BIO 202 Chapter 34, Spring 2018

© 2017 Pearson Education, Ltd. Lecture Presentation by Cindy S. Malone, PhD, California State University Northridge 34 Plant Form and Function Chapter 34 Roadmap. © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Introduction • Plants make their own food through photosynthesis, using the energy from sunlight • They stay in one place, extending their roots and shoots to harvest diffuse resources • The structure of a plant’s body is dynamic, because most plants grow throughout their lives and exhibit indeterminate growth © 2017 Pearson Education, Ltd. Introduction • We will look at three fundamental questions, focusing on angiosperms: 1. How is the plant body organized? 2. Why are plants so diverse in size and shape? 3. How do plants grow throughout their lives? © 2017 Pearson Education, Ltd. Plant Form • Plants use light energy to synthesize carbohydrates from carbon dioxide in the air and water from the soil • Plants must also obtain nitrogen, phosphorus, potassium, magnesium, and other nutrients • These key elements exist in nature primarily as ions dissolved in soil water © 2017 Pearson Education, Ltd. How Is the Plant Body Organized? • Belowground, the root system anchors the plant and takes in water and nutrients from the soil • Aboveground, the shoot system harvests light and carbon dioxide from the atmosphere to produce sugars • Together, these systems make up the plant body Figure 34.1 Apical bud Sunlight Node Internode Shoot system Node CO2 Leaves Axillary bud Branch Stem Transport of water, sugar, and nutrients through vascular tissue Lateral roots Root system Water Nutrients (such as N, P, K) Taproot © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Importance of Surface Area and Volume Relationships • Root and shoot systems both function in absorption • Roots absorb water and key nutrients • Shoots absorb light • Absorption takes place across a surface, but the cells that use the absorbed light and molecules occupy a volume • A plant is efficient in absorption if it has a large surface area relative to its volume Figure 34.2 Thick structure (64 cells) 50 µm Tubelike structure (64 cells) Flattened structure (64 cells) Surface area = 240,000 µm2 (96 cell surfaces x 2500 µm2/cell surface) Volume = 8,000,000 µm3 (64 cells x 125,000 µm3/cell) Surface area/volume = 0.0300/µm Surface area = 340,000 µm2 (136 cell surfaces x 2500 µm2/cell surface) Volume = 8,000,000 µm3 (64 cells x 125,000 µm3/cell) Surface area/volume = 0.0425/µm Surface area = 420,000 µm2 (168 cell surfaces x 2500 µm2/cell surface) Volume = 8,000,000 µm3 (64 cells x 125,000 µm3/cell) Surface area/volume = ? © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. The Root System • Many root systems have a taproot and lateral roots • There are many key functions of the root system: • Roots anchor the plant to the soil • Roots absorb ions and water from soil • Roots conduct water and ions to the shoot system • Roots obtain energy from the sugar in the shoot system • Roots store molecules produced in the shoot system © 2017 Pearson Education, Ltd. The Root System • Although most root systems contain the same general structures, root systems are diverse • This diversity can be analyzed on three levels: 1. Morphological diversity among species 2. Phenotypic plasticity in response to the environment 3. Modified roots that are specialized for unusual functions © 2017 Pearson Education, Ltd. Morphological Diversity in Root Systems • Consider prairie plants • Prairies are grassland ecosystems that support many herbaceous plants • The root systems of prairie grasses are perennial, surviving fire or drought • A new shoot system is produced each spring or after fire • Prairie grasses show diversity among species, minimizing competition for water and resources Figure 34.3 Pale purple coneflower Nonnative lawn grass, for comparison Lead plant Compass plant Switchgrass Junegrass Depth (m) Fibrous roots Taproot © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Phenotypic Plasticity in Root Systems • Morphological diversity in roots also occurs within species • Roots show phenotypic plasticity, with form that changes with environmental conditions • The environment can determine the type and depth of the root system • Roots die back in areas lacking resources • For example, spruce trees in waterlogged soils have shallow roots due to lack of oxygen © 2017 Pearson Education, Ltd. Modified Roots • Modified roots are formed by various types of plants • Adventitious roots grow from the shoot system • In ivy, adventitious roots act to anchor the plant • The prop roots of corn brace the plant • Root systems can produce modified roots • Mangroves produce pneumatophores for gas exchange • The taproots of beets and carrots store carbohydrates Table 34.1 © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. The Shoot System • The