Chapter 35

1. Chapter35 Plant Structure, Growth, and Development Kristin Spitz, Amanda Munoz, Caity Graham, Michelle Lamberta, and Sam Noto

2. Plant Structure • The plant body has a hierarchy of organs, tissues, and cells. • tissue: an integrated group of cells with a common structure, function, or both • organ: a specialized center of body function composed of several different types of tissues

3. Plant Structure: Organs • Plants have three basic organs: roots, shoots, and leaves.

4. Plant Structure: Organs • Root System: all of a plant′s roots that anchor it in the soil, absorb and transport minerals and water, and store food • Roots anchor vascular plants to the ground, absorb minerals and nutrients, and store organic nutrients. • Some plants have a taproot system that consists of one main vertical root that has many smaller lateral root branching off from it. • Seedless vascular plants and most monocots have a fibrous root system: a mat of generally thin roots spread out below the surface of the soil, with no root standing out as the main one.

5. Plant Structure: Organs • Absorption of water and minerals is greatly enhanced by root hairs: tiny extensions of a root epidermal cell, growing just behind the root tip and increasing surface area for absorption of water and minerals • Environmental conditions may result in roots being modified for a variety of functions. Many modified roots are aerial roots that are above the ground during normal development.

6. Plant Structure: Organs • Stem: a vascular plant organ consisting of an alternating system of nodes and internodes that support the leaves and reproductive structures • Node: a point along the stem of a plant at which leaves are attached • Internode:a segment of a plant stem between the points where leaves are attached

7. Plant Structure: Organs • Axiallary bud: a structure that has the potential to form a lateral shoot, or branch. The bud appears in the angle formed between a leaf and a stem • Terminal bud: embryonic tissue at the tip of a shoot, made up of developing leaves and a compact series of nodes and internodes • Apical dominance: concentration of growth at the tip of a plant shoot, where a terminal bud partially inhibits axillary bud growth • Modified stems, including stolons, rhizomes, tubers, and bulbs, have evolved in many plants as environmental adaptations.

8. Plant Structure: Organs • The leaf is the main photosynthetic organ of plants. • Leaves consist of a blade and a stalk. • Blade: the flattened portion of a typical leaf • Stalk or Petiole: joins the leaf to a node of the stem

9. Plant Structure: Organs • Vein: a vascular bundle in a leaf • Monocots have parallel major veins that run the length of the leaf blade. • Eudicots have a multibranched network of major veins. • Some modified leaves have evolved to function in support, protection, storage, and reproduction, not only in photosynthesis.

10. Plant Structure: Tissues

11. Plant Structure: Tissues • Plants have three main tissue systems: dermal, vascular, and ground • Tissue System: one or more tissues organized into a functional unit connecting the organs of a plant

12. Plant Structure: Tissues • Dermal Tissue System: the outer protective covering of plants • Epidermis: the dermal tissue system of nonwoody plants, usually consisting of a single layer of tightly packed cells • Periderm: the protective coat that replaces the epidermis in plants during secondary growth, formed of the cork and cork cambium • Cuticle: a waxy covering on the surface of stems and leaves that acts as an adaptation to prevent desiccation in terrestrial plants

13. Plant Structure: Tissues • Vascular Tissue System: a system formed by xylem and phloem throughout a vascular plant, serving as a transport system for water and nutrients, respectively • Xylem: vascular plant tissue consisting mainly of tubular dead cells that conduct most of the water and minerals upward from roots to the rest of the plant • Phloem: vascular plant tissue consisting of living cells arranged into elongated tubes that transport sugar and other organic nutrients throughout the plant

14. Plant Structure: Tissues • The vascular tissue of a root or stem is collectively called the stele. • The stele of the root is in the form of a vascular cylinder: the central cylinder of the vascular tissue in a root. • The stele of stems and leaves is divided into vascular bundles: a strand of vascular tissues (both xylem and phloem) in a stem or leaf

