PLANT DIVERSITY LAND PLANTS EVOLVED FROM GREEN ALGAE • The Origin of Plants from Algae – The closest modern relatives of the ancestors of plants are the green algae, charophytes. – Today this species is found around the edges of ponds and lakes. – The theory is that some ancient charophytes might have lived in similar locations that dried out and adapted to the new conditions as plants. CHAROPHYTES LAND PLANTS EVOLVED FROM GREEN ALGAE – The oldest plant fossil is approximately 475 million years ago. – Modern plants have since diversified and is now a multicellular autotroph in which the embryo develops within the female parent. • Challenges of Life on Land – The differences between plants and charophytes are related to living on land and amount to four challenges. LAND PLANTS EVOLVED FROM GREEN ALGAE – Obtaining Resources From Two Places at Once • Aquatic organisms and algae get their resources from the surrounding water. • Land plants (photosynthetic) get their resources from air and soil. – Light and carbon dioxide are available above ground. – Water and mineral nutrients are found in the soil. • Shoots and roots allow access to these two environments. ALGAE vs. PLANTS algae are simple and unspecialized possessing a holdfast that serves as an anchor, while plants have specialized tissues that perform specific tasks. Plants have a root system that holds it in place and extracts mineral and nutrients from the soil. Algae take up minerals and nutrients directly from the water via diffusion. Algae lack specific reproductive structure while plants have stamens and carpels. Both have photosynthetic properties. LAND PLANTS EVOLVED FROM GREEN ALGAE • The plant’s roots absorb water, minerals, and nutrients from the soil. • Plant shoots bear leaves above ground which use the sun as energy for photosynthesis. – Most plants have a vascular system that transports the minerals, nutrients, and water between the roots and shoots and leaves. – Staying “Afloat” in Air • Sea weed or kelp (both algae) stay upright because of the buoyancy of the water. LAND PLANTS EVOLVED FROM GREEN ALGAE • Plants stay upright because of the strong support system they have. • This terrestrial adaptation includes the chemical lignin, which hardens the plants’ cell walls. – Maintaining Moisture • Even though the plant is on soil and exposed to dry air, its cellular processes take place in an aqueous environment. • Plants possess the ability to maintain a watery internal environment. – Examples are the waxy surfaces of a cactus or apple. PLANT SUPPORT: LIGNIN LAND PLANTS EVOLVED FROM GREEN ALGAE • A waxy cuticle coats the leaves and other above ground parts, helping maintain water internally. – One negative of this is the slowing down of the exchange of CO₂ and O₂. • The result, therefore, is the exchange of gasses through the stomata, which are microscopic pores in a leaf’s surface. • Two surrounding cells regulate the stoma’s opening and closing., opening only when necessary to prevent evaporation. STOMATA LAND PLANTS EVOLVED FROM GREEN ALGAE • An Overview of Plant Diversity – There are four major periods of plant evolution. • The first dealt with the origin of plants from charophytes, their aquatic algal ancestors. • The bryophytes were the first to diversify. – These are the mosses. – These plants don’t have seeds and lack lignin-hardened material. • Vascular plants marked the second period of plant evolution and saw the development of vascular MAJOR PERIODS OF PLANT EVOLUTION LAND PLANTS EVOLVED FROM GREEN ALGAE tissue with lignin-hardened vascular tissue that transport water and nutrients. • The third period included the origin of the seed. – Seeds are embryos within a seed coat that also contains a store of food. – Seeds allow for the spread of plants to diverse areas without allowing the embryo to dry out. – Early seed plants included gymnosperms (Gr. for naked seed) that have seeds that develop without being enclosed within a chamber on specialized leaves. » Conifers that produce cones are examples of these. • The fourth period of plant evolution began with flowering plants, or angiosperms. LAND PLANTS EVOLVED FROM GREEN ALGAE – In angiosperms, the seed develops within a protective organ, the ovary, contrasting with the gymnosperm’s naked seeds. • Alternation of Generations – Plant generations alternate between diploid (2n) and haploid (n) forms. • Recall that diploids have two sets of chromosomes, one from each parent. • Haploids have one set as a result of meiosis. – In the plant life cycle, the haploid and diploid forms are distinct, multicellular generations. A PLANT’S LIFE CYCLE A PLANT’S LIFE CYCLE LAND PLANTS EVOLVED FROM GREEN ALGAE • Animals have a unicellular haploid stage, a single sperm or egg cell. – The haploid generation of plants produces gametes called gametophytes. – The diploid generation produces spores called sporophytes. • In the plant’s life cycle, each generation takes turns giving rise to the other. – The alternation between the haploid and diploid forms is called the alternation of generations. LAND PLANTS EVOLVED FROM GREEN ALGAES – Spores differ from gametes in two ways. • Spores can develop into new organisms without fusing with another cell. – Gametes must fuse together to produce a zygote. • Spores of some plants have tough coats that enable them to resist harsh environments and lay dormant. – Gametes cannot tolerate harsh environments and lay dormant. – Alternation of generations occurs only in life cycles of plants and certain algae. REVIEW: CONCEPT CHECK 19.1, page 424 1. Name the group of algae most closely related to plants. What is a major difference between plants and algae? 