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CAMPBELL BIOLOGY IN FOCUS Urry • Cain • Wasserman • Minorsky • Jackson • Reece 28 Plant Structure and Growth Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge © 2014 Pearson Education, Inc. Overview: Are Plants Computers? Romanesco grows according to a repetitive program The development of plants depends on the environment and is highly adaptive © 2014 Pearson Education, Inc. Figure 28.1 © 2014 Pearson Education, Inc. Angiosperms are primary producers and are important in agriculture Taxonomists split the angiosperms into two major clades: monocots and eudicots © 2014 Pearson Education, Inc. Figure 28.2 Monocots Eudicots One cotyledon Two cotyledons Veins usually parallel Veins usually netlike Vascular tissue scattered Vascular tissue usually arranged in ring Root system usually fibrous (no main root) Taproot (main root) usually present Pollen grain with one opening Pollen grain with three openings Floral organs usually in multiples of three Floral organs usually in multiples of four or five Embryos Leaf venation Stems Roots Pollen Flowers © 2014 Pearson Education, Inc. Figure 28.2a Monocots Eudicots One cotyledon Two cotyledons Veins usually parallel Veins usually netlike Embryos Leaf venation © 2014 Pearson Education, Inc. Figure 28.2b Monocots Eudicots Vascular tissue scattered Vascular tissue usually arranged in ring Root system usually fibrous (no main root) Taproot (main root) usually present Stems Roots © 2014 Pearson Education, Inc. Figure 28.2c Monocots Eudicots Pollen grain with one opening Pollen grain with three openings Floral organs usually in multiples of three Floral organs usually in multiples of four or five Pollen Flowers © 2014 Pearson Education, Inc. Concept 28.1: Plants 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 An organ consists of several types of tissues that together carry out particular functions A tissue is a group of cells consisting of one or more cell types that together perform a specialized function © 2014 Pearson Education, Inc. The Three Basic Plant Organs: Roots, Stems, and Leaves The basic morphology of vascular plants reflects their evolution as organisms that draw nutrients from below ground and above ground Plants take up water and minerals from below ground Plants take up CO2 and light from above ground © 2014 Pearson Education, Inc. Three basic organs evolved to acquire these resources: roots, stems, and leaves The root system includes all of the plant’s roots The shoot system includes the stems, leaves, and (in angiosperms) flowers © 2014 Pearson Education, Inc. Figure 28.3 Reproductive shoot (flower) Apical bud Node Internode Apical bud Vegetative shoot Shoot system Blade Petiole Axillary bud Leaf Stem Taproot Lateral (branch) roots © 2014 Pearson Education, Inc. Root system Roots rely on sugar produced by photosynthesis in the shoot system Shoots rely on water and minerals absorbed by the root system © 2014 Pearson Education, Inc. Roots A root is an organ with important functions Anchoring the plant Absorbing minerals and water Storing carbohydrates © 2014 Pearson Education, Inc. Tall, erect plants with large shoot masses have a taproot system, which consists of A taproot, the main vertical root Lateral roots branching off the taproot Small or trailing plants have a fibrous root system, which consists of Adventitious roots that arise from stems Lateral roots that arise from the adventitious roots and form their own lateral roots © 2014 Pearson Education, Inc. In most plants, absorption of water and minerals occurs through the root hairs, thin extensions of epidermal cells that increase surface area © 2014 Pearson Education, Inc. Figure 28.4 © 2014 Pearson Education, Inc. Most terrestrial plants form mycorrhizal associations, which increase mineral absorption Many plants have root adaptations with specialized functions © 2014 Pearson Education, Inc. Figure 