15. Plant Structure: Tissues • Ground Tissue System: plant tissues that are neither vascular nor dermal, fulfilling a variety of functions, such as storage, photosynthesis, and support • Pith: ground tissue that is internal to the vascular tissue in a stem; in many monocot roots, parenchyma cells that form the central core of the vascular cylinder • Cortex: ground tissue that is between the vascular tissue and dermal tissue in a root or dicot stem

16. Plant Structure: Cells • Plants, like any multicellular organism, are characterized by cellular differentiation. • Parenchyma cells: a relatively unspecialized plant cell type that carries out most of the metabolism, synthesizes and stores organic products, and develops into a more differentiated cell type • Collenchyma cells: a flexible plant cell type that occurs in strands or cylinders that support young parts of the plant without restraining growth • Sclerenchyma cells: a rigid, supportive plant cell type usually lacking protoplasts and possessing thick secondary walls strengthened by lignin at maturity

17. Plant Structure: Cells • Plants have special water-conducting cells of the xylem. • Tracheid: a long, tapered water-conducting cell that is dead at maturity and is found in the xylem of all vascular plants • Vessel Element: a short, wide, water-conducting cell found in the xylem of most angiosperms and a few nonflowering vascular plants. Dead at maturity, vessel elements are aligned end to end to form micropipes called vessels

18. Plant Structure: Cells • Plants also have special sugar-conducting cells of the phloem. • Sieve-Tube Member: a living cell that conducts sugars and other organic nutrients in the phloem of angiosperms. They form chains called sieve tubes • Sieve Plate: an end wall in a sieve-tube member, which facilitates the flow of phloem sap in angiosperm sieve tubes • Companion Cell: a type of plant cell that is connected to a sieve-tube member by many plasmodesmata and whose nucleus and ribosomes may serve one or more adjacent sieve-tube members

19. Plant Growth • Plants continue to grow throughout their whole life. This condition is known as indeterminate growth. At any given time, a typical plant consists of embryonic, developing, and mature organs. • The length of a plant’s life cycle classifies it as an annual, biennial, or perennial. • Annual: a flowering plant that completes its entire life cycle in a single year or growing season • Biennial: a flowering plant that requires two years to complete its life cycle • Perennial: a flowering plant that lives for many years

20. Plant Growth • Plants are capable of indeterminate growth because they have meristems: plant tissue that remains embryonic as long as the plant lives

21. Plant Growth • Apical Meristem: embryonic plant tissue in the tips of roots and in the buds of shoots that supplies cells for the plant to grow in length • The process of growth in length due to apical meristems is called primary growth. Primary growth allows roots to extend throughout the soil and shoots to increase exposure to light and CO2 • Lateral Meristem: a meristem that thickens the roots and shoots of woody plants. The vascular cambium and cork cambium are lateral meristems • The process of growth in thickness due to lateral meristems is called secondary growth.

22. Plant Growth • The cells within meristems divide relatively frequently. Some of the cells remain in the meristem and produce more cells while others differentiate and are incorporated into tissues and organs. • Cells that remain within an apical meristem as sources of new cells are called initials. • New cells that are displaced from an apical meristem and continue to divide until the cells they produce become specialized are called derivatives.

23. Plant Growth • Primary growth produces the primary plant body: the tissues produced by apical meristems, which lengthen stems and roots. • Herbaceous plants: primary plant body is the entire plant • Woody plants: primary plant body is only the youngest parts

24. Plant Growth • The root tip is covered by a root cap: a cone of cells at the tip of a plant root that protects the apical meristem • The root cap protects the delicate apical meristem as the root pushes through the abrasive soil during primary growth. • Growth occurs just behind the root tip, in three zones of cells at successive stages of primary growth. Moving away from the root tip, they are the zones of cell division, elongation, and maturation.

