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? qaY (3 min) qaY

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)

Refresher – Haploid & Diploid

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.

Meiosis

Alternation of Generation The diploid (2n) phase of a plant’s life cycle is known as the sporophyte (spore-producing plant). Spores are haploid (n) reproductive cells produced by meiosis in the sporophyte, and they can mature into new haploid individuals known as gametophytes (gamete-producing plant). Gametes are 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.

Adaptations to Sunlight

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.

Stomata

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. DNA sequences 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 alternation of 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.

Dependence on Water Bryophytes must live in areas where water is available at least part of the year, since the sperm must swim through water to meet the eggs.

Life Cycle of a Moss A spore lands in a moist place and germinates to form a mass of green filaments called a protonema – As the protonema grows, it grows rhizoids into the ground, and shoots into the air – These shoots grow into the moss plants we are familiar with  gametophyte stage

Life Cycle of a Moss Gametes are produced at the ends of these stalks – Sperm produced in antheridia – Eggs produced in archegonia

Life Cycle of a Moss When sperm and egg meet, form diploid zygote  matures into the sporophyte – Sporophyte grows directly out of the gametophyte, depends on it for nutrients & water – Sporophyte ends in a capsule, where haploid spores are produced by meiosis Spores are scattered by the wind, start the cycle over again.

Sporophytes

Human Uses of Mosses Sphagnum mosses are a group of mosses that thrive in the acidic water in bogs. In some environments, layers of dead sphagnum moss accumulate to form peat bogs. Peat can be harvested and then burned as fuel.

Peat Bog

Bog Mummies!

Peat Moss Dried sphagnum absorbs enormous amounts of water, so it acts like a natural sponge. For this reason, peat is also often harvested to be used as peat moss in gardens. Adding peat moss to soil helps to retain water, plus peat has a low pH, so it can be used to raise the acidity of the soil, which is beneficial for some kinds of plants.

Peat Moss

Crash Course: Non-Vascular Plants 6y9U (9:40) 6y9U Take notes during the video – this will go in your Logbook

Weekend Homework Textbook readings – Section 22-1 (pg ): q 1, 3, 4 (pg 555) – Section 22-2 (pg ); q 1, 3, 4 (pg 559) Article Assignment #2 – Due Monday night before midnight – Submit through Google classroom please! Don’t forget to submit your pre-reading questions and the copy of your article

Seedless Vascular Plants Since early plants were dependent on osmosis to move water from cell to cell, they were limited to just a few centimetres. For millions of years, this was the maximum height of all plants. About 420 million years ago, however, plants over a metre tall appeared – these were the first plants with vascular tissue.

Vascular Tissues Xylem is the transport system that moves water upward from the roots, to every part of the plant – Xylem is made up of cells called tracheids, which were an evolutionary innovation. These cells are hollow, with thick cell walls that can resist pressure. These cells contain lignin that stiffens the cell walls. This allows plants to grow upright and to great heights

Xylem These cells are connected end-to-end like a series of straws, and they allow water to move through them, throughout the plant body much more efficiently than by diffusion alone.

Phloem Phloem is the second transport system, which transports solutions of nutrients and the products of photosynthesis, from the leaves to other parts of the plant

Vascular Tissues Both forms of vascular tissue – xylem and phloem – can move fluids through the plant body, even against the force of gravity. They form an integrated transport system to move water, nutrients, and other dissolved materials from one part of the plant to the other.

Seedless Vascular Plants Seedless vascular plants include club mosses, horsetails, and ferns. These plants all have true roots, leaves, and stems.

Parts of Vascular Plants Roots – underground organs to absorb water and minerals. The centre of the root contains water- conducting tissues. Leaves – photosynthetic organs that contain one or more bundles of vascular tissues, gathered into veins made of xylem and phloem. Stems – supporting structures that connect roots and leaves, carrying water and nutrients between them

Parts of the Vascular Plant

Club Mosses Ancient club mosses made up some of Earth’s first forests, with giant trees over 35 m tall. These ancient forests now make up much of the coal deposits we have today.

Club Mosses The club mosses still around today are fairly small plants that live in moist forests. They look somewhat like miniature pine trees, and are sometimes called “ground pines”.

Horsetails This group of plants has scale- like leaves that are arranged in distinctive whorls at joints along the stem. They are also called “scouring rush” because in colonial times, they were used to scour pots and pans, since they contain crystals of abrasive silica.

Ferns Ferns evolved approximately 350 million years ago, and they have survived in greater numbers than any other seedless vascular plants. Over 11,000 species of ferns still exist today. In addition to true roots, ferns have creeping underground stems called rhizoids, from which their distinctive leaves called fronds grow.

Ferns Ferns can survive in areas with very low light, and are usually found in wet, or seasonally wet, habitats around the world. They are very common in many forests, particularly temperate and tropical rainforests. In tropical forests, ferns can grow as large as small trees.

Ferns

Fern Life Cycle In ferns and all vascular plants, the sporophyte is the dominant stage in the life cycle. The haploid spores form on the underside of the fronds (of the sporophyte) in tiny capsules called sporangia, grouped into clusters called sori.

Fern Life Cycle When the spores germinate, they develop into a small haploid gametophyte. The gametophyte grows a set of rootlike rhizoids, then flattens into a green heart-shaped structure. This gametophyte is tiny, but it does grow independently of the sporophyte. The antheridia and archegonia are found on the underside of the gametophyte. Fertilization requires a thin film of water so that the sperm can swim to the egg. After fertilization, a new sporophyte begins to develop, and the gametophyte withers away. The fern sporophyte can live for several years.

Fern Life Cycle

Homework No class tomorrow (Career fair) – I’ll move your Article Deadline to Tomorrow night – 1 Bonus mark for anyone who has it in by tonight There’s a time stamp, I can tell! Textbook Pages (for Wed) – Section 22-3 (pgs ) – Questions 1-4 on pg 563