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UT BIO 311D - Plant Structure
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BIO 311D 1st Edition Lecture 28 Outline of Last Lecture I. Hormones and the Endocrine SystemII. Intracellular CommunicationIII. Endocrine SignalingIV. Paracrine and Autocrine signalingV. Synaptic signalingVI. Endocrine Tissues and OrgansVII. Cellular Response PathwaysOutline of Current Lecture I. Plants have a hierarchical organization consisting of organs, tissues, and cellsII. The Three Basic Plant Organs: Roots, Stems, and LeavesIII. RootsIV. StemsV. LeavesVI. Dermal, Vascular, and Ground TissuesVI. Common Types of Plant CellsCurrent LecturePlants have a hierarchical organization consisting of organs, tissues, and cells• Plants have organs composed of different tissues, which in turn are composed of different cell types• A tissue is a group of cells consisting of one or more cell types that together perform a specialized function• An organ consists of several types of tissues that together carry out particular functionsThe Three Basic Plant Organs: Roots, Stems, and Leaves• Three basic organs evolved: roots, stems, and leaves• They are organized into a root system and a shoot system• Roots rely on sugar produced by photosynthesis in the shoot system, and shoots rely on water and minerals absorbed by the root system• Monocots and eudicots are the two major groups of angiospermsRoots• A root is an organ with important functions:– Anchoring the plant– Absorbing minerals and water– Storing carbohydrates• Most eudicots and gymnosperms have a taproot system, which consists of:– A taproot, the main vertical root– Lateral roots, or branch roots, that arise from the taproot• Most monocots have a fibrous root system, which consists of:– Adventitious roots that arise from stems or leaves– Lateral roots that arise from the adventitious roots• Most eudicots and gymnosperms have a taproot system, which consists of:– A taproot, the main vertical root– Lateral roots, or branch roots, that arise from the taproot• Most monocots have a fibrous root system, which consists of:– Adventitious roots that arise from stems or leaves– Lateral roots that arise from the adventitious roots• In most plants, absorption of water and minerals occurs near the root hairs, where vast numbers of tiny root hairs increase the surface areaStems• A stem is an organ consisting of – An alternating system of nodes, the points at which leaves are attached– Internodes, the stem segments between nodes• An axillary bud is a structure that has the potential to form a lateral shoot, or branch• An apical bud, or terminal bud, is located near the shoot tip and causes elongation of a young shoot• Apical dominance helps to maintain dormancy in most axillary buds• Many plants have modified stems (e.g., rhizomes, bulbs, stolons, tubers)Leaves• The leaf is the main photosynthetic organ of most vascular plants• Leaves generally consist of a flattened blade and a stalk called the petiole, which joins the leaf to a node of the stem• Monocots and eudicots differ in the arrangement of veins, the vascular tissue of leaves• Most monocots have parallel veins• Most eudicots have branching veins• In classifying angiosperms, taxonomists may use leaf morphology as a criterion• Some plant species have evolved modified leaves that serve various functionsDermal, Vascular, and Ground Tissues• Each plant organ has dermal, vascular, and ground tissues• Each of these three categories forms a tissue system• Each tissue system is continuous throughout the plant• In nonwoody plants, the dermal tissue system consists of the epidermis• A waxy coating called the cuticle helps prevent water loss from the epidermis• In woody plants, protective tissues called periderm replace the epidermis in older regions of stems and roots• Trichomes are outgrowths of the shoot epidermis and can help with insect defense• The vascular tissue system carries out long-distance transport of materials between roots and shoots• The two vascular tissues are xylem and phloem• Xylem conveys water and dissolved minerals upward from roots into the shoots• Phloem transports organic nutrients from where they are made to where they are needed• The vascular tissue of a stem or root is collectively called the stele• In angiosperms the stele of the root is a solid central vascular cylinder• The stele of stems and leaves is divided into vascular bundles, strands of xylem and phloem• Tissues that are neither dermal nor vascular are the ground tissue system• Ground tissue internal to the vascular tissue is pith; ground tissue external to the vascular tissue is cortex• Ground tissue includes cells specialized for storage, photosynthesis, and supportCommon Types of Plant Cells• Like any multicellular organism, a plant is characterized by cellular differentiation, the specialization of cells in structure and function• The major types of plant cells are:• Parenchyma• Collenchyma• Sclerenchyma• Water-conducting cells of the xylem• Sugar-conducting cells of the phloem• Mature parenchyma cells• Have thin and flexible primary walls• Lack secondary walls• Are the least specialized• Perform the most metabolic functions• Retain the ability to divide and differentiate• Collenchyma cells are grouped in strands and help support young parts of the plant shoot• They have thicker and uneven cell walls• They lack secondary walls• These cells provide flexible support without restraining growth• Sclerenchyma cells are rigid because of thick secondary walls strengthened with lignin• They are dead at functional maturity• There are two types:• Sclereids are short and irregular in shape and have thick lignified secondary walls• Fibers are long and slender and arranged in threads• The two types of water-conducting cells, tracheids and vessel elements, are dead at maturity• Tracheids are found in the xylem of all vascular


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