25. Plant Growth • Zone of Cell Division: the zone of primary growth in roots consisting of the root apical meristem and its derivatives. New root cells are produced in this region. • Zone of Elongation: the zone of primary growth in roots where new cells elongate, sometimes up to ten times their original length • Cell elongation is mainly responsible for pushing the root tip farther into the soil • Zone of Maturation: the zone of primary growth in roots where cells complete their differentiation and become functionally mature

26. Plant Growth

27. Plant Growth • Primary growth produces the epidermis, ground tissue, and vascular tissue. • The ground tissue of roots, consisting mostly of parenchyma cells, fills the cortex, the region between the vascular cylinder and epidermis. • Endodermis: the innermost layer of the cortex in plant roots; a cylinder one cell thick that forms the boundary between the cortex and the vascular cylinder • Pericycle: the outermost layer of the vascular cylinder of a root, where lateral roots originate

28. Plant Growth • Leaf Primordia: finger-like projections along the flanks of a shoot apical meristem, from which leaves arise • The apical meristem of a shoot is located in the terminal bud, where it gives rise to a repetition of internodes and leaf–bearing nodes.

29. Plant Growth • The epidermis covers stems as part of the continuous dermal tissue system. Vascular tissue runs the length of a stem in vascular bundles.

30. Plant Growth • Stoma: a microscopic pore surrounded by guard cells in the epidermis of leaves and stems that allows gas exchange between the environment and the interior of the plant • Guard Cells: the two cells that flank the stomatal pore and regulate the opening and closing of the pore • Mesophyll: the ground tissue of a leaf, sandwiched between the upper and lower epidermis and specialized for photosynthesis • Palisade Mesophyll: one or more layers of elongated photosynthetic cells on the upper part of a leaf; also called palisade parenchyma • Spongy Mesophyll: loosely arranged photosynthetic cells located below the palisade mesophyll cells in a leaf

31. Plant Growth • The vascular tissue of each leaf is continuous with the vascular tissue of the stem. • Leaf Trace: a small vascular bundle that extends from the vascular tissue of the stem through the petiole and into a leaf • The vascular structure also functions as a skeleton that reinforces the shape of the leaf. • Bundle Sheath: a protective covering around a leaf vein, consisting of one or more cell layers, usually parenchyma

32. Plant Growth • The secondary plant body consists of the tissues produced by the vascular cambium and cork cambium, or secondary growth. • A secondary xylem is added by the vascular cambium. • A thick, tough covering that consists mainly of cork cells is produced by the cork cambium. • The vascular cambium increases in circumference and also lays down successive layers of secondary xylem to its interior and secondary phloem to its exterior. • The vascular cambium is developed from undifferentiated cells that regain the capacity to divide. • The meristematic bands combine to become a continuous cylinder of dividing cells. This eventually becomes a cylinder.

33. Plant Growth • The vascular cambium appears as a ring, with interspersed regions of cells called fusiform initials and ray initials. When these initials divide, they increase the circumference of the cambium itself and add secondary xylem to the inside of the cambium and secondary phloem to the outside. • Fusiform Initials: cells within the vascular cambrium that produce elongated cells such as tracheids, vessel elements, fibers, and sieve–tube members • Ray Initials: cells within the vascular cambrium that produce xylem and phloem rays, radial files that consist mostly of parenchyma cells

34. Plant Growth • As secondary growth continues over the years, layers of secondary xylem (wood) accumulate, consisting mainly of tracheids, vessel elements, and fibers. • Heartwood: older layers of secondary xylem, closer to the center of a stem or root, that no longer transport xylem sap • Heartwoods are closer to the center of the sten or root. • Sapwood: outer layers of secondary xylem that still transport xylem sap

35. Plant Growth • The cork cambium gives rise to the secondary plant body′s protective covering, or periderm, which consists of the cork cambium plus the layers of cork cells it produces (phelloderm and suberin). • Most of the periderm is impermeable to water and gases. • Lenticels: small raised areas that dot the periderm in the bark of stems and roots that enable gas exchange between living cells and the outside air

36. Plant Growth • Cells of the cork cambium do not continue to divide so the thickening of the stem or roots splits the first cork cambium, which loses its meristematic activity and differentiates into cork cells. A new cork cambium forms to the inside, resulting in another layer of periderm. Older layers of periderm are aloughed off, as is evident in the cracked, peeling barks of many tree trunks. • Bark: all tissues external to the vascular cambium, consisting mainly of the secondary phloem and layers of periderm

