BIOL 425 8th Edition Exam 3 Study Guide Chapters 26 30 Chapter 26 Secondary Growth in Stems Secondary Growth An increase in thickness or girth of the plant body which results from the activity of the two lateral meristems the vascular cambium and the cork cambium Vascular Cambium Meristematic cells of the vascular cambium are highly vacuolated and exist in two forms vertically oriented fusiform initials which are several to many times longer than they are wide and as horizontally oriented ray initials which are slightly elongated or squarish Secondary xylem and phloem are produced through periclinal divisions of the fusiform and ray initials and their immediate derivatives The call plate that forms between these dividing cambial initials is parallel to the surface of the root or stem Axial system Composed of the xylem and phloem cells produced by the fusiform initials that have their long axes oriented vertically The ray initials produce horizontally oriented ray cells which form the vascular rays radial system The vascular rays are variable in length and serve as pathways for the movement of food substances from the secondary phloem to the secondary xylem and the movement of water from the secondary xylem to the secondary phloem Secondary Growth on the Primary Body The vascular cambium of the stem arises from the procambium that remains undifferentiated between the primary xylem and phloem as well as from parenchyma of the interfascicular regions regions between the vascular bundles or fascicles That portion of the cambium arising within the vascular bundles is known as fascicular cambium and that arising in the interfascicular regions is called interfascicular cambium Periderm The dermal tissue system of the secondary plant body In most woody stems periderm formation usually follows the initiation of secondary xylem and phloem production The periderm replaces the epidermis as the protective covering on those portions of the plant Structurally the periderm consists of three parts the cork cambium the meristem that produces the periderm cork the protective tissue formed to the outside by the cork cambium and the phelloderm a tissue that resembles cortical parenchyma formed to the inside by the meristem Lenticels Portions of the periderm with numerous intercellular spaces which allow gas exchange through the periderm Bark Refers to all the tissues outside the vascular cambium including the periderm when present All of the tissues outside the innermost cork cambium make up the outer bark which consists of entirely dead tissues The living part which is inside the innermost cork cambium and extends inward to the vascular cambium is called the inner bark The part of the inner bark that contains living functional sieve elements and is actively engaged in the transport of food substances is called conducting phloem Wood Wood is the secondary xylem and is classified as either hardwoods or softwoods Hardwoods are angiosperm woods and softwoods are conifer woods The structural basis for the visibility of growth layers in wood is the difference in the density of the wood produced early in the growing season and that produced later The early wood is less dense with wider cells and thinner walls than the late wood with narrower cells and thicker walls The change from early to late wood may be gradual Chapter 27 Regulating Growth and Development The Plant Hormones Hormones chemical signals that regulate metabolism growth and morphogenesis in plants and animals Plant hormones are known as phytohormones and different tissues require different amounts of hormones Auxins A class of plant hormones that control cell elongation and other effects The principal naturally occurring auxin is called indole 3 acetic acid IAA IAA closely resembles tryptophan and both are synthesized from indole but plants can produce IAA through multiple pathways They are primarily in leaf primordia and young leaves and developing seeds Auxin is transported both polarly and nonpolarly Auxins cause apical dominance tropic responses vascular tissue differentiation and promote cambial activity Cytokinins Cytokinins are N6 adenine derivatives phenyl urea compounds Zeatin is the most common cytokinin in plants and occurs primarily in root tips Cytokinins are transported in the xylem from roots to shoots Cytokinins promote cell division and shoot formation in tissue culture They also delay leaf senescence and can increase root development in arid conditions Ethylene The gas ethylene C2H4 is synthesized from methionine It is the only hydrocarbon with a pronounced effect on plants It occurs I most tissues in response to stress especially in tissues undergoing senescence or ripening Ethylene moves by diffusion from its site of synthesis Ethylene is responsible for fruit ripening leaf and flower senescence and fruit abscission Abscisic acid Abscisic acid is synthesized from a carotenoid intermediate The name is a misnomer because the hormone has little to do with abscission This acid occurs in mature leaves and roots especially in response to water stress It may also be synthesized in seeds Abscisic acid is exported from leaves in the phloem and from roots in the xylem Effects include stomatal closure induction of photosynthesis induction of storage protein synthesis in seeds and embryogenesis Gibberellins Gibberellic acid is the most widely studied fungal product They are synthesized via the terpenoid pathway They occur in young tissues of the shoot and developing seeds It is uncertain whether synthesis also occurs in roots Gibberellins are probably transported in the xylem and phloem This acid causes hyperelongation of shoots by stimulating both cell division and cell elongation producing tall as opposed to dwarf plants Also cause induction of seed germination Hormones alter cell growth and gene expression through signal transduction pathways At the molecular level hormones influence developmental processes through their interaction with receptors in the plant cell The plant hormones mediate many processes by activating or repressing sets of genes in the cell s nucleus At the cellular level hormones influence the rate and direction of cell expansion and rate of cell division These effects are mediated through complex biochemical reponse pathways in the cell often involving second messengers Chapter 28 External Factors and Plant Growth Tropism Tropism is the growth response involving bending of a plant part toward or away from an