Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 9: Plant Organization 9-1 Plant Organs The flowering plants, or angiosperms, have characteristic organs and tissues. An organ is a structure that contains different types of tissues and performs one or more specific functions. The vegetative organs of a flowering plant – the root, stem, and leaf – allow the plant to live and grow. The body of a plant has a root system and a shoot system. 9-2 Organization of the plant body 9-3 Roots The root system of a plant has a main root, or taproot, and many branch or lateral roots. The root system absorbs water and minerals from the soil for the plant. The cylindrical shape of the root allows it to penetrate the soil. Root hairs greatly increase the absorptive capacity of the root. 9-4 Roots can also have other functions. Roots produce hormones that stimulate the growth of stems and coordinate their size with the size of the root. Generally the root system is equivalent in size and extent to the shoot system. Perennial plants often store the products of photosynthesis in their roots. 9-5 Root system 9-6 Stems The shoot system of a plant includes both stems and leaves. A stem is the main axis of the plant along with its lateral branches. At the tip of the stem is tissue that allows the stem to elongate and produce leaves. A leaf attached to a stem at a node; and internode is the region beween nodes. 9-7 Shoot system 9-8 Stems also contain vascular tissue that transports water and minerals from roots to leaves, and also transports the products of photosynthesis in the opposite direction. A cylindrical stem can expand in girth as well as in length. Some stems have functions other than transport; some are specialized for storage. 9-9 Leaves A leaf is a broad, thin organ that carries on photosynthesis. This shape maximizes the surface area for collection of solar energy and absorption of carbon dioxide. The wide portion of a leaf is the blade, a petiole is the stalk of the leaf, and axillary buds are found at the leaf axil. Some leaves have other functions. 9-10 Leaves 9-11 Monocot Versus Dicot Plants Flowering plants are divided into two groups depending on their number of cotyledons (seed leaves). Monocots (monocotyledons) have one cotyledon; dicots (dicotyledons) have two. Cotyledons provide nutrients for seedlings before true leaves begin photosynthesizing. 9-12 The vascular (transport) tissue is organized differently in monocots and dicots. Monocot roots have vascular tissue in a ring; in stems, vascular bundles are scattered. Dicot roots have vascular tissue in a star shape with phloem located between arms of xylem. Dicot stems have vascular bundles in a ring. 9-13 Monocot and dicot traits 9-14 Leaf veins are vascular bundles within a leaf. Monocots usually have parallel venation. Dicots exhibit netted venation, which may be either pinnate or palmate. Adult monocots and dicots differ in the number of flower parts, and dicot pollen grains have three apertures while monocot pollen grains have one aperture. Important monocots are rice, wheat and corn; oak trees and dandelions are dicots. 9-15 9-16 Dicot leaves 9-17 Plant Tissues A plant grows throughout its lifespan because of meristem (embryonic tissue) in stem and root tips (apexes). Three specialized tissues are in plants: 1) Epidermal tissue – forms the outer protective covering 2) Ground tissue – fills interior of a plant 3) Vascular tissue – transports water and nutrients and provides support. 9-18 Epidermal Tissue Epidermal tissue forms the outer protective covering of a herbaceous plant and is modified in roots, stems, and leaves. Exposed epidermal cells are covered with waxy cuticle to minimize water loss. Epidermal cells in roots have root hairs. Lower leaf epidermal cells have guard cells and stomata. 9-19 Root hairs 9-20 Stoma of leaf 9-21 In older woody plants, the epidermis of the stem is replaced by cork tissue. Cork, a component of bark, is made up of dead cells that may be sloughed off. New cork cells are made by a meristem called cork cambium. As cork cells mature, they fill with suberin, a lipid that makes them waterproof and chemically inert. Cork protects woody plants and helps them resist fungi, bacteria, and animals. 9-22 Cork of older stem 9-23 Ground Tissue Ground tissue forms the bulk of the plant. Ground tissue contains parenchyma cells, which are thin-walled and capable of photosynthesis when they contain chloroplasts. Collenchyma cells have thicker walls for flexible support. Sclerenchyma cells are hollow, nonliving support cells with secondary walls. 