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Chapter 30

Chapter 30 Textbook Notes - Green Plants

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Christoph Richter

Notes From Reading CHAPTER 30:G REENP LANTS (PGS. 657-695) Key Concepts  The green plants include both the green algae and the land plants  Green algae are an important source of oxygen and provide food for aquatic organisms; land plants hold soil and water in place, build soil, moderate extreme temperatures and winds, and provide food for other organisms  Land plants were the first multicellular organisms that could live with most of their tissues exposed to the air. A series of key adaptations allowed them to survive on land. In terms of total mass, plants dominate today’s terrestrial environments  Once plants were able to grow on land, a sequence of important evolutionary changes made it possible for them to reproduce efficiently – even in extremely dry environments Introduction  The green plants dominate terrestrial and freshwater habitats  Comprise two major types of organisms: the green algae and the land plants  Green algae are important photosynthetic organisms in aquatic habitats (lakes, pond and other freshwater settings)  Land plants are the key photosynthesizes in terrestrial environments  They are the closet living relative to land plants and form a monophyletic groups with them  The transition from aquatic to terrestrial life occurred when land plants evolved from green algae 30.1 Why Do Biologists Study the Green Plants?  Agriculture, forestry and horticulture are among the mot important endeavors supported by biological science  Biologists study plants not only because they are captivating organisms but also because they keep us alive Plants Provide Ecosystem Services  An ecosystem consists of all the organisms in a particular area, along with physical components of the environment, such as the atmosphere, precipitation, surface water, sunlight, soil and nutrients  Plants provide ecosystem services because they add to the quality of the atmosphere, surface water, solid and other physical components of an ecosystem  Plants alter the landscape in ways that benefit other organism: o They produce oxygen via oxygenic photosynthesis o They build soil by providing food for decomposers o They hold soil and prevent nutrients from being lost by erosion by wind and water o They moderate the local climate by providing shade, reducing the impact of winds that dry out landscapes or make them colder  Perhaps the most important ecosystem service provided by plants involves food  They are the dominant primary producers in terrestrial ecosystems and provide the base of the food chain in the vast majority of terrestrial habitats  Plants are eaten by herbivores which are eaten by carnivores which eaten by omnivores – organisms that eat both plant and animals  Omnivores feed at several different levels in the terrestrial food chain  Green plants are the key to the carbon cycle on the continents Plants Provide Humans with Food, Fuel, Building Materials and Medicines  Plants provide most of our food supply as well as a significant percentage of the fuel, fibres, building materials and medicines that we use  Agricultural research began with the initial domestication of crop plants  Artificial selection for plants with certain properties can lead to dramatic changes in plant characteristic  Humans have relied on plant-based fuels such as wood and coal  Plants are a key source of drugs 30.2 How Do Biologists Study Green Plants?  To understand how green plants originated and diversified, biologists use three tools: Notes From Reading CHAPTER 30:G REEN PLANTS (PGS . 657-695) o They compare the fundamental morphological features of various green algae and green plants o They analyze the fossil record of the lineage o They assess similarities and difference in DNA sequences from homologous genes to estimate phylogenetic trees Analyzing Morphological Traits  Green algae have long been hypothesized to be closely related to plants on the basis of several key morphological traits  The green algae include species that are unicellular, colonial or multicellular and that live in marine or freshwater habitats  Based on morphology, the most important phyla of plants are groups into three categories: nonvascular plants, seedless vascular plants and seed plants  Nonvascular plants lack vascular tissue – specialized groups of cells that conduct water or dissolved nutrients from one part of the plant body to another  Seedless vascular plants have well-developed vascular tissue but do not make seeds  A seed consists of an embryo and a store of nutritive tissue, surrounded by a tough protective layer  Seed plants have vascular tissue and make seeds  Within the seed plants, gymnosperms produce seeds that do not develop in an enclosed structure  In the flowering plants or angiosperms, seeds develop inside a protective