2. Make a table listing the four major challenges to plants living on land. In the second column, list at least one plant adaptation for each challenge. 3. List the four main groups of plants and describe two characteristics of each. 4. List the difference between the sporophyte and gametophyte plant generations. POLLEN AND SEEDS EVOLVED IN GYMNOSPERMS • Gymnosperm Adaptations – Gymnosperms have adapted with a smaller gametophyte, pollen, and seeds for survival on land. – Gymnosperms are plants that bear seeds that are not enclosed in an ovary (naked). – Here the diploid sporophyte generation is more highly developed than the haploid generation. PINE TREE’S GAMETOPHYTES POLLEN AND SEEDS EVOLVED IN GYMNOSPERMS – Pine trees are sporophytes in which tiny gametophytes live in cones. – Pollen is a second adaptation of seed plants to dry land. – Pollen is male gametophytes that contain cells that develop into sperm. – Wind carries the pollen in many plants, including the conifer. – Evolution has allowed plants on dry land to POLLEN AND SEEDS EVOLVED IN GYMNOSPERMS – develop pollen so that sperm can reach eggs (female cone in the case of conifers) without swimming through water. – Seeds are plant embryos surrounded by a protective coat that includes a food supply within its confines. – In the pine tree’s cones, there are many spore sacs, thousands of haploid spores that develop into pollen grains (male gametophytes). LIFE CYCLE OF A PINE TREE (GYMNOSPERM) POLLEN AND SEEDS EVOLVED IN GYMNOSPERMS – The female gametophytes develop in the ovules. – Within each of the two ovules on each scale of the female cones, a large spore cell undergoes meiosis and produces four haploid cells. • One of these will survive and grow into the female gametophyte. – The pollen will travel from tree to tree by the wind. POLLEN AND SEEDS EVOLVED IN GYMNOSPERMS – If a pollen grain reaches the female cone, sperm cells mature and fertilize egg cells within the female gametophyte. – More often than not, both eggs in the ovule will be fertilized but only one develops into an embryo. • The embryo is then the new sporophyte plant. • The Diversity of Gymnosperms – Four gymnosperm phyla exist today. POLLEN AND SEEDS EVOLVED IN GYMNOSPERMS • Ginkgos – Date back to dinosaur era – Tolerates city pollution well • Gnetophytes – Usually located in desert locale • Cycads – Sago palms • Conifers – Evergreens like spruce, pine, firs, juniper, cedar, redwoods – Leaves replace only when old ones die. GYMNOSPERMS: GINKOS AND GNETOPHYTES GYMNOSPERMS: CYCADS AND CONIFERS REVIEW: CONCEPT CHECK 19.4, page 433 1. Name three adaptations of gymnosperms and the advantages they provide. 2. Make a table listing the four different gymnosperm groups and a beneficial use or fact about each one. FLOWERS AND FRUITS EVOLVED IN ANGIOSPERMS • Angiosperm Adaptations – These were the last major group of plants to evolve. – Angiosperms are flowering plants where the reproductive structures are flowers, not cones like the gymnosperms. – The gametophytes develop within the flowers of the sporophyte. – The flower is specialized to function in reproduction and is unique to angiosperms. FLOWERS AND FRUITS EVOLVED IN ANGIOSPERMS – Flowers are structured to attract insects and other animals to transfer pollen from one flower to the next. • The following contribute to the interactions between angiosperms and the animal pollinators: – – – – – Variety of flower Shape of flower Odor Texture Color POLLINATION BY BEES AND FLIES FLOWERS AND FRUITS EVOLVED IN ANGIOSPERMS – Grasses, an angiosperm, are wind pollinated but have flowers which are smaller and less of an attraction than those pollinated by animals. – The stamen is the male reproductive organ of a flower. • This consists of a filament on which the anther, which produces the pollen or male gametophyte, sits. – The carpel is the female reproductive organ and is composed of the stigma, style, and ovary. PARTS OF A FLOWER FLOWERS AND FRUITS EVOLVED IN ANGIOSPERMS – Within the ovary, ovules or female gametophytes (embryo sacs) develop. – Referring to the slides with the stages of the life cycle of an angiosperm, the following is a description. • During reproduction, the pollen lands on the sticky stigma. • A tube then grows from each pollen grain down the style toward an ovule in the ovary. LIFE CYCLE OF AN ANGIOSPERM ANGIOSPERM vs. GYMNOSPERM LIFE CYCLE FLOWERS AND FRUITS EVOLVED IN ANGIOSPERMS • Two sperm cells in the pollen are released into the female gametophyte. • One sperm fertilizes an egg cell, producing a zygote, which then develops into an embryo. • The second sperm cell fuses with nuclei in the larger center cell of the female gametophyte, which then develops into a nutrient-storing tissue called endosperm. – This endosperm nourishes the embryo as it grows. • This is “double fertilization” and simultaneously produces the zygote and endosperm. FLOWERS AND FRUITS EVOLVED IN ANGIOSPERMS • The seed then develops from this whole ovule, containing the zygote and endosperm. – Some flowers contain many ovules and can produce many seeds. – This development of seeds within ovaries is in contrast to the gymnosperm's “naked” seed development. • As the seeds develop from the ovules, the ovary wall thickens and forms a fruit the surrounds the seeds. • A fruit is the ripened ovary of a flower. – Fruits provide protection and a means of dispersal of the seeds. RIPENED OVARIES OR FRUIT FLOWERS AND FRUITS EVOLVED IN ANGIOSPERMS • The Diversity of Angiosperms – Biologists, at one time, divided angiosperms into monocots and dicots. • These differ in the structure of the leaves, flowers, seeds, roots, and vascular tissue – Recent advances have added information to expand the cladogram. – Some flowering plants descended from ancestors that evolved earlier than the oldest known monocot or dicot. MONOCOT vs. DICOT AMBORELLA AND WATER LILLIES PREDATE MONOCOTS FLOWERS AND FRUITS EVOLVED IN ANGIOSPERMS • Human Dependence on Angiosperms – All fruit and most vegetables are angiosperms that provide the food that supports life. – Corn, rice, wheat, and other grains are fruits of grasses. – Angiosperms provide furniture, medicines, perfumes, decorations, and cotton fiber. REVIEW: CONCEPT CHECK 19.5, page 437 1. Define and give examples of a fruit and a flower. 