28.5 Storage roots Pneumatophores “Strangling” aerial roots © 2014 Pearson Education, Inc. Figure 28.5a Pneumatophores © 2014 Pearson Education, Inc. Figure 28.5b Storage roots © 2014 Pearson Education, Inc. Figure 28.5c “Strangling” aerial roots © 2014 Pearson Education, Inc. Stems 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 © 2014 Pearson Education, Inc. An apical bud, or terminal bud, is located near the shoot tip and causes elongation of a young shoot An axillary bud is located in the upper angle formed by the leaf and the stem and has the potential to form a lateral branch, thorn, or flower Axillary buds are generally dormant © 2014 Pearson Education, Inc. Some plants have modified stems with additional functions © 2014 Pearson Education, Inc. Figure 28.6 Stolon Rhizome Root Rhizomes Stolons Tubers © 2014 Pearson Education, Inc. Figure 28.6a Rhizome Root Rhizomes © 2014 Pearson Education, Inc. Figure 28.6b Stolon Stolons © 2014 Pearson Education, Inc. Figure 28.6c Tubers © 2014 Pearson Education, Inc. Leaves The leaf is the main photosynthetic organ of most vascular plants Leaves have other functions including gas exchange, dissipation of heat, and defense Leaves generally consist of a flattened blade and a stalk called the petiole, which joins the leaf to the stem © 2014 Pearson Education, Inc. Monocots and eudicots differ in the arrangement of veins, the vascular tissue of leaves Most monocots have parallel veins Most eudicots have branching veins © 2014 Pearson Education, Inc. Some plant species have evolved modified leaves that serve various functions © 2014 Pearson Education, Inc. Figure 28.7 Spines Tendrils Storage leaves Stem Reproductive leaves © 2014 Pearson Education, Inc. Storage leaves Figure 28.7a Tendrils © 2014 Pearson Education, Inc. Figure 28.7b Spines © 2014 Pearson Education, Inc. Figure 28.7c Storage leaves Stem Storage leaves © 2014 Pearson Education, Inc. Figure 28.7d Reproductive leaves © 2014 Pearson Education, Inc. Dermal, Vascular, and Ground Tissue Systems 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 © 2014 Pearson Education, Inc. Figure 28.8 Dermal tissue Ground tissue © 2014 Pearson Education, Inc. Vascular tissue The dermal tissue system is the outer protective covering In nonwoody plants, the dermal tissue system consists of the epidermis, a layer of densely packed cells In leaves and most stems, a waxy coating called the cuticle helps prevent water loss from the epidermis © 2014 Pearson Education, Inc. 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 © 2014 Pearson Education, Inc. The vascular tissue system facilitates transport of materials through the plant and provides support The two vascular tissues are xylem and phloem Xylem conducts water and dissolved minerals upward from roots into the shoots Phloem transports organic nutrients from where they are made to where they are needed © 2014 Pearson Education, Inc. The vascular tissue of a root or stem is collectively called the stele In angiosperms, the root stele is a solid central vascular cylinder The stele of stems and leaves is divided into vascular bundles, strands of xylem and phloem © 2014 Pearson Education, Inc. 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 photosynthesis, short-distance transport, storage, or support © 2014 Pearson Education, Inc. Common Types of Plant Cells Plant cells have structural adaptations that make their specific functions possible The major types of plant cells are Parenchyma Collenchyma Sclerenchyma Water-conducting cells of the xylem Sugar-conducting cells of the phloem © 2014 Pearson Education, Inc. Animation: Tour of a Plant Cell Parenchyma cells Have thin and flexible primary walls Lack secondary walls Have a large central vacuole Perform the most metabolic functions Retain the ability to divide and differentiate © 2014 Pearson Education, Inc. Figure 28.9a Parenchyma cells with chloroplasts (in