CHAPTER 30
PLANT DIVERSITY II:
THE EVOLUTION OF SEED PLANTS
Introduction
·
The evolution of plants is highlighted by two important landmarks:
·
The evolution of seeds, which led to the gymnosperms and angiosperms,
the plants that dominate most modern landscapes.
·
The emergence of the importance of seed plants to animals, specifically
to humans.
·
Agriculture, the cultivation and harvest of plants (primarily seed
plants), began approximately 10,000 years ago in Asia, Europe, and the
Americas.
·
This was the single most important cultural change in the history of
humanity, for it made possible the transition from hunter-gatherer societies to
permanent settlements.
·
The seeds and other adaptations of gymnosperms and angiosperms enhanced
the ability of plants to survive and reproduce in diverse terrestrial
environments.
·
Plants became the main producers on land.
·
Seed plants are vascular plants that
produce seeds.
·
Contributing to the success of seed plants as terrestrial organisms are
three important reproductive adaptations:
·
Continued reduction of the gametophyte.
·
The advent of the seed.
·
The evolution of pollen.
1. Reduction of the gametophyte continued with the evolution of seed plants
·
An important distinction between mosses and other bryophytes and ferns
and other seedless vascular plants is a gametophyte-dominated life cycle for
bryophytes and a sporophyte-dominant life cycle for seedless vascular plants.
·
Continuing that trend, the gametophytes of seed plants are even more
reduced than those of seedless vascular plants such as ferns.
·
In seeds plants, the delicate female gametophyte and young embryos are
protected from many environmental stresses because they are retained within the
moist sporangia of the parental sporophyte.
·
The gametophytes of seed plants obtain nutrients from their parents,
while those of seedless vascular plants are free-living and fend for
themselves.
·
For the gametophyte to exist within the sporophyte has required extreme
miniaturization of the gametophyte of seed plants.
·
The gametophytes of seedless vascular plants are small but visible to
the unaided eye, while those of seed plants are microscopic.
·
Why has the gametophyte generation not been completely eliminated from
the plant life cycle?
·
The haploid generation may provide a mechanism for “screening” new
alleles, including mutations.
·
Gametophytes with deleterious mutations affecting metabolism or cell
division will not survive to produce gametes that could combine to start new
sporophytes.
·
Another possible reason is that all sporophyte embryos are dependent,
at least to some extent, on tissues of the maternal gametophyte.
·
The gametophyte nourishes the sporophyte embryo, at least during its
early development.
2. Seeds became an important means of dispersing offspring
·
In bryophytes and seedless vascular plants, spores from the sporophyte
are the resistant stage in the life cycle.
·
For example, moss spores can survive even if the local environment is
too extreme for the moss plants themselves to survive.
·
Because of their tiny size, the spores themselves might also be
dispersed in a dormant state to a new area.
·
Spores were the main way that plants spread over Earth for the first
200 millions years of life on land
·
The seed represents a different solution to resisting harsh
environments and dispersing offspring.
·
In contrast to a single-celled spore, a multicellular seed is a more
complex, resistant structure.
·
A seed consists of a
sporophyte embryo packaged along with a food supply within a protective coat.
·
There are evolutionary and developmental relationships between spores
and seeds.
·
The parent sporophyte does not release its spores, but retains them
within its sporangia.
·
Not only are the spores retained, but the gametophyte develops within
the spore from which it is derived.
·
All seed plants are heterosporous, producing two different types of
sporangia that produce two types of spores.
·
Megasporangia produce megaspores, which give rise to female
(egg-containing) gametophytes.
·
Microsporangia produce microspores, which give rise to male
(sperm-containing) gametophytes.
·
In contrast to heterosporous seedless vascular plants, the megaspores
and the female gametophytes of seed plants are retained by the parent
sporophyte.
·
Layers of sporophyte tissues, integuments,
envelop and protect the megasporangium
·
An ovule consists of
integuments, megaspore, and megasporangium.
·
A female gametophyte develops inside a megaspore and produces one or
more egg cells.
·
A fertilized egg develops into a sporophyte embryo.
·
The whole ovule develops into a seed.
·
A seed’s protective coat is derived from the integuments of the ovule.
·
Within this seed coat, a seed may remain dormant for days, months, or
even years until favorable conditions trigger germination.
·
When the seed is eventually released from the parent plant, it may be
close to the parent, or be carried off by wind or animals.
3. Pollen eliminated the liquid-water requirement for fertilization
·
The microspores, released from the microsporangium, develop into pollen
grains.
·
These are covered with a tough coat containing sporopollenin.
