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Chapter 35

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  1. Chapter 35 Plant Structure, Growth, and Development

  2. Concept 35.1: The plant body has a hierarchy of organs, tissues, and cells • Plants, like multicellular animals, have organs composed of different tissues, which are in turn are composed of cells • The Three Basic Plant Organs: Roots, Stems, and Leaves

  3. Three basic organs evolved: roots, stems, and leaves • They are organized into a root system and a shoot system • They are organized into a root system and a shoot system

  4. LE 35-2 Reproductive shoot (flower) Terminal bud Node Internode Terminal bud Shoot system Vegetable shoot Blade Leaf Petiole Axillary bud Stem Taproot Lateral roots Root system

  5. Roots • Functions of roots: • Anchoring the plant • Absorbing minerals and water • Often storing organic nutrients • In most plants, absorption of water and minerals occurs near the root tips, where vast numbers of tiny root hairs increase the surface area

  6. Many plants have modified roots • Prop root storage root pneumatophores • cyprus, mangrove

  7. LE 35-5a modified stems Stolons for asexual reproduction. Bulbs for asexual reproduction & storage Tubers for storage Rhizomes for asexual reproduction

  8. Leaves leaf -photosynthetic organ of most vascular plants • Leaves generally consist of • flattened blade (high surface area) & stalk • petiole joins leaf to a node of the stem • Node –distance between axillary buds • Simple leaf—one blade/bud compound leaf—more than 1 • blade (leaflets) per bud

  9. Some leaves are modified for other functions: tendrils, spines, storage leaves, asexual reproductive leaves • Some plant species have evolved modified leaves that serve various functions • Tendrils for climbing on vines, spines, storage, reproduction • Note that stems of cacti are photosynthetic!

  10. The Three Tissue Systems: Dermal, Vascular, and Ground Analogous to these animal tissues in function & location: Epidermis dermal Endodermis (like endothelia of blood vessel linings) vascular Mesodermal (connective tissues for storage, support And photosynthesis) ground

  11. In nonwoody plants, the dermal tissue system consists of the epidermis • In woody plants, protective tissues called periderm (bark) replace the epidermis in older regions of stems and roots

  12. Plant Vascular tissue Xylemconveys water & dissolved minerals from roots to shoots • Tracheids -spindley & connected only by pits—in pterophyta & lycophyta (seedless vascular plants)—dead at maturity • Vessel elements—in angiosperms & gymnosperms (+ tracheids) containing perforations that connect them on sides & between separate cells—dead at maturity (lignin & cellulose cell walls remain for rigid sides) • Phloem transports organic nutrients from where they are made to where they are needed seive tube elements—no Nucleus, carry solutions, companion cells nucleated caretakers of ste --

  13. 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

  14. Common Types of Plant Cells • Like any multicellular organism, a plant is characterized by cellular differentiation, the specialization of cells in structure and function • Some major types of plant cells: • Parenchyma --photosynthesis • Collenchyma—store, cover, protect • Sclerenchyma--support • Water-conducting cells of the xylem (vascular) • Sugar-conducting cells of the phloem (vascular)

  15. LE 35-9 WATER-CONDUCTING CELLS OF THE XYLEM PARENCHYMA CELLS 100 µm Tracheids Vessel Parenchyma cells in Elodea leaf, with chloroplasts (LM) 60 µm Pits COLLENCHYMA CELLS Cortical parenchyma cells Tracheids and vessels (colorized SEM) 80 µm Vessel element Vessel elements with perforated end walls Tracheids SUGAR-CONDUCTING CELLS OF THE PHLOEM Collenchyma cells (in cortex of Sambucus, elderberry; cell walls stained red) (LM) Sieve-tube members: longitudinal view (LM) SCLERENCHYMA CELLS 5 µm Companion cell Sclereid cells in pear (LM) Sieve-tube member 25 µm Plasmodesma Sieve plate Cell wall Nucleus Cytoplasm Companion cell 30 µm 15 µm Fiber cells (transverse section from ash tree) (LM) Sieve-tube members: longitudinal view Sieve plate with pores (LM)

  16. Concept 35.2: Meristems generate cells for new organs • Apical meristems are located at the tips of roots and in the buds of shoots=terminal bud • elongate shoots and roots=primary growth

  17. Lateral meristems add thickness (diameter) 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 (bark), which is thicker and tougher

  18. LE 35-10 Primary growth in stems Shoot apical meristems (in buds) Epidermis Cortex Primary phloem Primary xylem Vascular cambium Lateral meristems Pith Cork cambium Secondary growth in stems Periderm Cork cambium Pith Cortex Primary xylem Primary phloem Secondary xylem Root apical meristems Secondary phloem Vascular cambium

  19. In woody plants, primary and secondary growth occur simultaneously but in different locations

  20. LE 35-11 Terminal bud Bud scale Axillary buds Leaf scar This year’s growth (one year old) Node Stem Internode One-year-old side branch formed from axillary bud near shoot apex Leaf scar Last year’s growth (two years old) Scars left by terminal bud scales of previous winters Growth of two years ago (three years old) Leaf scar

