gametophyte generation).
The diploid (2n) sporophyte produces haploid (n) spores (not gametes) by meiosis.
The process of meiosis (reduction division) takes place in structures called
sporangia.
Within sporangia, diploid spore mother cells (sporocytes) undergo meiosis to each
produce 4 haploid spores.
Spores are the first cells of the gametophyte generation, and they divide by
mitosis to produce a multicellular haploid gametophyte.
The haploid gametophyte is the source of gametes (i.e., eggs and sperm) which will
be produced via mitosis (not meiosis).
