Plant Diversity II
1. Reduction of
the gametophyte continued with the evolution of seed plants
2. Seeds became an
important means of dispersing offspring
3. Pollen
eliminated the liquid-water requirement for fertilization
4. The two clades of seed plants are gymnosperms and angiosperms
The evolution of plants is marked by two
important facts :
(1) The evolution of seeds, which
lead to the gymnosperms and angiosperms, the plants that dominate most modern
landscapes
(2) The emergence of the importance
of seed plants to animals, specifically to humans.
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.
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.
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.
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.
3. The Mesozoic era was the age of gymnosperms
4. The four phyla of extant
gymnosperms are ginkgo, cycads, gnetophytes, and
conifers
5. The flower is
the defining reproductive adaptation of angiosperms
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 abut 250,000 known species of angiosperms.
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.
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 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.
A
fruit is a mature ovary.
As seeds
develop from ovules after fertilization, the wall of the ovary thickens to form
the fruit.
Fruits
protect dormant seeds and aid in their dispersal.
Various
modifications in fruits help disperse seeds.
In some
plants, such as dandelions and maples, the fruit functions like a kite or
propeller, enhancing wind dispersal.
Many
angiosperms use animals to carry seeds.
Fruits
are classified into several types depending on their developmental origin.
Simple fruits are derived from a single ovary.
These
may be fleshy, such as a cherry, or dry, such as a soybean pod.
An aggregate
fruit, such as a blackberry, results from a single flower with several
carpals.
A multiple
fruit, such as a pineapple, develops from an inflorescence, a tightly
clustered group of flowers.
One
hypothesis for the function of double fertilization is that it synchronizes the
development of food storage in the seed with development of the embryo.
Double
fertilization may prevent flowers from squandering nutrients on infertile
ovules.
Earths
landscape changed dramatically with the origin and radiation of flowering
plants.
The
oldest angiosperm fossils are found in rocks in the early Cretaceous, about 130
million years ago.
By the
end of the Cretaceous, 65 million years ago, angiosperms had become the
dominant plants on Earth.
Ever
since they colonized the land, animals have influenced the evolution of
terrestrial plants and vice versa.
The fact
that animals must eat affects the natural selection of both animals and plants.
Natural
selection must have favored plants that kept their spores and gametophytes far
above the ground, rather than dropping them within the reach of hungry ground
animals.
In turn,
this may have been a selective factor in the evolution of flying insects.