shoot system has many anatomical features: • Stems are vertical aboveground structures consisting of nodes, where leaves are attached, and internodes, or segments between nodes • Axillary (or lateral) buds form at nodes just above the site of leaf attachment • An axillary bud may grow into a branch • The tip of each stem and branch contains an apical bud • Apical and axillary buds may develop into flowers © 2017 Pearson Education, Ltd. The Shoot System • Shoot diversity can also be analyzed on three levels: 1. Morphological diversity among species 2. Phenotypic plasticity within individuals 3. Modified shoots with specialized functions © 2017 Pearson Education, Ltd. Morphological Diversity in Shoot Systems • Shoot systems range in size from tiny duckweed (< 5 mm diameter) to giant redwood trees (100 m tall) • Variation in the size and shape of the shoot system of various species minimizes competition for light • Silversword plants native to Hawaii have shoot systems that are particularly diverse • Lush environments and competition for light favors taller shoot systems, while dry, windblown habitats favor short stems that are more anchored to the soil © 2017 Pearson Education, Ltd. Phenotypic Plasticity in Shoot Systems • The size and shape of a plant’s shoot system can vary based on variation in growing conditions: temperature, wind exposure, and availability of water, nutrients, and light • Genetically identical herbaceous plants were grown from cuttings taken from the same plant • The cuttings were grown at different elevations from sea level to alpine habitats, resulting in a variety of shoot systems Figure 34.4 1. Take cuttings from individuals of Potentilla glandulosa growing in low-, medium-, and high- elevation habitats in the Sierra Nevada mountains. 2. Propagate genetically identical individuals. 3. Transplant individuals from each source population into each habitat (low, medium, and high elevation). Allow to grow and observe mature plants. How much does a plant’s growth form depend on its environment? (No explicit hypothesis—the goal of this experiment was to explore the interaction between genetic makeup versus environmental influence on size and shape.) High 3000 m Medium Low “High” plant grown at low elevation “High” plant grown at medium elevation “High” plant grown at high elevation Environmental conditions have a profound influence on body size and shape (genetically identical plants look different at each site). BUT, genetic makeup also has a large influence on plant morphology. (Though not pictured here, plants from the different source populations looked different, even when all were grown in the same habitat.) Examples of mature plants observed: (No explicit predictions.) © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Modified Stems • Stems of many plants serve specialized functions • Cactus stems are modified to store water and for photosynthesis • Stolons are modified stems that grow horizontally, producing adventitious roots and leaves at each node • Rhizomes are stems that grow horizontally and function in asexual reproduction • Tubers are underground, swollen rhizomes that function as carbohydrate-storage organs • Thorns are modified stems that protect plants Table 34.2 © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. The Leaf • The vast majority of photosynthesis occurs in leaves • The leaf’s relatively large surface area is available for absorbing photons and supporting photosynthesis • A simple leaf has two main structures, an expanded blade and a stalk called a petiole • In a compound leaf, the blade is divided into a series of leaflets • Needlelike leaves reduce water loss by transpiration Figure 34.5 (a) A simple leaf has a petiole and a single blade. (b) A compound leaf has a blade divided into leaflets. (c) A doubly compound leaf is large yet rarely damaged by wind or rain. (d) Needlelike leaves are characteristic of species adapted to very cold or hot climates. Blade Petiole © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Morphological Diversity in Leaves • Not all leaves are thin, with a large surface area • Plants that thrive in deserts and in cold, dry habitats tend to have small, needlelike leaves, interpreted as adaptations to minimize transpiration in water-scarce habitats • Two observations support this hypothesis: 1. Water is scarce in deserts and frozen (and thus unavailable) in cold habitats 2. Leaves with a large surface area lose large amounts of water through transpiration © 2017 Pearson Education, Ltd. Morphological Diversity in Leaves • The arrangement of leaves on a stem may vary • Leaves may • Alternate on either side of the stem • Pair opposite each other on the stem • Be arranged in a whorl • Form a compact basal arrangement where internodes are extremely short—leading to a rosette growth form Figure 34.6 (a) Alternate leaves (b) Opposite leaves (c) Whorled leaves (d) Rosette © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Phenotypic Plasticity in Leaves • Although leaves do not grow continuously, they do exhibit phenotypic plasticity • Oak tree leaves vary depending on the amount of sunlight they are exposed to: • Sun leaves have a relatively small surface area, reducing water loss in areas of the body where light is abundant • Shade leaves are relatively large and broad, providing a high surface area to maximize photon absorption Figure 34.7 © 2017 Pearson Education, Ltd. Grown in shade Grown in sun © 2017 Pearson Education, Ltd. Modified Leaves • Onion bulbs store nutrients • The leaves of succulents store water • Tendrils enable vines to climb • The bright red leaves of poinsettias attract pollinators • The tubelike leaves of the pitcher plant trap insects • Cactus spines protect the stem Table 34.3 © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Plant Cells • Although eukaryotic cells share many key features, plant cells have several features not found in animal cells • Plant cells are surrounded by a cellulose-rich primary cell wall • Some plant cells have a rigid secondary cell wall • Plant cells are connected by plasmodesmata Figure 34.8 © 2017 Pearson Education, Ltd. (a) Plant cells have cell walls, vacuoles, and chloroplasts. (b) Adjacent plant cells are connected by plasmodesmata. Cell 2 Smooth ER Cell wall Plasmodesma Plasma membrane Vacuole Cell 1 Plasma membrane Cell wall Cell wall Plasma membrane Chloroplast Mitochondrion Rough endoplasmic reticulum Smooth endoplasmic reticulum Golgi apparatus © 2017 Pearson Education, Ltd. Plant Cells • Plant cells may contain chloroplasts and a large vacuole that fills most of the cell’s volume • Chloroplasts are the site of photosynthesis • Non-photosynthetic cells may have organelles that are similar to chloroplasts, but modified for storage of pigments, starch, oil, or proteins • Vacuoles contain an aqueous solution called cell sap and store waste, water, and nutrients © 2017 Pearson Education, Ltd. Plant Tissue Systems • Plant tissues are grouped into tissue systems, based on their structural features and location within the plant • Simple tissues consist of a single cell type; complex tissues contain several types of cells • Three tissue systems are found in plants: 1. The dermal tissue system 2. The ground tissue system 3. The vascular tissue system © 2017 Pearson Education, Ltd. The Dermal Tissue System • Dermal tissue is made up of several different cell types and thus represents a complex tissue • Epidermal cells protect the surface of the plant • They secrete a waxy cuticle to reduce water loss and protect against pathogens • Stomata regulate gas exchange and water loss • A stoma is surrounded by two specialized guard cells, which change shape to open and close the stoma Figure 34.9 Guard cells Pore Stoma closed Stoma open 10 µm © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Trichomes Perform an Array of Functions • Trichomes are hairlike appendages made up of specialized epidermal cells that perform various functions: • They keep the plant’s surface cool by reflecting sunlight • They reduce water loss by limiting transpiration • They provide barbs or toxins to protect from herbivores • They trap and digest insects Figure 34.10 200 µm © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. The Ground Tissue System • Most photosynthesis and carbohydrate storage takes place in the ground tissue system • Ground tissue is a complex tissue made up of three distinct tissues: 1. Parenchyma 2. Collenchyma 3. Sclerenchyma © 2017 Pearson Education, Ltd. Parenchyma Consists of “Workhorse” Cells • Parenchyma is a simple tissue • Parenchyma cells have relatively thin primary cell walls • They are the most abundant and versatile plant cells • In leaves, parenchyma tissue is the primary site of photosynthesis • In roots, parenchyma cells store starch granules © 2017 Pearson Education, Ltd. Parenchyma Consists of “Workhorse” Cells • Many parenchyma cells are totipotent, retaining the capacity to divide and develop into a complete plant • This allows the plant to heal wounds and reproduce asexually • The totipotency of parenchyma cells allows gardeners to clone plants by making cuttings • The parenchyma cells form a mass of undifferentiated cells called a callus • Roots develop from the callus, and the new individual can be planted in soil Figure 34.11 © 2017 Pearson Education, Ltd. (a) In leaves: photosynthesis and gas exchange (b) In roots: carbohydrate storage Chloroplasts 25 µm Starch granules (stained purple) 25 µm © 2017 Pearson Education, Ltd. Collenchyma Functions Primarily in Shoot Support • Collenchyma is a simple tissue • Collenchyma cells have unevenly thickened primary cell walls and provide flexible structural support to actively growing parts of the plant • Collenchyma is often found just