Elodea leaf) (LM) © 2014 Pearson Education, Inc. 60 m Collenchyma cells Are grouped in strands Help support young parts of the plant shoot Have thicker and uneven cell walls Lack secondary walls Provide flexible support without restraining growth © 2014 Pearson Education, Inc. Figure 28.9b Collenchyma cells (in Helianthus stem) (LM) © 2014 Pearson Education, Inc. 5 m Sclerenchyma cells are rigid due to thick secondary walls containing lignin, a strengthening polymer 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 grouped in strands © 2014 Pearson Education, Inc. Figure 28.9c 5 m Sclereid cells (in pear) (LM) 25 m Cell wall Fiber cells (cross section from ash tree) (LM) © 2014 Pearson Education, Inc. Figure 28.9ca 5 m Sclereid cells (in pear) (LM) Cell wall © 2014 Pearson Education, Inc. Figure 28.9cb 25 m Fiber cells (cross section from ash tree) (LM) © 2014 Pearson Education, Inc. Water-conducting cells of the xylem are dead at functional maturity There are two types of water-conducting cells Tracheids are long, thin cells with tapered ends that move water through pits Vessel elements align end to end to form long micropipes called vessels © 2014 Pearson Education, Inc. Tracheids occur in the xylem of all vascular plants Vessel elements are common to most angiosperms, a few gymnosperms, and a few seedless vascular plants © 2014 Pearson Education, Inc. Figure 28.9d Vessel Tracheids 100 m Pits Tracheids and vessels (colorized SEM) Perforation plate Vessel element Vessel elements, with perforated end walls © 2014 Pearson Education, Inc. Tracheids Figure 28.9da Vessel Tracheids 100 m Tracheids and vessels (colorized SEM) © 2014 Pearson Education, Inc. Sugar-conducting cells of the phloem are alive at functional maturity In seedless vascular plants and gymnosperms, sugars are transported through sieve cells © 2014 Pearson Education, Inc. In angiosperms, sugars are transported in sieve tubes, chains of cells called sieve-tube elements Sieve plates are the porous end walls that allow fluid to flow between cells along the sieve tube Sieve-tube elements lack organelles, but each has a companion cell whose nucleus and ribosomes serve both cells © 2014 Pearson Education, Inc. Figure 28.9e 3 m Sieve-tube elements: longitudinal view (LM) Sieve plate Sieve-tube element (left) Companion and companion cell: cells cross section (TEM) Sieve-tube elements Plasmodesma Sieve plate 30 m Nucleus of companion cell 15 m Sieve-tube elements: longitudinal view © 2014 Pearson Education, Inc. Sieve plate with pores (LM) Figure 28.9ea 3 m Sieve-tube element (left) and companion cell: cross section (TEM) © 2014 Pearson Education, Inc. Figure 28.9eb Sieve-tube elements: longitudinal view (LM) Sieve plate Companion cells Sieve-tube elements 30 m © 2014 Pearson Education, Inc. Figure 28.9ec Sieve plate 15 m Sieve plate with pores (LM) © 2014 Pearson Education, Inc. Concept 28.2: Meristems generate new cells for growth and control the developmental phases and life spans of plants A plant can grow throughout its life; this is called indeterminate growth Indeterminate growth is enabled by meristems, which are perpetually undifferentiated tissues Some plant organs cease to grow at a certain size; this is called determinate growth © 2014 Pearson Education, Inc. Different Meristems Produce Primary and Secondary Growth There are two main types of meristems Apical meristems Lateral meristems © 2014 Pearson Education, Inc. Figure 28.10 Primary growth in stems Epidermis Cortex Primary phloem Primary xylem Shoot tip (shoot apical meristem and young leaves) Pith Vascular cambium Cork cambium Axillary bud meristem Secondary growth in stems Lateral meristems Cork cambium Periderm Pith Cortex Primary phloem Root apical meristems © 2014 Pearson Education, Inc. Primary xylem Secondary xylem Secondary phloem Vascular cambium Figure 28.10a Shoot tip (shoot apical meristem and young leaves) Axillary bud