Levetin−McMahon: Plants and Society, Fifth EditionII. Introduction to Plant Life: Botanical Principles9. Diversity of Plant Life © The McGraw−Hill Companies, 2008139 9 Diversity of Plant Life CHAPTER OUTLINE The Three-Domain System 140 Survey of The Plant Kingdom 140A CLOSER LOOK 9.1 Alternationof Generations 142Mosses, Liverworts, and Hornworts 144 Fern-Allies and Ferns 146 Gymnosperms 149 A CLOSER LOOK 9.2 Amber: A Glimpse Into The Past 152 Angiosperms 154 Chapter Summary 154 Review Questions 154 Further Reading 154 KEY CONCEPTS 1. Living organisms are classified into three domains: Archaea, Bacteria, and Eukarya.2. Land plants occur in the kingdom Planta of the domain Eukarya and include a diverse group of organisms that impacts our lives in many economically and ecologically important ways. 3. All plants have an alternation of diploid and haploid generations although the structure of each generation differs in the various divisions. CHAPTER Cypress swamps are characterized by a rich diversity of plants representing various divisions of the Kingdom PlantaeLevetin−McMahon: Plants and Society, Fifth EditionII. Introduction to Plant Life: Botanical Principles9. Diversity of Plant Life © The McGraw−Hill Companies, 2008140UNIT IIIntroduction to Plant Life: Botanical Principles With the overwhelming diversity of life on Earth, scientists have long sought to categorize these organisms into a meaningful system. For many years, organisms were classified as either plants or animals. As our knowledge increased, it became evident that many organisms could not be conveniently classified as either. Other kingdoms have been suggested to solve the problems with the two-kingdom system. One system proposed by Robert Whitaker in 1969 classified organisms into five kingdoms: Animalia, Plantae, Fungi, Protista, and Monera. In this system, the kingdom Monera contained organisms with prokaryotic cells while the other four kingdoms had eukaryotic cells. Recall that a prokaryotic cell lacks a nucleus and membrane-bound organelles that occur in eukaryotes. Research during the 1970s and 1980s changed the scientific understanding about relation-ships among prokaryotic organisms; two distinctly different types of prokaryotic organisms were recognized. This research led to the current classification of living organisms into three domains. A domain is a taxonomic category above the rank of kingdom. THE THREE-DOMAIN SYSTEM In 1990 microbiologist Carl Woese proposed the reorga-nization of life into the domains Eukarya, Archaea, andBacteria (Eubacteria). The domain Eukarya includes all the eukaryotic kingdoms while Archaea and Bacteria are pro-karyotic domains. Although archaea look like bacteria, they represent a distinct evolutionary line. The majority of prokaryotic organisms are in the domain Bacteria. The Archaea include methane producers as well as organisms that live in hot springs and environments with high salt content. It has been suggested that these extreme environments may be similar to conditions that existed during early Earth history. The two domains of prokaryotic organ-isms have differences in ribosomal RNA as well as several other molecular and biochemical characteristics. In fact for some of the molecular characteristics, archaea are similar to eukaryotic cells. The domain Eukarya includes the kingdoms Protista, Plantae, Fungi, and Animalia although some researchers have suggested splitting the kingdom Protista into several smaller kingdoms. Organisms in the kingdom Protista consist of unicellular and simple multicellular organisms that can be plant-like, fungus-like, or animal-like. The remaining three kingdoms are all multicellular in organization and can be distinguished by their modes of nutrition. Members of the kingdom Plantae are land plants that are autotrophic , capable of manufacturing their own food through photosynthesis. The organisms in the kingdoms Animalia and Fungi cannotmake their own food and rely on external sources of nutrition. They are, therefore, considered heterotrophic. Animals,from primitive sponges to highly evolved mammals, are ingestive heterotrophs, engulfing their food and digesting it internally. The fungi, from molds to mushrooms, are absorptive heterotrophs, secreting into their surroundings enzymes that break down food, which is then absorbed. Although historically fungi were considered members of the plant kingdom, recent molecular evidence suggests a closer evolutionary relationship between fungi and animals. Organisms that were once regarded as plants in the old two-kingdom system now are included in three eukaryotic kingdoms and the domain Bacteria. In particular, the algae and fungi are not included in the kingdom Plantae. The algae consist of a diverse grouping of photosynthetic organisms that have been classified according to pigment types, storage products, and ultrastructural features. They range from pro-karyotic microscopic forms to giant kelps and can be found in marine and freshwater habitats where they form the base of the food chains. The cyanobacteria, which were previously known as bluegreen algae, are in the domain Bacteria, and all the other groups of algae are in the kingdom Protista of the Eukarya. The algae will be discussed in detail in Chapter 22. Organisms traditionally called fungi are also included in two kingdoms. In the kingdom Protista are several groups of fun-guslike organisms that evolved along separate evolutionary pathways. The majority of fungi are classified in the kingdom Fungi. The fungi will be covered in Chapters 23–25. SURVEY OF THE PLANT KINGDOM The kingdom Plantae includes a diverse group of complex photosynthetic organisms ranging from mosses to flowering plants. Recall from Chapter 8 that large groupings of simi-lar organisms are called divisions (or phyla). This kingdom includes 12 divisions of plants with living representatives; however, other groups are extinct and known only from the fossil record. These 12 divisions are often referred to as land plants to distinguish them from the algae. One of the features of all land plants is the retention of the embryo. After fertil-ization, the zygote develops into a multicellular embryo while still enclosed in the female gametangium (reproductive struc-ture). Land plants are often called embrophytes to reflect this trait although some algae are found on land (damp soil), none