2. Name three examples of monocots and three examples of dicots. 3. Make a list of benefits angiosperms offer humans. 4. Explain what is meant by the term double fertilization. REPRODUCTIVE ADAPTATIONS CONTRIBUTE TO ANGIOSPERM SUCCESS • Flowers and Reproduction – Most flowers share the same basic pattern. – Flowers are specialized shoots found only in angiosperms the usually consist of four different rings of modified leaves: sepals, petals, stamens and carpels. • The sepals cover and protect the flower bud before it opens. • The next ring is the petals, which are colorful, and in some cases, have markings to direct the insect toward the reproductive parts. PARTS OF A FLOWER REPRODUCTIVE ADAPTATIONS CONTRIBUTE TO ANGIOSPERM SUCCESS • The stamens and carpels, the reproductive parts, are closest to the center of the flower. • Most flowers have multiple stamens and one carpel. • Some plants, like wild roses, have more than one carpel. • The male gametophytes are produce within the stamens. • The stamen is composed of the filament and the anther. • In the anther, meiosis produces spores that become pollen, the male haploid gametophytes. ANGIOSPERM’S LIFE CYCLE REPRODUCTIVE ADAPTATIONS CONTRIBUTE TO ANGIOSPERM SUCCESS • In the anther, meiosis produces spores that become pollen, the male haploid gametophytes. – Each pollen grain consists of two cells surrounded by a thick protective wall. • Female gametophytes are produced in the carpels. – At the base is the ovary where the ovules are located. – In each ovule, the diploid cell undergoes meiosis and produces four haploid spores. » Unfortunately, three of the four spores die. – The survivor enlarges and undergoes three cycles of mitosis, with the result becoming the female gametophyte, or embryo sac. REPRODUCTIVE ADAPTATIONS CONTRIBUTE TO ANGIOSPERM SUCCESS • In the embryo sac are seven cells including an egg cell and a large, central cell with two haploid nuclei. • Part of the carpel is the style and stigma. • The stigma is the tip which is sticky and the style is a long tube leading to the ovary. • During pollination, pollen grains from another plant of the same species land on the stigmata. • The pollen absorbs water and then extends a structure called the pollen tube, which grows toward the ovary through the style. REPRODUCTIVE ADAPTATIONS CONTRIBUTE TO ANGIOSPERM SUCCESS • One of the pollen cells divides once, forming two haploid sperm. • When the pollen tube reaches the ovary, it enters the embryo sac and releases both sperm cells. • Next two fertilizations take place. – One sperm cells fertilizes the egg cell producing a zygote. – The zygote develops, then, into the sporophyte embryo. – The other sperm cell fertilizes the large central cell with the two haploid nuclei, with the result being a triploid cell (3n). REPRODUCTIVE ADAPTATIONS CONTRIBUTE TO ANGIOSPERM SUCCESS • The triploid cell develops into the nutrient rich tissue called the endosperm which gives nourishment to the embryo. • Seed Development and Dispersal – After this double fertilization, the ovule develops into the seed. – The outer layer, the seed coat, protects the embryo and endosperm. – One can then see a miniature root and shoot. SEED DEVELOPMENT SEED DEVELOPMENT REPRODUCTIVE ADAPTATIONS CONTRIBUTE TO ANGIOSPERM SUCCESS – The cotyledon develops, which functions in the storage and transfer of nutrients to the embryo. • Monocots have one cotyledon and dicots, two. – Several cycles of mitosis takes place in the embryo and, then, growth is suspended (dormancy). – At this point, the seed is ready to leave the parent. SEED DISPERSAL REPRODUCTIVE ADAPTATIONS CONTRIBUTE TO ANGIOSPERM SUCCESS – Seed dispersal occurs in many ways. • • • • Sticking to an animals fur In fleshy fruits, attractive to animals Like coconuts, traveling on water Like the dandelion, by the wind • Seed Germination – Germination, or the growth of the seed again, occurs when conditions are favorable. – This occurs after the seed soaks up water, SEED GERMINATION SEED GERMINATION REPRODUCTIVE ADAPTATIONS CONTRIBUTE TO ANGIOSPERM SUCCESS expanding and splitting the seed coat, and triggering metabolic changes in the embryo causing growth. – Adaptations • Once out of the seed coat, the shoots are susceptible to “abrasion” by the surrounding soil. • Some dicots have a hooked shoot tip. – Once out of the soil, this, then, straightens out. • In monocots, a sheath surrounding the shoot pushes straight upward through the soil, and, after REPRODUCTIVE ADAPTATIONS CONTRIBUTE TO ANGIOSPERM SUCCESS • emerging into the light, expands the leaves and begins the process of photosynthesis. • This is the stage where the plant is called a seedling. – Environmental Conditions • Conditions for germinations vary among different plant species. • Desert