meristem Root apical meristems © 2014 Pearson Education, Inc. Vascular cambium Cork cambium Lateral meristems Figure 28.10b Primary growth in stems Epidermis Cortex Primary phloem Primary xylem Pith © 2014 Pearson Education, Inc. Figure 28.10c Secondary growth in stems Cork cambium Periderm Pith Cortex Primary phloem Primary xylem Secondary xylem © 2014 Pearson Education, Inc. Secondary phloem Vascular cambium Apical meristems are located at the tips of roots and shoots and at the axillary buds of shoots Apical meristems elongate shoots and roots, a process called primary growth © 2014 Pearson Education, Inc. Lateral meristems add thickness to woody plants, a process called secondary growth There are two lateral meristems: the vascular cambium and the cork cambium The vascular cambium adds layers of vascular tissue called secondary xylem (wood) and secondary phloem The cork cambium replaces the epidermis with periderm, which is thicker and tougher © 2014 Pearson Education, Inc. In woody plants, primary growth extends the shoots and secondary growth increases the diameter of previously formed parts © 2014 Pearson Education, Inc. Figure 28.11 Apical bud Bud scale Axillary buds This year’s growth (one year old) Leaf scar Bud scar Last year’s growth (two years old) Node One-year-old branch formed Internode from axillary bud near shoot tip Leaf scar Stem Bud scar Growth of two years ago (three years old) © 2014 Pearson Education, Inc. Leaf scar Meristems give rise to Initials, also called stem cells, which remain in the meristem Derivatives, which become specialized in mature tissues © 2014 Pearson Education, Inc. Gene Expression and Control of Cell Differentiation Cells of a developing organism synthesize different proteins and diverge in structure and function even though they have a common genome Cellular differentiation depends on gene expression, but is determined by position Positional information is communicated through cell interactions © 2014 Pearson Education, Inc. Gene activation or inactivation depends on cell-tocell communication For example, Arabidopsis root epidermis forms root hairs or hairless cells depending on the number of cortical cells it is touching © 2014 Pearson Education, Inc. Figure 28.12 Cortical cells GLABRA-2 is expressed, and the cell remains hairless. 20 m GLABRA-2 is not expressed, and the cell will develop a root hair. The root cap cells will be sloughed off before root hairs emerge. © 2014 Pearson Education, Inc. Meristematic Control of the Transition to Flowering and the Life Spans of Plants Flower formation involves a transition from vegetative growth to reproductive growth It is triggered by a combination of environmental cues and internal signals Reproductive growth is determinate; the production of a flower stops primary growth of that shoot © 2014 Pearson Education, Inc. Flowering plants can be categorized based on the timing and completeness of the switch from vegetative to reproductive growth Annuals complete their life cycle in a year or less Biennials require two growing seasons Perennials live for many years © 2014 Pearson Education, Inc. Concept 28.3: Primary growth lengthens roots and shoots Primary growth arises from the apical meristems and produces parts of the root and shoot systems © 2014 Pearson Education, Inc. Primary Growth of Roots The root tip is covered by a root cap, which protects the apical meristem as the root pushes through soil Growth occurs just behind the root tip, in three zones of cells Zone of cell division Zone of elongation Zone of differentiation, or maturation © 2014 Pearson Education, Inc. Video: Root Time Lapse Figure 28.13 Cortex Vascular cylinder Epidermis Root hair Dermal Ground Vascular Zone of differentiation Zone of elongation Zone of cell division (including root apical meristem) Root cap © 2014 Pearson Education, Inc. Mitotic cells 100 m Figure 28.13a Mitotic cells 100 m © 2014 Pearson