Chapter 2: Plant Diversification Bryophytes: Ancestral green algae evolved and led to the diversification of land plants. This phylogeny occurred through plant morphology, biochemistry, and genetics. From one ancestral green alga species, 10 phyla of extant plants arose. These ten phyla can be broken down The bryophyte clade is composed of three phyla of non-vascular plants: phylum Hepatophyta (liverworts), phylum Anthocerophyta (hornworts), and phylum Bryophyta (mosses).
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Chapter 2: Plant
Diversification
Bryophytes:
Ancestral green algae evolved and led to the diversification of land plants.
This phylogeny occurred through plant morphology, biochemistry, and genetics.
From one ancestral green alga species, 10 phyla of extant plants arose. These
ten phyla can be broken down into two categories: Bryophytes (nonvascular
plants), and Vascular plants.
The bryophyte clade is composed of three phyla of non-vascular plants:
phylum Hepatophyta (liverworts), phylum Anthocerophyta (hornworts), and
phylum Bryophyta (mosses).
Liverworts are described as
“thalloid” because of the flattened
shape of their gametophytes. Some
liverworts are called “leafy” because
their gametophytes have stemlike
structures with leaflike parts.
Hornworts have grass like
sporophytes and there is a
sporangium which extends along its
length and releases spores at the
tip of its “horn”. Gametophytes
grow horizontally and are often
attached to multiple sporophytes.
Hornworts(Phylum
Anthocerophyta)
Mosses(Phylum Bryophyta)
Moss gametophytes grow vertically and they are the structures that
primarily make up a carpet of moss. Moss sporophytes turn brown when they
are ready to release spores.
A wetland moss genus called Sphagnum is common and it forms
widespread deposits of partially decayed organic material known as peat. It is
estimated that 400 billion tons of organic carbon are stored in peat worldwide.
These reservoirs of carbon help to stabilize global atmospheric CO2
concentrations. In addition, peat is a fuel source, notably in Canada and Ireland.
Peat also serves as a soil conditioner, so it is often used for packing plant roots
during shipping. Unfortunately, current overproduction of Sphagnum may
reduce its ecological benefits.
Ecological importance:
Liverworts
(Phylum
Hepatophyta)
This is a peat bog being harvested.
“Tolland Man” is a bog mummy dating from 405- 100 BC. Sphagnum can preserve a human body for thousands of years due to its acidity and lack of oxygen.
Vascular plants:
Seedless vascular plants:
Vascular plants dominate today’s landscape. Fossils indicate that the
earliest vascular plant ancestors were on earth more than 400 million years ago.
Unlike Bryophytes, these species had sporophytes which were branched and not
dependent on gametophytes for growth. This branching also led to more complex
bodies with multiple sporangia. This evolutionary development increased sperm
production and increased survival despite herbivory. The main traits which
distinguish vascular plants are: life cycles with dominant sporophytes, xylem and
phloem, and roots and leaves. Vascular plants can be divided into two groups:
seedless vascular plants and seed plants.
Pterophytes(Phylum Pterophyta)
There are three types of Pterophytes: whisk ferns and relatives, horsetails,
and ferns. Nearly all seedless plants are homosporous.
Ferns:Ferns sporophytes often have horizontal stems which led to the formation
of fronds (large leaves). Most species of fern have megaphylls, stalked
sporangia with spring-like devices to catapult spores into the air, and are
homosporous. Some species produce more than a trillion spores over their
lifetime.
Seedless vascular plants have a sporophyte- dominant life cycle. The fern
life cycle (on the next page) is representative of all Pterophytes.
Vascular seed plants:
Gymnosperms:
The fern life
cycle:
Seed plants can be divided into two groups: gymnosperms and angiosperms. They have had an enormous impact on human society. The domestication of these plants, especially angiosperms, led to one of the most important cultural changes in human history, the shift from hunter- gatherer societies to permanent farming settlements.
Heterospory:Seed plants evolved from plants that had megasporangia and
microsporangia. Megasporangia produce spores that give rise to female
gametophytes and microsporangia produce microspores that give rise to male
gametophytes.
A seed is a sporophyte embryo, along with its food supply, packaged into
a protective coat. Seeds are more resistant than spores and can be widely
distributed by wind or animals.
Seeds:
Gymnosperms are plants which have “naked” seeds, for their seeds are
not enclosed in ovaries. Their seeds are exposed, usually in the form of cones
(strobili). Of the ten plant phyla of land plants, four are gymnosperms:
Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta.
Gymnosperms are plants which have “naked” seeds, for their seeds are
not enclosed in ovaries. Their seeds are exposed, usually in the form of cones
(strobili). Of the ten plant phyla of land plants, four are gymnosperms:
Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta.
Phylum Cycadophyta
Ginkgo biloba is the only species
of this phylum. It is the source of
herbal extract that reportedly
improves memory. It also tolerates
high amounts of air pollution. And it
has fan-like leaves that turn gold in
the fall.
Phylum Ginkgophyta
Cycads have large cones and
palmlike leaves. Only 130 species
survive today, but they thrived
during the Mesozoic era, or the “Age
of Cycads”.
Phylum Gnetophyta
Gnetophytes consist of three genera: Gnetum, Ephedra, and
Welwitschia. Although these species are different in appearance, the genera
are grouped based on molecular data.
Welwitschia mirabilis lives only in the deserts of southwestern Africa. Its strap- like leaves are among the largest leaves known.
Gnetum includes about
35 species of tropical trees,
shrubs, and vines.
Ephedra are desert shrubs, commonly called “Mormon tea”. They produce ephedrine which is used medically as a decongestant.
Phylum Coniferophyta
This phylum is by far the largest of the gymnosperm phyla. Most
conifers are evergreens; they retain their leaves throughout the year.
Angiosperms:
Douglas fir provides more timber than any other North
American tree species.
The sequoia is one of the largest living organisms and also among the most ancient.
The “berries” of the common juniper are actually ovule- producing cones consisting of fleshy sporophylls.
Gymnosperm evolution:Gymnosperms appear early in fossil record and dominated Mesozoic
terrestrial. Early gymnosperms lived in Carboniferous ecosystems still
dominated by seedless vascular plants. Flora and fauna changed dramatically
as many groups of organisms disappeared and others became prominent.
Gymnosperms, such as pines and furs, adapted to arid conditions, and though
they do not dominate terrestrial ecosystems, gymnosperms remain an
important part of the Earth’s flora.
The life cycle of a pine:
gymnosperm life cycle is the long
lived sporophyte. The female
gametophyte, the ovule, is
nutritionally dependent on the
sporophyte. Fertilization of the sperm
and egg cell produces a woody naked
seed.
All angiosperms are classified under phylum Anthophyta. The key
adaptations of angiosperms are flowers and fruits. Flowers generally consist of
four whorls of modified leaves: sepals, petals, stamens (which produce pollen),
and carpels (which produce ovules). Ovaries ripen into fruits, which are often
carried by wind, water, or animals to new locations. In the angiosperm lifecycle,
double fertilization occurs when a pollen tube discharges two sperm into the
female gametophyte. One sperm fertilizes the egg, while the other combines
with two nuclei at the center of the female gametophyte and starts the
development of food- storing endosperm tissue. The endosperm nourishes the