plants germinate only after a heavy rainfall to allow the seedling to push through the moistened soil and provides a temporary water supply. REPRODUCTIVE ADAPTATIONS CONTRIBUTE TO ANGIOSPERM SUCCESS • Some seeds will germinate only after long periods of cold in harsh winter climates. • Some seeds require intense heat to germinate. • Asexual Reproduction in Plants – Obstacles exist to successful sexual reproduction in plants. – Pollen may not reach the correct species of flower. – Seeds can get damaged during dispersal. – Seedlings may not survive. REPRODUCTIVE ADAPTATIONS CONTRIBUTE TO ANGIOSPERM SUCCESS – Asexual reproduction in plants is called vegetative reproduction. – These plants produce genetically identical plants to themselves. – This can be done naturally or with help. – Some cacti drop sections its stems that produce new, identical plants. – Some plants, like the strawberry or aspen send out runners that produce new, identical plants. VEGETATIVE REPRODUCTION WITH STOLENS REPRODUCTIVE ADAPTATIONS CONTRIBUTE TO ANGIOSPERM SUCCESS • How Long Does a Plant Live? – Annuals complete their life cycles in one year. – Biennials complete their life cycle in two years and usually flower only in the second year. – Perennials live and reproduce for many years. ANNUALS, BIENNIALS, AND PERENNIALS REVIEW: CONCEPT CHECK 20.1, page 446 1. Diagram the reproductive structures of a flower. For each structure, include a label stating a brief description of its function. 2. Describe three different methods of seed dispersal. 3. Explain how two different adaptations of seed germination in dicots and monocots protect the developing shoot. 4. Give two examples of vegetative reproduction in plants. 5. Compare and contrast annuals, biennials, and perennials. STRUCTURE FITS FUNCTION IN THE PLANT BODY • A Plant’s Root System and Shoot System – Roots function as a support and anchor system. – Roots absorb minerals and water. – Most monocots have a fibrous root system which consists of mats of thin roots spread out below the soil surface. – This increases the surface area to absorb those nutrients and water. FIBROUS vs. TAPROOT SYSTEMS ROOT vs. SHOOT SYSTEM STRUCTURE FITS FUNCTION IN THE PLANT BODY – Most dicots have a taproot system characterized by one large vertical root with branches off of it. • Examples would includes carrots, turnips, and beets with starchy taproots. – The shoot system consists of stems, leaves, and flowers. – Stems provide support to the leaves and flowers. – Nodes are where leaves are attached. STRUCTURE FITS FUNCTION IN THE PLANT BODY • Internodes are the parts of the stem between the nodes. – Vascular tissue runs vertically in the stems, transporting water and nutrients up the stem and food down. – Some photosynthesis and storage occur in the stems. – Shoots that have yet to develop are called buds. – Terminal buds are found at the tip of the stem. STRUCTURE FITS FUNCTION IN THE PLANT BODY – Axillary buds are found in the angles (axils) formed by a leaf and the main stem. • Growth from this area forms the plant’s branches. – Leaves are mostly flattened and thin. – The main part is the blade. – The petiole connects the leaf to the stem. – Leaf veins carry water and nutrients and consist of vascular tissue and support tissue. – Leaves can be modified, depending on the plant. LEAVES MODIFIED LEAVES STRUCTURE FITS FUNCTION IN THE PLANT BODY • • • • Grasses lack petioles. The celery stalk is the petiole. Spines on a cactus are leaves. Tendrils on pea plants or grape vines attach to things so that they can climb. • A Plants Main Tissue Systems – There are three main tissue systems: • Dermal • Vascular • Ground tissue TISSUE SYSTEMS OF A PLANT LEAF TISSUE ANATOMY STRUCTURE FITS FUNCTION IN THE PLANT BODY – Dermal Tissue • This is the outer layer of the plant (skin). • Epidermis, the dermal tissue of nonwoody organs such as young roots, consists of one or more layers of cells. – This covers all the young parts of a plant. – This is what secretes the waxy cuticle. – Vascular Tissue • This is what transports water, minerals, and organics from the roots to the shoots. STRUCTURE FITS FUNCTION IN THE PLANT BODY • It also adds to the structure of the plant. • This comes under the term tracheid and consists of xylem and phloem. • Xylem transports water and minerals from the roots to the shoots. • Phloem transports food from the mature leaves to the other parts of the plant, shoots and roots, where photosynthesis doesn’t take place. • In the roots, the vascular tissue is in the center. • In the stems, it is arranged in vascular bundles, or separate strands. MONOCOT vs. DICOT VASCULAR SYSTEMS VASCULAR BUNDLES: MONOCOT VASCULAR BUNDLES: DICOT STRUCTURE FITS FUNCTION IN THE PLANT BODY – Monocots have vascular bundles scattered throughout the ground tissue. – Dicots’ are arranged in a ring. – Ground Tissue • Between the dermal and vascular tissue is the ground tissue which makes up most of a young, nonwoody plant and functions in photosynthesis, storage, and support. • The ground tissue of the root consists primarily of a mass of cells called the cortex. STRUCTURE FITS FUNCTION IN THE PLANT BODY • Types of Plant Cells – Parenchyma Cells are the most abundant. • They have thin cell walls and large central vacuoles. • They provide storage, photosynthesis, and cellular respiration. • Fruits are composed mostly of parenchyma cells. • Phloem is composed of parenchyma cells. – Collenchyma cells have unevenly thickened cell walls. PARENCHYMA CELLS COLLENCHYMA CELLS STRUCTURE FITS FUNCTION IN THE PLANT BODY • These are usually grouped in strands or cylinders and provide support for the growing parts of the plant. • These include young stems and petioles where it is located below the surface. – Celery stalk strings • These cells elongate with the stem and leaves as they grow. – Sclerenchyma cells are for support. • They grow and die within a mature part of a plant. • They are rich in lignin, which is left behind, to create a skeleton to support the plant. SCLERENCHYMA CELLS STRUCTURE FITS FUNCTION IN THE PLANT BODY – The xylem is sclerenchyma cells. – Plants are composed of more than one of these cell types. REVIEW: CONCEPT CHECK 20.2, page 451 1. Compare and contrast the functions of roots and shoots. 