Education, Inc. The primary growth of roots produces the epidermis, ground tissue, and vascular tissue In angiosperm roots, the stele is a vascular cylinder In most eudicots, the xylem is starlike in appearance with phloem between the “arms” In many monocots, a core of parenchyma cells is surrounded by rings of xylem then phloem © 2014 Pearson Education, Inc. Figure 28.14 Epidermis Cortex Endodermis Vascular cylinder Pericycle Core of parenchyma cells 100 m (a) Root with xylem and phloem in the center (typical of eudicots) Xylem Phloem (b) Root with parenchyma in the center (typical of monocots) Endodermis Pericycle Xylem Phloem Dermal Ground Vascular 70 m © 2014 Pearson Education, Inc. 100 m Figure 28.14a Epidermis Cortex Endodermis Vascular cylinder Pericycle Xylem Phloem 100 m (a) Root with xylem and phloem in the center (typical of eudicots) Dermal Ground Vascular © 2014 Pearson Education, Inc. Figure 28.14b Epidermis Cortex Endodermis Vascular cylinder Pericycle Core of parenchyma cells Xylem Phloem 100 m Dermal Ground © 2014 Pearson Education, Inc. Vascular (b) Root with parenchyma in the center (typical of monocots) Figure 28.14c Endodermis Pericycle Xylem Phloem Dermal Ground Vascular 70 m © 2014 Pearson Education, Inc. The ground tissue, mostly parenchyma cells, fills the cortex, the region between the vascular cylinder and epidermis The innermost layer of the cortex is called the endodermis The endodermis regulates passage of substances from the soil into the vascular cylinder © 2014 Pearson Education, Inc. Lateral roots arise from within the pericycle, the outermost cell layer in the vascular cylinder © 2014 Pearson Education, Inc. Figure 28.15-1 Emerging lateral root 100 m Cortex Vascular cylinder © 2014 Pearson Education, Inc. Pericycle Figure 28.15-2 Emerging lateral root Epidermis 100 m Cortex Vascular cylinder © 2014 Pearson Education, Inc. Pericycle Lateral root Figure 28.15-3 Emerging lateral root Epidermis 100 m Cortex Vascular cylinder © 2014 Pearson Education, Inc. Pericycle Lateral root Figure 28.15a Emerging lateral root 100 m Cortex Vascular cylinder © 2014 Pearson Education, Inc. Pericycle Figure 28.15b Epidermis Lateral root © 2014 Pearson Education, Inc. Figure 28.15c Epidermis Lateral root © 2014 Pearson Education, Inc. Primary Growth of Shoots A shoot apical meristem is a dome-shaped mass of dividing cells at the shoot tip Leaves develop from leaf primordia, projections along the sides of the apical meristem Shoot elongation results from lengthening of internode cells below the shoot tip © 2014 Pearson Education, Inc. Figure 28.16 Shoot apical meristem Leaf primordia Young leaf Developing vascular strand Axillary bud meristems 0.25 mm © 2014 Pearson Education, Inc. Axillary buds are dormant due to apical dominance, inhibition from an active apical bud When released from dormancy, axillary buds give rise to lateral shoots (branches) © 2014 Pearson Education, Inc. Tissue Organization of Leaves The epidermis in leaves is interrupted by stomata, which allow CO2 and O2 exchange between the air and the photosynthetic cells in a leaf Each stomatal pore is flanked by two guard cells, which regulate its opening and closing The ground tissue in a leaf, called mesophyll, is composed of parenchyma cells sandwiched between the upper and lower epidermis © 2014 Pearson Education, Inc. Figure 28.17 Guard cells Dermal Ground Vascular Cuticle Sclerenchyma fibers Stoma 50 m Stomatal pore Epidermal cell (b) Surface view of a spiderwort (Tradescantia) leaf (LM) Upper epidermis Palisade mesophyll Bundlesheath cell 100 m Spongy mesophyll Lower epidermis Cuticle Xylem Phloem (a) Cutaway drawing of leaf tissues © 2014 Pearson Education, Inc. Vein Guard cells Vein Air spaces Guard cells (c) Cross section of a lilac (Syringa) leaf (LM) Figure 28.17a Dermal Ground Vascular Cuticle Sclerenchyma fibers Stoma Upper epidermis Palisade mesophyll Spongy mesophyll