9-24 Ground tissue cells 9-25 Vascular Tissue There are two types of vascular (transport) tissue that extend from roots to leaves. Xylem transports water and minerals from roots to leaves and contains two types of conducting cells: tracheids and vessel elements. Phloem transports organic nutrients from leaves to roots and has sieve-tube elements with companion cells; plasmodesmata extend between cells at sieve plates. 9-26 Xylem structure 9-27 Phloem structure 9-28 Organization of Roots Within a root are zones where cells are in various stages of differentiation. The root apical meristem is in the zone of cell division; the root cap is a protective covering for the root tip. In the zone of elongation, cells become longer as they specialize. In the zone of maturation, mature cells are differentiated and epidermal cells have root hairs. 9-29 Dicot root tip 9-30 Tissues of a Dicot Root Epidermis – single layer of thin-walled, rectangular cells; root hairs present in zone of maturation Cortex – thin-walled, loosely-packed parenchyma; starch granules store food Endodermis – between cortex and vascular cylinder, single layer of endodermal cells bordered by the Casparian strip; regulates entrance of minerals into the vascular cylinder 9-31 Movement of materials into vascular cylinder 9-32 Vascular Tissue – has star-shaped xylem in dicots with phloem in separate regions between arms of xylem; the pericycle gives rise to lateral roots 9-33 Branching of a dicot root 9-34 Organization of Monocot Roots Monocot roots have the same growth zones as a dicot root but they do not undergo secondary growth. In a monocot root’s centrally located pith, ground tissue is surrounded by a vascular ring composed of alternating xylem and phloem bundles. Monocot roots also have pericycle, endodermis, cortex, and epidermis. 9-35 Monocot root 9-36 Root Diversity Roots have special adaptations and associations to better perform the functions of anchorage, absorption of water and minerals, and carbohydrate storage. In some dicot plants, a primary root, or taproot, grows straight down and is the dominant root; it can be fleshy and stores food. 9-37 Taproot 9-38 Monocots have no single main root but instead have a large number of slender roots. These slender roots and their lateral branches make up a fibrous root system. Fibrous roots develop from the lower nodes of the stem rather than from the root system, and are known as adventitious roots. Adventitious roots that emerge from the surface to help anchor the plant are called prop roots. 9-39 Fibrous roots and prop roots 9-40 Some plants (dodders and broomrapes) are parasitic on other plants. Their stems have rootlike projections called haustoria that grow into the host plant and extract water and nutrients from the host. Mycorrhizae are a mutualistic association between roots and fungi that aid the plant in extraction of nutrients and water from soil. Peas and other legumes have root nodules in which nitrogen-fixing bacteria live. 9-41 Dodder 9-42 Organization of Stems During primary growth, the shoot apical meristem at the shoot tip produces new cells that elongate and add length to the stem. The shoot apical meristem is protected within a terminal bud where leaf primordia envelope it. In the temperate zone, a terminal bud stops growing in winter and is protected by bud scales. 9-43 Leaf primordia are produced at nodes; the stem between two nodes is called an internode. Internodes increase in length as the stem grows. Axillary buds, which are dormant but may develop into branch shoots or flowers, form at the axes of leaf primordia. 9-44 Shoot tip 9-45 In addition to leaf primordia, three specialized types of primary meristem that contribute to shoot length develop from shoot apical meristem. Protoderm gives rise to epidermis. Ground meristem produces parenchyma in the pith and parenchyma in the cortex. Procambium produces primary xylem and primary phloem; later, vascular cambium occurs between xylem and phoem. 9-46 Fate of primary meristems 9-47 Herbaceous Stems Mature nonwoody stems, or herbaceous stems, exhibit only primary growth. The outermost tissue is the epidermis, which is covered by waxy cuticle. These stems have distinctive vascular bundles, with xylem toward the inside and phloem toward the outside. In dicot stems, vascular bundles are in a distinct ring; monocot vascular bundles are scattered throughout. 