2. List the functions of dermal, ground, and vascular tissues. 3. Describe the characteristics of the three main plant cell types. PLANTS ACQUIRE NUTRIENTS FROM THE SOIL AND AIR • Seeking the Source of a Plant’s “Substance” – Aristotle originally hypothesized that soil provides all the mass necessary for plant growth. – Van Helmont, in the 1600s, tested this and added nothing other than water to the soil. – He noted that the plant gained more mass than the soil lost, proving the hypothesis wrong. PLANTS ACQUIRE NUTRIENTS FROM THE SOIL AND AIR – His new hypothesis was that plants gain substance (mass) from water added to the soil. – Hales, an English botanist, proposed another hypothesis, that plants gain substance from the air. – Recent studies have indicated that these early ideas about plant nutrition might have some truth. • Air supplies carbon dioxide to the plant. Van HELMONT’S EXPERIMENT PLANTS ACQUIRE NUTRIENTS FROM THE SOIL AND AIR • CO₂ is used in photosynthesis to produce sugar which might lead to cellulose. • Hydrogen comes from water which also is a transport mechanism. • The soil gives up minerals that the plant needs. • The Mineral Requirements of Plants – Most plants need 17 chemical elements to complete their life cycles. – CO₂ provides carbon and oxygen and comes from air. PLANTS ACQUIRE NUTRIENTS FROM THE SOIL AND AIR – Water provides hydrogen and comes from the soil. – The other elements are mineral nutrients absorbed in ionic form from the soil. – Plants can suffer from nutritional deficiencies. • • • • Stunted plant growth No flowers produced No synthesis of chlorophyll Yellowing of young leaves SOME ESSENTIAL PLANT MINERAL NUTRIENTS MINERAL NUTRIENT FUNCTIONS IN PLANT Nitrogen Protein and nucleic acid synthesis Sulfur Protein synthesis Phosphorus Nucleic acid and ATP synthesis Potassium Protein synthesis; regulation of osmosis Calcium Cell wall formation; enzyme activity Magnesium Chlorophyll synthesis; enzyme activity PLANTS ACQUIRE NUTRIENTS FROM THE SOIL AND AIR • A Closer Look at Nitrogen – Plants use nitrogen to produce proteins, nucleic acids, and hormones. – Plants do suffer from nitrogen deficiency, even though 80% of the atmosphere is nitrogen. – The plants must absorb it from the soil and not as a gas. – Nitrogen is first converted to ammonium ions (NH₄⁺) or nitrate ions (NO₃⁻). PLANTS ACQUIRE NUTRIENTS FROM THE SOIL AND AIR – Nitrogen fixation, by bacteria, converts the gas to the ionic forms. – Other bacteria act as decomposers. – A third group of bacteria convert the NH₄⁺ ions to NO₃⁻. – Legumes such as peas, peanuts, alfalfa, and beans contain nitrogen fixing bacteria on their roots as lumps called root nodules. – Farmers use crop rotation to add nitrogen to the soil. NITROGEN CYCLE PLANTS ACQUIRE NUTRIENTS FROM THE SOIL AND AIR • Fertilizers – These are products used by farmers amateur gardeners that contain nitrogen, phosphorus, and potassium. – The bags have three-numbered codes such as 10-12-8, meaning 10% nitrogen, 12% phosphorus, and 8% potassium. – Except when they are applied, these fertilizers run off when it rains, contributing to pollution. REVIEW: CONCEPT CHECK 21.1, page 463 1. Did van Helmont’s experiment support or disprove Aristotle’s hypothesis? Explain. 2. List at least three mineral nutrients required by plants, and describe their contributions to plant function. 3. Describe the role of three different kinds of bacteria in making nitrogen available to plants. 4. Describe the benefits and possible problems from fertilizer use. VASCULAR TISSUE TRANSPORTS SAP WITHIN A PLANT • How Roots Absorb Water and Minerals – Root hairs increase the surface area of the roots to improve the absorption of water and dissolved minerals. • They are very small projections of the roots epidermal cells and grow into spaces between soil particles. – Remember that mycorrhizae work in symbiotic relationship to allow the absorption of the dissolved minerals, especially phosphate. ROOT HAIRS VASCULAR TISSUE TRANSPORTS SAP WITHIN A PLANT – Two forces id in moving the water from the roots to the rest of the plants. – The first, root pressure, pushes the water up the xylem and works during the night. – The epidermal cells and ground tissue cells use ATP to accumulate certain minerals, which then move from cell to cell through cytoplasmic channels. This is the way water and minerals move through the xylem. VASCULAR TISSUE TRANSPORTS SAP WITHIN A PLANT – Endodermis is a layer of cells that surrounds the vascular tissue. • It is composed of waxy cells which prevents the water and minerals from backflow out of the xylem. – The minerals accumulate in the xylem, water enters by osmosis, and the xylem sap is pushed upward. • The Upward Movement of Xylem Sap – Root pressure is only one way the sap gets to the top of the plant. VASCULAR TISSUE TRANSPORTS SAP WITHIN A PLANT – There is a force that pulls