Bundlesheath cell Lower epidermis Cuticle Xylem Phloem (a) Cutaway drawing of leaf tissues © 2014 Pearson Education, Inc. Vein Guard cells Figure 28.17b Guard cells 50 m Stomatal pore Epidermal cell Dermal © 2014 Pearson Education, Inc. (b) Surface view of a spiderwort (Tradescantia) leaf (LM) Figure 28.17c Upper epidermis Palisade mesophyll 100 m Spongy mesophyll Lower epidermis Dermal Ground © 2014 Pearson Education, Inc. Vein Air spaces Guard cells (c) Cross section of a lilac (Syringa) leaf (LM) The mesophyll of eudicots has two layers The palisade mesophyll in the upper part of the leaf consists of elongated cells for photosynthesis The spongy mesophyll in the lower part of the leaf consists of loosely arranged cells for gas exchange © 2014 Pearson Education, Inc. The vascular tissue of each leaf is continuous with the vascular tissue of the stem Veins are the leaf’s vascular bundles, which function in fluid transport and provide structure to the leaf Each vein in a leaf is enclosed by a protective bundle sheath © 2014 Pearson Education, Inc. Tissue Organization of Stems Stems are covered in epidermis and contain vascular bundles that run the length of the stem Lateral shoots develop from axillary buds on the stem’s surface In most eudicots, the vascular tissue consists of vascular bundles arranged in a ring © 2014 Pearson Education, Inc. Figure 28.18 Sclerenchyma (fiber cells) Phloem Xylem Ground tissue connecting pith to cortex Pith Ground tissue Epidermis Dermal Cortex Epidermis Vascular bundle Vascular 1 mm (a) Cross section of stem with vascular bundles forming a ring (typical of eudicots) © 2014 Pearson Education, Inc. Ground Vascular bundles 1 mm (b) Cross section of stem with scattered vascular bundles (typical of monocots) Figure 28.18a Sclerenchyma (fiber cells) Phloem Xylem Ground tissue connecting pith to cortex Pith Dermal Cortex Epidermis Vascular bundle Vascular 1 mm (a) Cross section of stem with vascular bundles forming a ring (typical of eudicots) © 2014 Pearson Education, Inc. Ground Figure 28.18b Ground tissue Epidermis Dermal Ground Vascular Vascular bundles 1 mm (b) Cross section of stem with scattered vascular bundles (typical of monocots) © 2014 Pearson Education, Inc. In most monocot stems, the vascular bundles are scattered throughout the ground tissue, rather than forming a ring © 2014 Pearson Education, Inc. Concept 28.4: Secondary growth increases the diameter of stems and roots in woody plants Secondary growth is characteristic of gymnosperms and many eudicots, but not monocots Secondary growth occurs in stems and roots of woody plants but rarely in leaves The secondary plant body consists of the tissues produced by the vascular cambium and cork cambium © 2014 Pearson Education, Inc. In woody plants, primary growth and secondary growth occur simultaneously but in different locations © 2014 Pearson Education, Inc. Figure 28.19 (a) Primary and secondary growth in a two-year-old woody stem Pith Primary xylem Vascular cambium Primary phloem Cortex Epidermis Epidermis Cortex Primary phloem Vascular cambium Vascular ray Periderm Secondary Cork phloem cambium Vascular Cork cambium Bark Secondary Late wood xylem Early wood Primary xylem Pith Cork 1 mm Secondary xylem Secondary phloem First cork cambium Periderm (mainly cork cambia and cork) Vascular Growth ray ring 1.4 mm Secondary phloem Most recent cork cambium Secondary xylem © 2014 Pearson Education, Inc. Cork Layers of periderm Bark (b) Cross section of a three-yearold Tilia (linden) stem (LM) Figure 28.19a-1 (a) Primary and secondary growth in a two-year-old woody stem Epidermis Cortex Primary phloem Vascular cambium Primary xylem Pith Periderm (mainly cork cambia and cork) Secondary phloem Secondary xylem © 2014 Pearson Education, Inc. Pith Primary xylem Vascular cambium Primary phloem Cortex Epidermis Figure 28.19a-2 (a) Primary and secondary growth in a two-year-old woody stem