·
They are carried away by wind or animals until pollination occurs when they land in the vicinity of an ovule.
·
The pollen grain will elongate a tube into the ovule and deliver one or
two sperm into the female gametophyte.
·
While some primitive gymnosperms have flagellated sperm cells, the
sperm in most gymnosperms and all angiosperms lack flagella.
·
In seed plants, the use of resistant, far-traveling, airborne pollen to
bring gametes together is a terrestrial adaptation.
·
In bryophytes and pteridophytes, flagellated sperm must swim through a
film of water to reach eggs cells in archegonia.
·
The evolution of pollen in seed plants led to even greater success and
diversity of plants on land.
4. The two clades of seed plants are gymnosperms and angiosperms
·
Like other groups of organisms, our understanding of plant taxonomy is being revised to
reflect new data, new methods, and new ideas.
·
The current data support a phylogeny of the seed plants with two main
monophyletic branches - the gymnosperms and the angiosperms.
· Both probably evolved from different ancestors in an extinct group of plants, the progymnosperms, some of which had seeds.
·
The most familiar gymnosperms are the conifers, the cone-bearing plants
such as pines.
·
The ovules and seeds of gymnosperms (“naked seeds”) develop on the
surfaces of specialized leaves called sporophylls.
·
In contrast, ovules and seeds of angiosperms develop in enclosed
chambers (ovaries).
·
Gymnosperms appears in the fossil record much earlier than angiosperms.
1. The
Mesozoic era was the age of gymnosperms
·
The gymnosperms probably descended from progymnosperms, a group of
Devonian plants.
·
While the earliest progymnosperms lacked seeds, by the end of the
Devonian, some species had evolved seeds.
·
Adaptive radiation during the Carboniferous and early Permian produced
the various phyla of gymnosperms.
·
The flora and fauna of Earth changed dramatically during the formation
of the supercontinent Pangaea in the Permian.
·
This likely led to major environmental changes, including drier and
warmer continental interiors.
·
Many groups of organisms disappeared and others emerged as their
successors.
·
For example, amphibians decreased in diversity while reptiles
increased.
·
Similarly, the lycophytes, horsetails, and ferns that dominated in
Carboniferous swamps were largely replaced by gymnosperms, which were more
suited to the drier climate.
·
The change in organisms was so dramatic that geologists use the end of
the Permian, about 245 million years ago, as the boundary between the Paleozoic
and Mesozoic eras.
·
The terrestrial animals of the Mesozoic, including dinosaurs, were
supported by a vegetation consisting mostly of conifers and cycads, both
gymnosperms.
·
The dinosaurs did not survive the environmental upheavals at the end of
the Mesozoic, but many gymnosperms persisted and are still an important part of
Earth’s flora.
2. The four phyla of extant gymnosperms are ginkgo, cycads, gnetophytes, and conifers
·
There are four plant phyla grouped as gymnosperms.
·
Phylum Ginkgophyta consists of only a single
extant species, Ginkgo biloba.
·
This popular ornamental species has fanlike leaves that turn gold
before they fall off in the autumn.
·
Landscapers usually plant only male trees because as the seed coats on
female plants decay, they produce a repulsive odor (to humans, at least).
·
Cycads (phylum Cycadophyta)
superficially resemble palms.
·
Palms are actually flowering plants.
·
Phylum Gnetophyta consists of three very
different genera.
·
Weltwitschia plants, from deserts in southwestern
Africa, have straplike leaves.
·
Gentum species are tropical trees
or vines.
·
Ephedra (Mormon tea) is a shrub of
the American deserts.
·
The conifer, phylum
Coniferophyta, is the largest gymnosperm phylum.
·
The term conifer comes from
the reproductive structure, the cone, which is a cluster of scalelike
sporophylls.
·
Although there are only about 550 species of conifers, a few species
dominate vast forested regions in the Northern Hemisphere where the growing
season is short.
·
Conifers include pines, firs, spruces, larches, yews, junipers, cedars,
cypresses, and redwoods.
·
Most conifers are evergreen, retaining their leaves and
photosynthesizing throughout the year.
·
Some conifers, like the dawn redwood and tamarack, are deciduous,
dropping their leaves in autumn.
·
The needle-shaped leaves of some conifers, such as pines and firs, are
adapted for dry conditions.
·
A thick cuticle covering the leaf and the placement of stomata in pits
further reduce water loss.
·
Much of our lumber and paper comes from the wood (actually xylem
tissue) of conifers.
·
This tissue gives the tree structural support.
·
Coniferous trees are amongst the largest and oldest organisms of Earth.
·
Redwoods from northern California can grow to heights of over 100m.