it to the top, like a straw. – Transpiration, evaporation of water through the leaves, creates this pull (transpirationpull). • This occurs because of cohesion, the quality of the molecules of the same kind to sick together, and adhesion, the quality of attraction between unlike molecules. • Both of these forces counteract the pull of gravity, with adhesion preventing the falling back of the TRANSPIRATION TRANSPIRATION TRANSPIRATION VASCULAR TISSUE TRANSPORTS SAP WITHIN A PLANT • • • • • water due to gravity, at night. There are two types of xylem “straws” in which water travels through the plant. Tracheids are long cells with tapered ends Vessel elements are wider, shorter, and less tapered. These cells can overlap, forming tubes which are hollow since the cells have died. The lignin cell walls remain which create the tubes. XYLEM STRUCTURE VASCULAR TISSUE TRANSPORTS SAP WITHIN A PLANT • Regulating Water Loss – Heat denatures protein, thus disabling enzymes for photosynthesis. – Transpiration allows for evaporative cooling preventing this but, yet, allows for a large amount of water loss from the plant. – When transpiration exceeds water intake, leaves wilt. – The leaves’ stomata help regulate VASCULAR TISSUE TRANSPORTS SAP WITHIN A PLANT transpiration according to environmental conditions. – Guard cells surround each stoma opening and open and close by changing shape. • During daylight, stomata are open allowing CO₂ to enter. • Sunlight and low CO₂ levels trigger the guard cells to gather K⁺ ions that facilitates water entering the guard cells. • The guard cells then swell. GUARD CELLS GUARD CELLS VASCULAR TISSUE TRANSPORTS SAP WITHIN A PLANT • They, then, buckle away from their centers so that a gap opens. • At night, the stomata close which also happens when the plant is losing water from transpiration faster than it is gaining water from the soil. • K⁺ ions are lost from the guard cells with water following. • The guard cells droop, losing water, closing the stoma. • The Flow of Phloem Sap – Phloem transports sucrose and other organic compounds along with water. SIEVE TUBES VASCULAR TISSUE TRANSPORTS SAP WITHIN A PLANT – This occurs through cells called sieve-tube members. • Their end walls are like sieves, allowing fluid to flow through pores. • A chain of these forms a sieve tube. – These cells are alive, in contrast to xylem cells. – In maturing, the sieve-tube members lose their nuclei along with some other organelles. VASCULAR TISSUE TRANSPORTS SAP WITHIN A PLANT – As a result, they lose their ability to perform some of the necessary cell functions. – Companion cells, alongside the sieve tubes, provide the proteins and other resources to the tubes. – From Source to Sink • Phloem moves sugars from their source to their need. • Where sugar is produced or stored in the leaves is referred to as the sugar sources. VASCULAR TISSUE TRANSPORTS SAP WITHIN A PLANT • The sugar sink is the site where the sugar will be used or further stored. – These include roots, developing shoot tips, and fruits. • Sugar sources can change within seasons. – Beet taproots or potato tubers are sinks in the summer. – The following spring, they become sugar sources. – The Pressure-Flow Mechanism • The pressure-flow mechanism drives the movement of phloem sap. • Production of sugar at the source, such as a leaf, is actively transported into a sieve-tube member of the phloem. PRESSURE-FLOW MECHANISM PRESSURE-FLOW MECHANISM VASCULAR TISSUE TRANSPORTS SAP WITHIN A PLANT • A high concentration of sugar is then noted at the source. • Water enters the phloem by osmosis, which causes a higher water pressure at the source than at the sink. • The reverse happens at the sink. Sugars leave the sieve-tube, water follows, and pressure falls. • Water flows from higher to lower with this process called the pressure-flow mechanism. • Water gets back to the source via the xylem. REVIEW: CONCEPT CHECK 21.2, page 468 1. Why is root pressure a pushing force? Explain 2. Explain the role of transpiration in water movement. 3. Describe the mechanism that opens and closes stomata. 4. Explain how phloem sap flow from a sugar source to a sugar sink. HORMONES COORDINATE PLANT FUNCTIONS • Discovery of a Plant Hormone – Darwin, in addition to natural selection, studied with his son how plants grow toward light. – In the late 1800s, they observed how grass seedlings would bend toward light while they were growing. – If they cut off the tips, the shoots would grow straight up. PLANT HORMONES PLANT HORMONES PLANT HORMONES HORMONES COORDINATE PLANT FUNCTIONS – Placing dark caps on the tips would do the same thing. – Placing clear caps over the tips or shielding the lower parts of the seedlings, the plants would bend toward the light. – The Darwin's’ hypothesized that a shoot tip can detect light and then transmits a signal to the growing region of the shoot. – This was later proven to be caused by a chemical messenger in the shoot tip. HORMONES COORDINATE PLANT FUNCTIONS – The Darwin’s and other scientists discovered other types of chemical messengers called plant hormones. – Hormones control a plant’s germination from a seed, growth, flowering, and fruit production. – Hormones are produced in small amounts but only a small amount can have a large effect. • Hormones can turn genes on and off, inhibiting enzymes, or changing plasma membrane properties. HORMONES COORDINATE PLANT FUNCTIONS • Functions of the Five Major Hormones – There are several types of plant hormones: auxins, cytokinins, gibberlins, abscisic acid, and ethylene. – No one hormone acts