  21. Concept 35.3: Primary growth lengthens roots and shoots • Primary growth produces the primary plant body, the parts of the root and shoot systems produced by apical meristems

  22. 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 maturation Video: Root Growth in a Radish Seed (time lapse)

  23. LE 35-12 Vascular cylinder Cortex Epidermis Key Zone of maturation Root hair Dermal Ground Vascular Zone of elongation Apical meristem Zone of cell division Root cap 100 µm

  24. LE 35-13 Epidermis Cortex Vascular cylinder Endodermis Pericycle Core of parenchyma cells Xylem Phloem 100 µm 100 µm Transverse section of a typical root. In the roots of typical gymnosperms and eudicots, as well as some monocots, the stele is a vascular cylinder consisting of a lobed core of xylem with phloem between the lobes. Transverse section of a root with parenchyma in the center. The stele of many monocot roots is a vascular cylinder with a core of parenchyma surrounded by a ring of alternating xylem and phloem. Endodermis Key Dermal Pericycle Ground Vascular Xylem Phloem 50 µm

  25. Tissue Organization of Stems • In gymnosperms and most eudicots, the vascular tissue consists of vascular bundles that are arranged in a ring

  26. In most monocot stems, the vascular bundles are scattered throughout the ground tissue, rather than forming a ring

  27. LE 35-16 Phloem Xylem Ground tissue Sclerenchyma (fiber cells) Ground tissue connecting pith to cortex Pith Epidermis Key Vascular bundles Cortex Epidermis Dermal Vascular bundles Ground Vascular 1 mm 1 mm A monocot (maize) stem. Vascular bundles are scattered throughout the ground tissue. In such an arrangement, ground tissue is not partitioned into pith and cortex. (LM of transverse section) A eudicot (sunflower) stem. Vascular bundles form a ring. Ground tissue toward the inside is called pith, and ground tissue toward the outside is called cortex. (LM of transverse section)

  28. Tissue Organization of Leaves • The epidermis in leaves is interrupted by stomata, which allow CO2 exchange between the air and the photosynthetic cells in a leaf • The ground tissue in a leaf is sandwiched between the upper and lower epidermis • The vascular tissue of each leaf is continuous with the vascular tissue of the stem

  29. LE 35-17 Key to labels Guard cells Dermal Stomatal pore Ground Vascular Epidermal cells Sclerenchyma fibers 50 µm Cuticle Surface view of a spiderwort (Tradescantia) leaf (LM) Stoma Upper epidermis Palisade mesophyll Bundle- sheath cell Spongy mesophyll Lower epidermis Guard cells Cuticle Vein Xylem Vein Air spaces Guard cells Phloem Guard cells 100 µm Cutaway drawing of leaf tissues Transverse section of a lilac (Syringa) leaf (LM)

  30. Concept 35.4: Secondary growth adds girth to stems and roots in woody plants • 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

  31. LE 35-18a Primary and secondary growth in a two-year-old stem Epidermis Pith Cortex Primary xylem Primary phloem Vascular cambium Primary phloem Vascular cambium Cortex Epidermis Primary xylem Phloem ray Growth Pith Xylem ray Primary xylem Secondary xylem Vascular cambium Secondary phloem Primary phloem Cork First cork cambium Periderm (mainly cork cambia and cork) Growth Primary phloem Secondary phloem Secondary xylem (two years of production) Vascular cambium Secondary xylem Vascular cambium Secondary phloem Bark Primary xylem Most recent cork cambium Layers of periderm Pith Cork

  32. LE 35-18b Secondary phloem Vascular cambium Cork cambium Late wood Secondary xylem Periderm Early wood Cork Transverse section of a three-year- old Tilia (linden) stem (LM) Xylem ray Bark 0.5 mm 0.5 mm

  33. The Vascular Cambium and Secondary Vascular Tissue • The vascular cambium is a cylinder of meristematic cells one cell thick • It develops from undifferentiated cells and parenchyma cells that regain the capacity of divide

  34. In transverse section, the vascular cambium appears as a ring, with regions of dividing cells called fusiform initials and ray initials • The initials increase the vascular cambium’s circumference and add secondary xylem to the inside and secondary phloem to the outside

  35. LE 35-19 Vascular cambium Types of cell division Accumulation of secondary growth

  36. 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

  37. LE 35-20 Growth ring Vascular ray Heartwood Secondary xylem Sapwood Vascular cambium Secondary phloem Bark Layers of periderm

  38. Cork Cambia and the Production of Periderm • The cork cambium gives rise to the secondary plant body’s protective covering, or periderm • Periderm consists of the cork cambium plus the layers of cork cells it produces • Bark consists of all the tissues external to the vascular cambium, including secondary phloem and periderm

  39. Gene Expression and Control of Cellular Differentiation • Morphogenesis in plants, as in other multicellular organisms, is often controlled by homeotic genes • In cellular differentiation, cells of a developing organism synthesize different proteins and diverge in structure and function even though they have a common genome • Cellular differentiation to a large extent depends on positional information and is affected by homeotic genes

  40. Location and a Cell’s Developmental Fate • A cell’s position in a developing organ determines its pathway of differentiation

  41. LE 35-28 Cortical cells 20 µm