Chapter 35 Plant Structure Growth and Development Concept 35 1 Plants have a hierarchical organization consisting of organs tissues and cells 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 functions The three basic plant organs roots stems and leaves Chapter 35 Plant Structure Growth and Development 1 These organs form a root system and a shoot system The ability to acquire these resources efficiently is traceable to the evolution of three basic organs roots stems and leaves Roots typically are not photosynthetic they starve unless photosynthates the sugars and other carbohydrates produced during photosynthesis are imported from the shoot system Roots A root is an organ that anchors a vascular plant in the soil absorbs minerals and water and often stores carbohydrates Most eudicots and gymnosperms have a taproot system consisting of one main vertical root the taproot which develops from an embryonic root The taproot gives rise to lateral roots also called branch roots Taproot systems generally penetrate deeply and are therefore well adapted to deep soils where the groundwater is not close to the surface Fibrous root systems a mat of generally thin roots spreading out below the soil surface Fibrous root systems usually do not penetrate deeply and are therefore best adapted to shallow soils or regions where rainfall is light and does not moisten the soil much below the surface layer Although the entire root system helps anchor a plant in most plants the absorption of water and minerals occurs primarily near the tips of roots where vast number of root hairs emerge and increase the surface area of the root enormously A root hair is a thin tubular extension of a root epidermal cell A stem is an organ that raises or separate leaves exposing them to sunlight Stems also raise reproductive structures facilitating dispersal of pollen and fruit Each stem consists of an alternating system of nodes the points at which leaves are attached and internodes the stem segments between nodes Axillary buds are structures that form a lateral shoot commonly called a branch Apical bud or terminal bud is composed to developing leaves and a compact series of nodes and internodes The inhibition of axillary buds by an apical bud is called apical dominance A growing axillary bud gives rise to a lateral shoot complete with its own apical bud leaves and axillary buds Removing the apical bud stimulates the growth of axillary buds resulting in more lateral shoots Some plants have stems with additional functions such as food storage and asexual reproduction These modified stems which include rhizomes bulbs stolons and tubers are often mistaken for roots In most vascular plants the leaf is the main photosynthetic organ although green stems also perform photosynthesis Leaves are extensively in form but generally consists of flattened blade and a stalk the petiole which joins the leaf to the stem at a node Monocots and eudicots differ in the arrangement of veins the vascular tissues of leaves Monocots have parallel major veins that run the length of the blade Eudicots generally have branched network of major veins Stems Leaves Dermal Vascular and Ground Tissues Each of these three categories forms a tissue system a functional unit connecting all of the plants organs Each plant organ root stem or leaf has dermal vascular and ground tissues The dermal tissue system is the plant s outer protective covering It is the first line of defense against physical damage and pathogents In nonwoody plants it is usually a single tissue called the epidermis a layer of tightly packed cells In leaves and most stems the cuticle a waxy coating on the epidermal surface helps prevent water loss In woody plants protective tissues called periderm replace the epidermis in older regions of stems and roots A root hair is an extension of an epidermal cell near the tip of the root The vascular tissue system carries out long distance transport of materials between the root and shoot system Xylem conducts water and dissolved minerals upward from roots into the shoots Phloem transports sugars the products of photosynthesis from where they are made to where they are needed The vascular tissue of a root or stem is collectively called the stele In angiosperms the root stele is a solid central vascular cylinder of xylem and phloem Chapter 35 Plant Structure Growth and Development 2 The stele of stems and leaves consists of vascular bundles separate strands containing xylem and phloem Tissues that are neither dermal nor vascular are part of the ground tissue system Ground tissue that is internal to the vascular tissues is known as the pith and ground tissue that is external to the vascular tissue is called cortex Common Types of Plant Cells A plant is characterized by cell differentiation the specialization of cells in structure and function Cell differentiation may involve changes both in the cytoplasm and its organelles and in the cell wall Concept 35 1 Summary Plants have a hierarchical organization consisting of organs tissues and cells Vascular plants have shoots consisting of stems leaves and in angiosperms flowers Roots anchor the plant absorb and conduct water and minerals and store food Leaves are attached to stem nodes and are the main organs of photosynthesis Axillary buds in axils of leaves and stems give rise to branches Plant organs may be adapted for specialized functions Vascular plants have three tissue systems dermal vascular and ground which are continuous throughout the plant Dermal tissue protects against pathogens herbivores and drought and aids in the absorption of water minerals and carbon dioxide Vascular tissues facilitate the long distance transport of substance of substances Ground tissues function in storage metabolism and regeneration Parenchyma cells are relatively unspecialed and thin walled cels that retain the ability to divide they perform most of the plant s metabolic functions of synthesis and storage Collenchyma cells have unevenly thickend walls they support young growing parts of the plant Sclerenchyma cells fibers and sclerids have thick lignified walls that help support mature nongrowing parts of the plant Tracheids and vessel elements the water conducting cells of xylem have thick walls and are dead at functional maturity Sieve tube elements are living but highly