alone, instead they work as a balance of hormones controls the plant’s life. – Auxins • Auxins are hormones that promote plant growth, and is from Greek and means “to increase.” HORMONES COORDINATE PLANT FUNCTIONS • They are produced in the apical meristems at the tips of the shoots and promote elongation. • Exposing a seedling to light from one direction causes the auxin to build up on the shaded side stimulating growth beneath the tip. • Those cells on the shaded side elongate further than those on the lighted side, which causes the shoot to bend toward the light. – This works, hypothetically, by the auxins loosening the bonds holding the cell walls together. – The cell takes up more water by osmosis and then elongates. HORMONES COORDINATE PLANT FUNCTIONS • Seeds can secrete auxins that promotes the ovary development into a fruit, especially in some plants where fruits develop without pollination and seed development, such as seedless tomatoes and cucumbers. – Cytokinins • These hormones stimulate cell division. • They are produced on actively growing tissues such as embryos, roots, and fruits. • Cytokinins can slow the aging of flowers and fruits. • There can be and is counter-effects of cytokinins and auxins. HORMONES COORDINATE PLANT FUNCTIONS • Cytokinins from the roots affect the shoots in promoting cell division in axillary buds, encouraging branching, but auxins from the terminal bud inhibits branching. • This results in fewer and shorter branches near the tip of the plant. • One can trim the terminal bud to induce a more bushy plant. – Gibberlins • These are produced at the tips of both stems and roots. AUXIN vs. HORMONES COORDINATE PLANT FUNCTIONS • They stimulate growth of stems by promoting both cell division and cell elongation. • While similar to auxins, botanists do not understand their relationship. • In combination with auxins, they stimulate fruit development. • Applying gibberlins to the Thompson variety of grapes makes them grow larger. • They also promote germination, especially in some cereal grains. BENEFICIAL USES OF GIBBERLINS HORMONES COORDINATE PLANT FUNCTIONS – Abscisic Acid • When there are droughts or severe cold in winter, plants tend to become dormant. • During these times, abscisic acid (ABA) inhibits primary and secondary growth by inhibiting cell division in buds. • These hormones are washed out after a downpour. • Abscisic acid inhibits the germinating effect of gibberlins. • Abscisic acid also reacts to stress, especially when HORMONES COORDINATE PLANT FUNCTIONS • The plant is dehydrated, causing the stomata to close and reduce transpiration. – Ethylene • This is a naturally occurring plant hormone that stimulated fruit ripening. • Burning kerosene release ethylene, which artificially causes fruit to ripen. • Ethylene also promotes leaves to drop from deciduous trees. • Leaf drop is caused by a shift in amounts of ethylene and auxin in leaf petioles. ETHYLENE AND LEAF DROP HORMONES COORDINATE PLANT FUNCTIONS • With the shorter days as autumn progresses, auxin decrease and ethylene increases with leaf drop occurring. • Preservation of water is the goal of leaf drop. REVIEW: CONCEPT CHECK 22.1, page 479 1. Explain why the Darwins did not observe any bending of the seedlings when they covered the tips of the seedlings with dark caps. 2. List five major plant hormones and state one effect of each. 3. Explain how leaf drop is an adaptive response to winter. PLANTS RESPOND TO CHANGES IN THE ENVIRONMENT • Rapid Plant Movements – Plants respond to stimuli such as light, temperature, gravity, and touch. – Most plant responses are slow but in some instances they can be rapid as in the tropical plant Mimosa pudica, which folds up when touched. • Touch causes cells at the base of the leaflet to lose ions, with water following the ions out of the leaf causing it to look wilted. RAPID PLANT MOVEMENTS: Mimosa pudica PLANTS RESPOND TO CHANGES IN THE ENVIRONMENT – This rapid movement can bump off or scare off insects looking to eat the plant. • Tropisms – These are growth responses that cause parts of a plant to grow slowly toward or away from a stimulus. – These are usually controlled by hormones, typically auxins. – Tropisms are not rapidly reversible. PLANTS RESPOND TO CHANGES IN THE ENVIRONMENT – Responses to Touch • Thigmotropism is a change in plant growth due to touch. • Climbing plants have tendrils that wrap around things, like wires. • The area touching something slows in growth, while the opposite side grows faster, therefore wrapping itself around something. • The seedling’s response to stress is an example where one sees a plant growing away from an object, avoiding that object and preventing damage. THIGMOTROPISM PLANTS RESPOND TO CHANGES IN THE ENVIRONMENT • Researchers have shown that ethylene plays a role in thigmotropism. – Responses to Light • Phototropism is the growth of a plant toward or away from light. • This was seen in the Darwins’ experiments with the shoot tips which contains a protein that when activated by light signals molecules that affect auxin transport down from the shoot tip. – Responses to Gravity • Gravitropism is the plant’s growth response to gravity. GRAVITROPISM PLANTS RESPOND TO CHANGES IN THE ENVIRONMENT • This is seen in germinating seeds where the shoot grows upward and the root downward in the soil, no matter where the seed is planted in the soil. • Scientists do not know how the plant knows “up” from “down.” • It could be uneven distribution of organelles containing starch grains that signals auxins to affect the cell’s direction of