under the epidermis of stems, especially outside vascular bundles Figure 34.12 © 2017 Pearson Education, Ltd. (a) Cross section of celery stalk (b) Close-up of “string,” in cross section (c) Collenchyma cells, in cross section One cell Cytoplasm Cell wall 100 µm 10 µm © 2017 Pearson Education, Ltd. Sclerenchyma: Two Types of Specialized Support Cells • The cells of sclerenchyma tissue have a thin primary cell wall and a thick, rigid secondary cell wall • The secondary cell wall contains tough, rigid lignin as well as cellulose • Sclerenchyma is a complex tissue with two types of cells: 1. Fibers are elongated 2. Sclereids are short, with variable shapes, and function in protection Figure 34.13 © 2017 Pearson Education, Ltd. (a) Fibers (b) Sclereids 20 µm 50 µm © 2017 Pearson Education, Ltd. The Ground Tissue System • Schlerenchyma and collenchyma cells differ • Collenchyma cells have expandable primary cell walls and support growing tissue in stems • Sclerenchyma cells are dead at maturity and function to support stems and other structures after growth has ceased © 2017 Pearson Education, Ltd. The Vascular Tissue System • The vascular tissue system is made up of two complex tissues: xylem and phloem • Xylem conducts water and dissolved nutrients from the root system to the shoot system • Phloem conducts sugars, amino acids, hormones, and other substances from roots to shoots and from shoots to roots © 2017 Pearson Education, Ltd. Xylem Structure • Xylem is a complex tissue containing water- conducting cells, parenchyma cells, and fibers • There are two types of water-conducting cells: 1. The xylem of all vascular plants contains tracheids with pits through which water moves 2. In angiosperms, xylem also contains vessel elements with perforations for water transport Figure 34.14 (a) Tracheids are long, tapered, and have pits. (b) Vessel elements are short and wide and have perforations as well as pits. (c) Tracheids and vessel elements are found together in vascular tissue. Pits Perforations Pits Pits 25 µm 50 µm 100 µm Tracheids Vessel elements © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Phloem Structure • Phloem is a complex tissue containing two types of specialized cells: 1. Sieve-tube elements are long, thin cells with perforated ends called sieve plates 2. Companion cells maintain the cytoplasm and plasma membrane of sieve-tube elements • Sieve-tube elements lack nuclei and are directly connected to adjacent companion cells by plasmodesmata Figure 34.15 Sieve-tube element (few organelles) Companion cell (many organelles) Sieve plate Companion cell Sieve plate 25 µm Companion cell Longitudinal section Cross section Sieve plate 50 µm © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. Primary Growth Extends the Plant Body • Plants grow throughout their lives because they have many meristems—populations of undifferentiated cells that retain the ability to undergo mitosis • Apical meristems are found at the tip of each root and shoot and are responsible for primary growth • The cells and tissues derived from apical meristems make up the primary plant body © 2017 Pearson Education, Ltd. Primary Growth Extends the Plant Body • Apical meristems give rise to three distinct primary meristems: 1. Protoderm gives rise to the dermal tissue system 2. Ground meristem gives rise to the ground tissue system 3. Procambium gives rise to the vascular tissue system Figure 34.16 (a) Apical and primary meristems in a shoot (b) Apical and primary meristems in a root Newly forming leaves Apical meristem at tip of shoot Primary meristems: Procambium Protoderm Apical meristem in axillary bud Ground meristem 100 µm 300 µm Apical meristem at tip of root © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. How Is the Primary Root System Organized? • The root apical meristem is protected by a group of cells called the root cap • Cells produced by the meristem constantly replenish the cap, which regularly loses cells • The root cap also senses gravity to determine the direction of growth and secretes a slimy lubricant to reduce friction as the apical meristem is pushed through soil © 2017 Pearson Education, Ltd. How Is the Primary Root System Organized? • Three distinct populations of cells are found behind the root cap: 1. The zone of cellular division contains the apical meristem, as well as the protoderm, ground meristem, and procambium 2. Cells in the zone of cellular elongation increase in length 3. Cells in the zone of cellular maturation differentiate into dermal, vascular, or ground tissue, and absorb water and nutrients through root hairs Figure 34.18a (a) Longitudinal section of a root Lateral root Root hairs Zone of cellular elongation Vascular tissue Ground tissue Epidermal tissue Zone of cellular division Apical meristem Sloughed-off root cap cells Root cap Zone of cellular maturation © 2017 Pearson Education, Ltd.