9-48 Herbaceous dicot stem 9-49 Monocot stem 9-50 Woody Stems A woody plant has both primary and secondary tissues. Primary tissues are new tissues formed each year from primary meristems right behind apical meristem. Secondary tissues develop during the second and subsequent years of growth from lateral meristems (vascular cambium and cork cambium). 9-51 Primary growth, which occurs in all plants, increases the length of the plant. Secondary growth, which occurs in conifers and some dicots, increases the girth of a plant. Trees undergo secondary growth because of a change in vascular cambium. The secondary tissues produced by the vascular cambium, called secondary xylem and secondary phloem, add to the girth of trunks, stems, branches, and roots. 9-52 Dicot stems 9-53 Secondary growth in a dicot stem 9-54 As a result of secondary growth, a woody dicot stem has an entirely different type of organization. A woody stem now has three distinct areas: the pith, the wood, and the bark. Pith rays are composed of living parenchyma cells that allow materials to move laterally. The bark of a tree contains cork, cork cambium, and phloem. Cork cambium replaces epidermis with cork cells impregnated with suberin. 9-55 Section of woody stem 9-56 Annual Rings In trees that have a growing season, vascular cambium is dormant during winter. In spring, with plentiful moisture, xylem contains wide vessels with thin walls in spring wood; summer wood has a lower proportion of vessels. Spring wood followed by summer wood makes up one year’s growth or annual ring. 9-57 Tree trunk 9-58 Stem Diversity Plants use stems for such functions as reproduction, climbing, survival, and food storage. Modified stems aid adaptation to different environments. Examples of stem modifications include: Stolons Rhizomes Tubers Corms 9-59 Stolons and rhizomes 9-60 Tubers and corms 9-61 Organization of Leaves Leaves are the organs of photosynthesis in vascular plants. Leaves have a flattened blade, that may be single or composed of leaflets, attached to a petiole. The epidermal layers may bear protective hairs or glands that produce irritating substances; a waxy cuticle reduces water loss and permits gas exchange. 9-62 Guard cells surrounding stomata in the lower epidermis allow gases to enter and exit the leaf. The body of the leaf is composed of mesophyll. Palisade mesophyll has elongated cells, and spongy mesophyll has irregular cells surrounded by air spaces. Parenchyma cells of these mesophyll layers house chloroplasts. 9-63 Leaf structure 9-64 Leaves are adapted to environmental conditions and may be broad and wide or reduced with sunken stomata. The leaves of a cactus are spines attached to a succulent stem. Climbing leaves, such as those of peas, are modified into tendrils. The leaves of a few plants are specialized for catching insects. 9-65 Classification of leaves 9-66 Leaf diversity 9-67 Chapter Summary A flowering plant (angiosperm) has three vegetative organs: the root absorbs water and minerals, the stem supports and services leaves, and the leaf carries on photosynthesis. Flowering plants can be divided into monocots and dicots based on structural differences. 9-68 Three types of meristem divide and produce specialized tissues. Dermal tissue consists of epidermis composed of epidermal cells. Ground tissue contains parenchyma, collenchyma, and sclerenchyma cells. Vascular tissue consists of xylem (vessel elements and tracheids) that transports water and minerals, and phloem (sievetube elements and companion cells) that transports organic nutrients. 9-69 A root tip has three zones: the zone of cell division, the zone of elongation, and the zone of maturation. A herbaceous root has epidermis, cortex, endodermis, and a vascular cylinder. The dicot vascular cylinder is star-shaped, while the monocot root has a ring of vascular tissue with alternating bundles of xylem and phloem surrounding pith. Roots have diverse structures, including taproots, adventitious roots, and prop roots. 9-70 Primary growth of a stem is due to the shoot apical meristem, which is protected within a terminal bud. Stems have nodes and internodes. In cross section, a nonwoody dicot has epidermis, cortex, vascular bundles in a ring, and an inner pith. Monocots have scattered vascular bundles. Secondary growth is due to vascular cambium; wood contains annual rings of xylem. 9-71 Stems are diverse; humans find many uses for stems. A leaf has an upper and a lower epidermis, and mesophyll tissue forms the bulk of the leaf. Stomata regulate the passage of gases in and out of leaves. Leaves are diverse and are adapted to environmental conditions. 9-72