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Plant Diversity. Chapter 22. What is a Plant?. Plants are multicellular eukaryotes with cell walls made of cellulose , and they carry out photosynthesis. Diversity of Plants.
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Plant Diversity Chapter 22
What is a Plant? • Plants are multicellular eukaryotes with cell walls made of cellulose, and they carry out photosynthesis.
Diversity of Plants • There is a wide diversity of plants, from tiny mosses, to huge trees such as giant redwoods that can grow to over 100m tall.
What is a Plant? • https://www.youtube.com/watch?v=In7Y4G1FqaY (3 min)
The Role of Plants • Plants play a key role in any ecosystem – providing the basis for most food webs, providing habitat, cleaning the air, preventing erosion, and much more. • While plants are different from animals in many ways (including being stationary autotrophs), they have found unique ways to deal with the challenges of life.
Plant Life Cycle • Plant life cycles include what is known as the “alternation of generations”, where there is a haploid and a diploid phase, each in their own generation.
Science 9 Refresher • Haploid refers to any cell that contains only 1 set of chromosomes (n), and diploid refers to any cell having a double set of chromosomes (2n)
Humans • In humans and most other animals, only our sex cells (gametes: sperm and egg cells) are haploid. These cells contain only 1 set of our 23 chromosomes. • The rest of the cells in our bodies are diploid, and they contain 2 complete sets of 23 chromosomes, for a total of 46.
Refresher – Mitosis • When cells reproduce (duplicate), they usually do so by mitosis, a type of cell division that produces 2 daughter cells that are identical to each other and to the parent cell (clones).
Refresher – Meiosis • To produce a haploid cell from the original diploid cell however, a different form of cell division is required – meiosis. Meiosis includes 2 rounds of cell division, and results in 4 daughter cells that are all different from each other and from the parent cells.
Alternation of Generation • The diploid (2n) phase of a plant’s life cycle is known as the sporophyte (spore-producing plant). • Sporesare haploid (n) reproductive cells produced by meiosis in the sporophyte, and they can mature into new haploid individuals known as gametophytes (gamete-producing plant). • Gametesare haploid(n) reproductive cells produces in the gametophyte by mitosis, and they fuse with other gametes during fertilization to produce a diploid cell which can mature into the diploid sporophyte.
Water? • The simplest plants, mosses and ferns, require water to reproduce (usually to bring gametes together for fertilization), while the more recent seed plants can carry out reproduction without water. • Many plants can also carry out asexual reproduction.
Ingredients for Plant Survival • As stationary organisms on land, plants face a special series of challenges. Plants have developed many adaptations to get the sunlight, water, minerals, and gasses that they need, and to transport water and nutrients throughout their bodies.
Sunlight • Plants need the energy from sunlight in order to carry out photosynthesis. • This has led to many adaptations to gather more sunlight, including broad, flat leaves, arranged on the stem to maximize light absorption. • Plants may also grow taller to out-compete others for sunlight, or grow in areas with fewer competitors.
Gas Exchange • Plants require carbon dioxide to carry out photosynthesis, as well as oxygen for cellular respiration. They must exchange these gasses with the atmosphere through their leaves, without losing too much extra water through transpiration.
Water • All cells require a continuous supply of water – for plants, this means they must transport the water obtained by their roots to all other parts of the plant, including those found far above the ground.
Water • Plants also need water as a key ingredient in photosynthesis, so when the sun is shining, plant cells use up water very quickly, and tissues can dry out easily. Plants have developed adaptations such as waxy coatings and stomata that can be closed, to prevent excess water loss.
Minerals • When plants soak up water through their roots, they also gain minerals from the soil. These minerals include nutrients that are required for plant growth.
Transporting Water & Nutrients • Plants soak up water and nutrients from their roots, but produce food in their leaves via photosynthesis. • Most plants (vascular plants) have specialized tissues to carry water and nutrients upward, and distribute the products of photosynthesis throughout the body tissues of the plant.
Transport of Water and Nutrients • Simpler types of plants (non-vascular plants) don’t have these specialized tissues, so they simply use diffusion to transport these substances.
Evolution of Plants • For most of Earth’s history, almost all life was in the water, and plants did not exist. • Photosynthetic algae and prokaryotes added oxygen to the atmosphere, and provided food for aquatic animals and microbes.
Evolution of Plants • The first plants evolved from an organism much like multicellular green algae that exist today. • These algae have similar reproductive cycles to plants, and have cellulose cell walls and use the same photosynthetic pigments as plants. • DNAsequences confirm that plants are closely related to certain groups of green algae that are found in freshwater environments.
Fossil Plants • The oldest known fossils of plants (450 million years old) were very similar to mosses found today. These early plants grew close to the ground, and were dependent on water for their reproduction, similar to mosses today.
Changes Over Time • Over time, natural selection favoured adaptations that allowed plants to move away from the water - to be more resistant to drying out, more capable of conserving water, and able to reproduce without water.
Evolution of Plants • This first group of plants led to all the current groups of plants.
Groups of Plants • One lineage developed into non-vascular plants including mosses and their relatives. Another line led to all other plants, including ferns, seed-bearing plants, cone-bearing plants, and flowering plants.
Plants Today • Plants are still classified into these groups by botanists (scientists who study pants) today. • Each of these groups has been successful on land, but they have all developed very different adaptations to a wide range of environments.
Bryophytes: Non-Vascular Plants • Plants that do not have specialized tissues for transporting water and nutrients (vascular tissues) are known as bryophytes, or non-vascular plants. • This includes mosses and their relatives: liverworts and hornworts.
Bryophytes • Bryophytes depend on water for reproduction. As a result, they can only live in areas where there is significant rainfall or dew for at least part of the year. • Without vascular tissues, they must use osmosis to transport water, which can only work for a maximum of a few centimeters.
Bryophytes • These two features together means that bryophytes are low-growing plants found in moist or shaded areas.
Mosses • Mosses are the most common bryophytes • They are found in areas with lots of water – swamps and bogs, near streams, and in rain forests • Well adapted wet habitats and nutrient-poor soils – this allows them to grow in harsh environments where other plants can’t survive
Mosses • Mosses are the most common plants in the polar regions
Mosses • The moss plants we are most familiar with are the gametophyte stage • The sporophyte stage of a moss is a thin, upright shoot topped with a capsule
Mosses • The tissues of mosses are only 1 cell thick, so they lose water very quickly if the air becomes too dry. • Mosses do not have true roots – instead they have rhizoids, long, thin cells that anchor them to the ground and absorb water and nutrients from the soil
Liverworts • Look like flat leaves attached to the soil; this is the gametophyte stage • Named liverworts because in some species this “leaf” is shaped like a liver
Liverworts • This broad, thin structure absorbs moisture and nutrients directly from the soil • Only found in areas where the soil is damp year-round
Liverworts • To reproduce, the gametophyte produces structures that look like tiny umbrellas, which is where the eggs and sperm are produced
Liverworts • Some liverworts can also reproduce asexually • Small multicellular reproductive structures called gemmae are produced • When washed away from the main plant by water, they can divide by mitosis to produce a new individual
Horworts • Gametophyte looks similar to the liverwort gametophyte • Also only found in areas where the soil is damp year-round • Hornwort sporophyte looks like a tiny horn, which is how it got its name
Life Cycle of Bryophytes • Like all plants, bryophyte life cycles include alternationof generations. • The gametophyte is the dominant form of the plant, and this is the phase where most of the photosynthesis takes place. • The sporophyte is dependent on the gametophyte to supply water and nutrition.