·
One bristlecone pine, also from California, is more than 4,600 years
old.
3. The life cycle of a pine demonstrates the key reproductive adaptations of seed plants
·
The life cycle of a pine illustrates the three key adaptations to
terrestrial life in seed plants:
·
Increasing dominance of the sporophyte.
·
Seeds as a resistant, dispersal stage.
·
Pollen as an airborne agent bringing gametes together.
·
The pine tree, a sporophyte, produces its sporangia on scalelike
sporophylls that are packed densely on cones.
·
Conifers, like all seed plants, are heterosporous, developing male and
female gametophytes from different types of spores produced by separate cones.
·
Each tree usually has both types of cones.
·
Small pollen cones produce microspores that develop into male
gametophytes, or pollen grains.
·
Larger ovulate cones make megaspores that develop into female
gametophytes.
·
It takes three years from the appearance of young cones on a pine tree
to the formation mature seeds.
·
The seeds are typically dispersed by the wind.
·
Reproduction in pines begins with the appearance of cones on a pine
tree.
·
1) Most species produce both pollen cones and ovulate cones.
·
2) A pollen cone contains hundreds of microsporangia held on small
sporophylls.
·
Cells in the microsporangia undergo meiosis to form haploid microspores
that develop into pollen grains.
·
3) An ovulate cone consists of many scales, each with two ovules.
·
Each ovule includes a megasporangium.
• 4) During
pollination, windblown pollen falls on the ovulate cone and is drawn into the
ovule through the micropyle.
·
The pollen grain germinates in the ovule, forming a pollen tube that
digests its way through the megasporangium.
·
5) The megaspore mother cell undergoes meiosis to produce four haploid
cells, one of which will develop into a megaspore.
·
The megaspore grows and divides mitotically to form the immature female
gametophyte.
·
6) Two or three archegonia, each with an egg, then develop within the
gametophyte.
·
7) At the same time that the eggs are ready, two sperm cells have
developed in the pollen tube which has reached the female gametophyte.
·
Fertilization occurs when one of the sperm nuclei fuses with the egg
nucleus.
·
8) The pine embryo, the new sporophyte, has a rudimentary root and
several embryonic leaves.
·
The female gametophyte surrounds and nourishes the embryo.
· The ovule develops into a pine seed, which consists of an embryo (new sporophyte), its food supply (derived from gametophyte tissue), and a seed coat derived from the integuments of the parent tree (parent sporophyte).
C. Angiosperms
(Flowering Plants)
·
Angiosperms, better known as flowering plants, are vascular seed plants
that produce flowers and fruits.
·
They are by far the most diverse and geographically widespread of all
plants.
·
There are about 250,000 known species of angiosperms.
1. Systematists are identifying the angiosperm clades
·
All angiosperms are placed in a single phylum, the phylum Anthophyta.
·
As late as the 1990s, most plant taxonomists divided the angiosperms
into two main classes, the monocots
and the dicots.
·
Most monocots have leaves with parallel veins, while most dicots have
netlike venation.
·
Recent systematic analyses have upheld the monocots as a monophyletic
group.
·
They include lilies, orchids, yuccas, grasses, and grains.
·
However, molecular systematics has indicated that plants with the dicot
anatomy do not form a monophyletic group.
·
One clade, the eudicots, does include the majority of dicots.
·
It includes roses, peas, sunflowers, oaks, and maples.
·
Some other dicots actually belong to angiosperm lineages that diverged
earlier that the origin of either monocots or eudicots.
·
These include the star anise, the water lilies, and Amborella trichopoda from the oldest
angiosperm branch.
·
While most angiosperms belong to either the monocots (65,000 species)
or eudicots (165,000 species) several other clades branched off before these.
·
Based on molecular analyses, Arborella
is the only survivor of a branch at the base of the angiosperm tree.
·
Refinements in vascular tissue, especially xylem, probably played a
role in the enormous success of angiosperms in diverse terrestrial habitats.
·
Like gymnosperms, angiosperms have long, tapered tracheids that
function for support and water transport.
·
Angiosperms also have fibers cells, specialized for support, and vessel
elements (in most angiosperms) that develop into xylem vessels for efficient
water transport.
2. The flower is the defining reproductive adaptation of angiosperms
·
While evolutionary refinements of the vascular system contributed to
the success of angiosperms, the reproductive adaptations associated with
flowers and fruits contributed the most.
·
The flower is an angiosperm
structure specialized for reproduction.
·
In many species, insects and other animals transfer pollen from one
flower to female sex organs of another.
·
Some species that occur in dense populations, like grasses, rely on the
more random mechanism of wind pollination
·