growth. • Coping With Stressful Environments – Water content, salt content, temperature can be stressful to the plant. PLANTS RESPOND TO CHANGES IN THE ENVIRONMENT – Drought • Prolonged inadequate rainfall produces drought. • The plant will become stressed and weakened in these conditions. • More water can be lost by transpiration than taken up by the roots. • The growth of young leaves is inhibited. • Leaves wilt and photosynthesis is reduced. • Cacti and similar plants (succulents) store water in their stems and have a thick cuticle and spines instead of leaves to prevent transpiration. DROUGHT AND COLD PLANTS RESPOND TO CHANGES IN THE ENVIRONMENT • Plants in cold regions are adapted by having small leaves and growing low to the ground, which reduces transpiration during the limited growing season and harsh winds. – Flooding • Too much water in the soil will lesson the air spaces that provide oxygen for cellular respiration in the roots. • Mangroves have adapted to this by having their roots and “knuckles” above the water. • Having too much water in the soil causes the plant MANGROVES “SNORKELS” or AIR TUBES PLANTS RESPOND TO CHANGES IN THE ENVIRONMEN cells to release ethylene which causes some of the submerged roots to die. • This creates air tubes that function as snorkels, which carries oxygen to the submerged roots. – Salt Stress • Too much salt in the soil causes the root cells to lose water to the soil through osmosis. • Other than halophytes, most plants cannot survive salt stress for long times. • Halophytes are salt-tolerant plants with salt glands as adaptations. PICKLEWEED, SALT TOLERANT PLANT PLANTS RESPOND TO CHANGES IN THE ENVIRONMEN • The glands pump salt out over the leaves and the rain washes it away. • A marsh plant called the pickleweed pumps the salt to the stems at the tips of the plant and the plant then sheds the stems, eliminating the salt. • Defending Against Disease – Plants are subject to infections by bacteria, viruses, and fungi. – The plant’s epidermis is the first line of defense. PLANTS RESPOND TO CHANGES IN THE ENVIRONMEN – Sometimes the stomata or plant wound can let these pathogens in. – Chemicals are the second line of defense. – Some chemicals are antimicrobials. – Some attack the cell walls of the bacteria. – Some signal lignin production, hardening the cell walls around the infected area and sealing it off. – Certain pathogens can be recognized and attacked by certain plants. PLANTS RESPOND TO CHANGES IN THE ENVIRONMEN – Horticulturists will select these plants or engineer them to introduce disease resistant genes into the pool. – Some plants have developed poisons or thorns to keep from being eaten. REVIEW: CONCEPT CHECK 22.2, page 485 1. Describe the role of osmosis in controlling the rapid plant movements of Mimosa pudica. 2. Distinguish among thigmotropism, phototropism, and gravitropism. 3. Contrast a desert plant’s adaptations with the adaptations of a houseplant experiencing a temporary drought. 4. Describe the two main adaptations in a plant’s defense against disease. PLANTS KEEP TRACK OF THE HOURS AND SEASONS • Circadian Rhythms – Experiencing jet lag can make you too tired or too wired. – Your pulse, body temperature, and blood pressure change with the time of the day. – Plants experience the same rhythm over a 24 hour period. – Biological cycles that occur over 24 hours are called circadian rhythms. PRAYER PLANT & MIMOSA, CIRCADIAN RHYTHMS PLANTS KEEP TRACK OF THE HOURS AND SEASONS – The biological clocks of organisms are set by daily signals from the environment, especially light. – Being kept in the dark does not change circadian rhythms except for the cycle lengths, being shorter or longer that 24 hours. – Changing from night to day is required to set the clock exactly to a 24 hour cycle. PLANTS KEEP TRACK OF THE HOURS AND SEASONS • Day Length and Seasons – Plants respond to not only a 24 hour cycle but changes in seasons. – Production of flowers, germinating seeds, dormancy occur at specific times during the year. – The length of the day light and night time determine the time of the year for the plant. – This ability to use this environmental stimulus PLANTS KEEP TRACK OF THE HOURS AND SEASONS to time seasonal activities is known as photoperiodism. • Chrysanthemums and poinsettias flower in the fall or winter when the dark period exceeds a certain length, called the critical night length. – These are examples of short-day plants (long-night plants). • Long-day plants (short-night plants), including spinach, lettuce, and irises flower in late spring or early summer when dark periods shorten. PLANTS KEEP TRACK OF THE HOURS AND SEASONS – Day-neutral plants flower when a certain stage of maturity is reached, with no dependency to day length. • Examples are dandelions, tomatoes, and rice. – Flower growers can use the information about photoperiodism to produce flowers out of season. – Plants can monitor day length as seasons change, including sunrise and sunset, with pigment proteins called phytochromes. DAY LENGTH AFFECTING PLANTS DAY LENGTH AFFECTING PLANTS DAY LENGTH AFFECTING PLANTS PLANTS KEEP TRACK OF THE HOURS AND SEASONS • Phytochromes absorb red light at sunrise changing their shape to an active form triggering certain plant responses. • At sunset, the phytochromes change back to their inactive form. PHYTOCHROMES REVIEW: CONCEPT CHECK 22.3, page 487 1. Give an example of a circadian rhythm in a plant or animal. 2. Describe the difference between a shortand a long-day plant and give an example of each. 3. Explain how phytochromes are activated.