Transcript The Significance of Seedless Vascular Plants

Gametophyte-sporophyte relationships in different plant groups.
The Significance of Seedless Vascular Plants
The ancestors of living lycophytes, horsetails, and ferns,
along with their extinct seedless vascular relatives, grew to
great heights during the late Devonian and early
Carboniferous, forming the first forests.
How did their dramatic growth affect Earth and its other
life? With the evolution of vascular tissue, roots, and leaves,
these plants accelerated their rate of photosynthesis,
dramatically increasing the removal of CO2 from the
atmosphere.
Scientists estimate that CO2 levels dropped by as much as a
factor of five during the Carboniferous, causing global cooling
that resulted in widespread glacier formation.
The seedless vascular plants that formed the first forests
eventually became coal. In the stagnant waters of Carboniferous
swamps, dead plants did not completely decay. This organic
material turned to thick layers of peat, later covered by the sea.
Marine sediments piled on top, and over millions of years, heat
and pressure converted the peat to coal. In fact, Carboniferous
coal deposits are the most extensive ever formed.
Coal was crucial to the Industrial Revolution, and people
worldwide still burn 6 billion tons a year. It is ironic that coal,
formed from plants that contributed to a global cooling now
contributes to global warming by returning carbon to the
atmosphere.
Growing along with the seedless plants in Carboniferous
swamps were primitive seed plants. Though seed plants were not
dominant at that time, they rose to prominence after the swamps
began to dry up at the end of the Carboniferous period
Gametophyte-sporophyte relationships in different plant groups.
A
. seed consists of an embryo and its food supply,
surrounded by a protective coat.
When mature, seeds are dispersed from their parent by
wind or other means. Because it nourishes and protects the
embryo-yet can move away from the mother plant
Seeds are a key adaptation that helped seed plants to
become the dominant producers on land and to make up the
vast majority of plant biodiversity today’s society.
Starting about 13,000 years ago, humans began to cultivate
wheat, figs, maize (commonly called corn), bananas, and
other wild seed plants.
This practice emerged independently in various parts of the
world, including the Near East, East Asia, Africa, and the
Americas. One piece of evidence, the well-preserved squash
The domestication of seed plants, particularly angiosperms,
produced the most important cultural change in human
history, transforming most human societies from roving
bands of hunter-gatherers to permanent settlements
anchored by agriculture.
Seed & pollen grains are key adaptations for life on
land
The terrestrial adaptations that seed plants added to those
already present in nonvascular plants (bryophytes) and
seedless vascular plants. The additional characters were :
Reduced gametophytes, heterospory, ovules, and pollen.
These adaptations provided new ways for seed plants to
cope with terrestrial conditions such as drought and
exposure to the ultraviolet (UV) radiation in sunlight.
Novel adaptations also freed seed plants from requiring
water for fertilization, enabling reproduction to occur under
a brooder range of conditions than in seedless plants.
Gymnosperms are plants that have "naked" seeds that are
not enclosed in ovaries.
Their seeds are exposed on modified leaves (sporophylls)
that usually form cones (strobili). (In contrast. Angiosperm
seeds are enclosed in fruits, which are mature ovaries.)
Products from Seed Plants
Most of our food comes from angiosperms. Just six crops
wheat, rice, maize, potatoes, cassava, and sweet potatoes yield
80% of all the calories consumed by humans.
We also depend on angiosperms to feed livestock: It takes 57 kg of grain to produce 1 kg of grain-fed beef.
Today's crops are the products of a relatively recent burst of
genetic change, resulting from artificial selection after humans
began domesticating plants approximately 13,000 years ago.
To appreciate the scale of the transformation, note how the
number and size of seeds in domesticated plants is greater
than those of their wild relatives, as in the case of maize
At the genetic level, scientists can glean information about
domestication by comparing the genes of crops with those of
wild relatives.
With maize, dramaticchanges, such as increased cob size
and loss of the hard coating around teosinte kernels, may have
been initiated by as few as five mutations.
How did wild plants change 50 dramatically in such a
relatively short time?
For thousands of years, farmers have selected the seeds of
plants with desirable traits (large fruits, for example) to plant
for the next year's crops.
Humans may also have selected the traits of some plants
indirectly, as in the case of wild almonds. Almonds contain a
bitter compound called amygdalin that repels birds and other
animals. Amygdalin breaks down into cyanide, 50 eating a
large number of wild almonds can be fatal. But mutations can
reduce the level of amygdalin, making almonds sweet rather
than bitter.
Wild birds eat almonds from trees with such mutations.
According to one hypothesis, humans may have observed
birds eating the almonds and then eaten the almonds
themselves-ultimately using such seeds to grow trees that
produced sweeter, less dangerous almonds.
In addition to staple crops, flowering plants provide other
edible products. Two of the world's most popular beverages
come from tea leaves and coffee beans, and you can thank the
tropical cacao tree for cocoa and chocolate.
 Spices are derived from various plant parts, such as flowers
(cloves, saffron), fruits and seeds (vanilla, black pepper,
mustard, cumin), leaves (basil, mint, sage), and even bark
(cinnamon).
Many seed plants, both gymnosperms and angiosperms, are
sources of wood, which is absent in all living seedless plants.
Wood consists of an accumulation of tough-walled xylem cells .
Wood is the primary source of fuel for much of the world,
and wood pulp, typically derived from conifers such as fir and
pine, is used to make paper. Worldwide, wood also remains the
most widely used construction material.
For centuries, humans have also depended on seed
plants for medicines.
Many cultures have a long tradition of using herbal
remedies, and scientific research has identified the relevant
secondary compounds in many of these plants, leading to
the synthesis of medicines.
Willow leaves and bark, for instance, have been used
since ancient times in pain relieving remedies, including
prescriptions by the Greek physician Hippocrates.
In early nineteenth century, scientists traced the willow's
medicinal property to the chemical salicin. A synthesized
derivative, acetylsalicylic acid, is what we call aspirin.
Although modern chemistry facilitates laboratory
synthesis, plants remain an important direct source of
medicinal compounds.
In the United States, for example, about
25%ofprescription drugs contain one or more active
ingredients extracted or derived from plants, typically from
seed plants.
Other ingredients were first discovered in seed plants and
then synthesized artificially as secondary compounds of
seed plants