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 landThe 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 PlantsMost 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 5-7 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