structure called a carpel Using the Fossil Record  The fossil record for green algae began 700-725 million years ago  The fossil record for land plants began 475 million years ago  The fossil record for plants is massive and is broken up into five segments, each of which encompasses a major event in the diversification of land plants  According to the fossil record, the green algae first, followed y the nonvascular plants, seedless vascular plants and seed plants Evaluating Molecule Phylogenies  The phylogenetic tree has several important points 1. Land plants probably evolved from green algae 2. The green algal group called Charales is the sister group to land plants – meaning that Charales are the closet living relative 3. The green plants are monophyletic, meaning that a single common ancestor gae rise to all of the green algae and land plants 4. The green algae group is paraphyletic; the land plants are monophyletic 5. The nonvascular plants are the most basal groups among land plants 6. Morphological simplicity of the whisk ferns is probably a derived trait, meaning that complex structures have been lost in this lineage 7. Seeds and flowers evolved only once 30.3 What Themes Occur in the Diversification of Green Plants?  The story of land plants is the story of adaptations that allowed photosynthetic organisms to move from aquatic to terrestrial environments The Transition of Land, I: How Did Plants Adapt to Dry Conditions?  Plants had to adapt to conditions in which only a portion of their tissues are wet  The adaptation of the water problem arose in two steps o Prevention of water loss from cells o Transportation of water from tissues with access to water to tissues without access Preventing Water Loss: Cuticle and Stomata  Cuticle is a waxy, watertight sealant that gives plants the ability to survive in dry environments  Gas exchange is accomplished by stomata, which have a pore that opens and closes as the guard cells change shape Transporting Water: Vascular Tissue and Upright Growth  The first land plants probably lacked rigidity and were low and sprawling Notes From Reading CHAPTER 30:G REEN PLANTS (PGS. 657-695)  The evolution of vascular tissue allowed early plants to support erect stems and transport water from roots to aboveground tissues  Vascular tissue evolved in a series of gradual steps that provided an increasing level of structural support, allowing plants to grow upright Mapping Evolutionary Changes on the Phylogenetic Tree  Land plants adapted to dry conditions by mapping where major innovations occurred as the group diversified The Transition to Land II: How Do Plants Reproduce in Dry Conditions?  Innovations that were instrumental for efficient in plant reproduction in a dry environment include: o Development of spore that resist drying o Gametes were produced in complex, multicellular structure o The embryo was retained on the parent plant and nourished by it Retaining and Nourishing Offspring: Land Plants as Embryophytes  The gametophytes of early land plants contain specialized reproductive organs called gametangia that protected gametes from drying and mechanical damage  Individuals produce distinctive male and female gametangia  The sperm-producing structure is the antheridium  The egg-producing structure is the archegonium  In land plants, the zygote is retained on the gametophyte after fertilization and begins to develop on the parent plant to form a multicellular embryo that remains attached to the parent and is nourished by it Alteration of Generations  All land plants undergo alteration of generations  They have a multicellular haploid phased called the gametophyte and a multicellular diploid phase known as the sporophyte  The relationship between gametophyte and sporophyte is variable  Alteration of generations involves the same basic sequence of events o Gametophytes produce gametes by mitosis  both the gametophyte and gametes are haploid o Two gametes unite during fertilization to form a diploid zygote o The zygote divides by mitosis and develops into a multicellular, diploid sporophyte o The sporophyte produces spores by meiosis  spores are haploid o Spores divide by mitosis and develop into a haploid gametophyte The Gametophyte-Dominant to Sporophyte-Dominant Trend in Life Cycles  In land plants, the relationship between gametophyte and sporophyte is highly variable  Gametophyte-dominated life cycles evolved early; sporophyte-dominated life cycles evolved later Heterospory  Another important innovation found in seed plant is heterospory, the production of two distinct types of spore-producing structure and thus two distinct types of spores  Homosporous plants produce a single type of spore that develops into a bisexual gametophytes that produce both eggs and sperm  The two types of spore-producing structures in heterosporous species are microsporangia and macrosporangia  Mi
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