37. Plant Development • Morphogenesis: the development of body shape and organization • Three developmental processes act to transform the fertilized egg into a plant: growth, morphogenesis, and cellular differentiation

38. Plant Development • Modern molecular techniques are helping plant biologists explore how growth, morphogenesis, and cellular differentiation give rise to a plant. • Arabidopsis thaliana was the first plant to have its entire genome sequenced. • A quest has been launched to determine the function of every one of the plant’s genes. • Systems Biology: an approach to studying biology that aims to model the dynamic behavior of whole biological systems • A goal of systems biology is the establish a blueprint for how plants are built.

39. Plant Development • The plane (direction) and symmetry of cell division are immensely important in determining plant form. • Asymmetrical cell division is when one daughter cell receives more cytoplasm than the other during mitosis. • Plane is determined during interphase. • The plane in which a cell divides is determined during late interphase. The first sign of this spatial orientation is a rearrangement of the cytoskeleton. Microtubules in the cytoplasm become concentrated into a ring called the preprophase band: microtubules in the cortex (outer cytoplasm) of a cell that are concentrated into a ring

40. Plant Development • Animal cells grow mainly by synthesizing protein–rich cytoplasm, a metabolically expensive process. Growing plant cells also produce additional protein–rich material in their cytoplasm, but water uptake typically accounts for about 90% of expansion. • Plant cells rarely expand equally in all directions. Their greatest expansion is usually oriented along the plant′s main axis. • Mutants have abnormal microtubule arrangements.

41. Plant Development • Morphogenesis must occur for development to proceed properly. • Pattern Formation: the ordering of cells into specific three–dimensional structures, an essential part of shaping an organism and its individual parts during development. • Positional Information: signals to which genes regulating development respond, indicating a cell′s location relative to other cells in an embryonic structure.

42. Plant Development • One type of positional information is associated with polarity, the condition of having structural differences at opposite ends of an organism. • Morphogenesis in plants is often under the control of master regulatory genes called homeotic genes. • Homeotic genes regulate major events, such as the formation of an organ.

43. Plant Development • Cellular differentiation depends to a large extent on gene expression and positional information. • The challenge of understanding cellular differentiation is explaining how cells with matching genomes diverge into various cell types. • A cell′s position in a developing organ determines its pathway of differentiation.

44. Plant Development • Plants pass through phases, developing from a juvenile phase to an adult vegetative phase to an adult reproductive phase. In plants the phases occur within a single region, the shoot apical meristem. • Phase Change: a shift from one developmental phase to another. • During the transition from a juvenile phase to an adult phase, the most obvious morphological changes typically occur in leaf size and shape. • Any new leaves that develop on branches that emerge from axillary buds at juvenile nodes will also be juvenile. • Phase changes are examples of plasticity in plant development.

45. Plant Development • The transition from vegetative growth to flowering is associated with the switching-on of floral meristem identity genes. • Meristem Identity Gene: a plant gene that promotes the switch from vegetative growth to flowering. • The protein products of these genes are transcription factors that regulate the genes required for the conversion of the indeterminate vegetative meristems into determinate floral meristems.

46. Plant Development • Organ Identity Genes: regulate the characteristic floral pattern: -floral organs – stamen, carpal, sepal, and petal – develop into whorls based on position. • Organ identity genes, also called plant homeotic genes, code for transcription factors. • ABC Model: a model of flower formation identifying three classes of organ identity genes that direct formation of the four types of floral organs.

47. Plant Development • According to the ABC Model, A genes are switched on in the two outer whorls (sepals and petals); B genes are switched on in the two middle whorls (petals and stamens); and C genes are switched on in the two inner whorls (stamens and carpels). • Sepals arise from those parts of the floral meristems in which only A genes are active; petals arise where A and B genes are active; stamens where B and C genes are active; and carpels where only C genes are active. • The ABC model can account for the phenotypes of mutants lacking A, B, or C gene activity.