Epidermis Cortex Primary phloem Vascular cambium Primary xylem Pith Pith Primary xylem Vascular cambium Primary phloem Cortex Epidermis Vascular ray Secondary xylem Secondary phloem First cork cambium Cork Periderm (mainly cork cambia and cork) Secondary phloem Secondary xylem © 2014 Pearson Education, Inc. Figure 28.19a-3 (a) Primary and secondary growth in a two-year-old woody stem Epidermis Cortex Primary phloem Vascular cambium Primary xylem Pith Pith Primary xylem Vascular cambium Primary phloem Cortex Epidermis Vascular ray Secondary xylem Secondary phloem First cork cambium Cork Periderm (mainly cork cambia and cork) Secondary phloem Secondary xylem © 2014 Pearson Education, Inc. Most recent cork cambium Cork Bark Layers of periderm Figure 28.19b Vascular cambium Secondary xylem Periderm Cork cambium Secondary phloem Bark Cork Late wood 1 mm Early wood Vascular ray 1.4 mm © 2014 Pearson Education, Inc. Growth ring (b) Cross section of a three-yearold Tilia (linden) stem (LM) The Vascular Cambium and Secondary Vascular Tissue The vascular cambium is a cylinder of meristematic cells one cell layer thick In cross section, the vascular cambium appears as a ring of initials (stem cells) The initials increase the vascular cambium’s circumference and add secondary xylem to the inside and secondary phloem to the outside © 2014 Pearson Education, Inc. Figure 28.20 Vascular cambium Growth Vascular cambium Secondary xylem After one year of growth © 2014 Pearson Education, Inc. Secondary phloem After two years of growth Elongated, parallel-oriented initials produce tracheids, vessel elements, fibers of xylem, sievetube elements, companion cells, axially oriented parenchyma, and fibers of the phloem Shorter, perpendicularly oriented initials produce vascular rays, radial files of parenchyma cells that connect secondary xylem and phloem © 2014 Pearson Education, Inc. Secondary xylem accumulates as wood and consists of tracheids, vessel elements (only in angiosperms), and fibers Early wood, formed in the spring, has thin cell walls to maximize water delivery Late wood, formed in late summer, has thick-walled cells and contributes more to stem support In temperate regions, the vascular cambium of perennials is inactive through the winter © 2014 Pearson Education, Inc. Tree rings are visible where late and early wood meet and can be used to estimate a tree’s age Dendrochronology is the analysis of tree ring growth patterns and can be used to study past climate change © 2014 Pearson Education, Inc. As a tree or woody shrub ages, the older layers of secondary xylem, the heartwood, no longer transport water and minerals The outer layers, known as sapwood, still transport materials through the xylem Only young secondary phloem transports sugar; older secondary phloem sloughs off and does not accumulate © 2014 Pearson Education, Inc. Figure 28.21 Growth ring Vascular ray Heartwood Secondary xylem Sapwood Vascular cambium Secondary phloem Bark Layers of periderm © 2014 Pearson Education, Inc. The Cork Cambium and the Production of Periderm The cork cambium gives rise to cork cells that accumulate to the exterior of the cork cambium Cork cells deposit waxy suberin in their walls and then die Periderm consists of the cork cambium and the cork cells it produces Periderm functions as a protective barrier, impermeable to water and gasses © 2014 Pearson Education, Inc. Bark consists of all the tissues external to the vascular cambium, including secondary phloem and periderm Lenticels in the periderm allow for gas exchange between living stem or root cells and the outside air © 2014 Pearson Education, Inc. Figure 28.UN01 © 2014 Pearson Education, Inc. Figure 28.UN02 Shoot tip (shoot apical meristem and young leaves) Axillary bud meristems Root apical meristems © 2014 Pearson Education, Inc. Vascular cambium Cork cambium Lateral meristems Figure 28.UN03 © 2014 Pearson Education, Inc.