Chapter 35

1. Chapter 35 Plant Structure, Growth, and Development Modified for IB HL by me!

2. Overview: Plants show a huge variety of structural forms • Plant physical structure can be highly variable • Even plants of the same species may not resemble each other • The plant species below exhibits a high degree of plasticity • The ability to alter itself in response to its environment

3. In addition to plasticity • Entire plant species have by natural selection accumulated characteristics of morphology that vary little among plants within the species

4. 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 composed of cells

5. The Three Basic Plant Organs: Roots, Stems, and Leaves • The basic morphology of vascular plants • Reflects their evolutionary history as terrestrial organisms that draw nutrients from two very different environments: below-ground and above-ground

6. Reproductive shoot (flower) Terminal bud Node Internode Terminal bud Shoot system Vegetative shoot Blade Petiole Leaf Axillary bud Stem Taproot Root system Lateral roots Figure 35.2 • Three basic organs evolved: roots, stems, and leaves • They are organized into a root system and a shoot system

7. Roots • A root • Is an organ that anchors the vascular plant • Absorbs minerals and water • Often stores organic nutrients

8. Figure 35.3 • In most plants • The absorption of water and minerals occurs near the root tips, where vast numbers of tiny root hairs increase the surface area of the root

9. (c) “Strangling” aerialroots (a) Prop roots (b) Storage roots Figure 35.4a–e (d) Buttress roots (e) Pneumatophores • Many plants have modified roots

10. 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

11. An axillary bud • Is a structure that has the potential to form a lateral shoot, or branch • A terminal bud • Is located near the shoot tip and causes elongation of a young shoot

12. (a) Stolons. Shown here on a strawberry plant, stolons are horizontal stems that grow along the surface. These “runners” enable a plant to reproduce asexually, as plantlets form at nodes along each runner. Storage leaves (d) Rhizomes. The edible base of this ginger plant is an example of a rhizome, a horizontal stem that grows just below the surface or emerges and grows along the surface. Stem Node Root (b) Bulbs. Bulbs are vertical, underground shoots consisting mostly of the enlarged bases of leaves that store food. You can see the many layers of modified leaves attached to the short stem by slicing an onion bulb lengthwise. Rhizome (c) Tubers. Tubers, such as these red potatoes, are enlarged ends of rhizomes specialized for storing food. The “eyes” arranged in a spiral pattern around a potato are clusters of axillary buds that mark the nodes. Root Figure 35.5a–d • Many plants have modified stems Stem tuber

13. Leaves • The leaf • Is the main photosynthetic organ of most vascular plants

14. Leaves generally consist of • A flattened blade and a stalk • The petiole, which joins the leaf to a node of the stem

15. (a) Simple leaf. A simple leafis a single, undivided blade.Some simple leaves are deeply lobed, as in anoak leaf. Petiole (b) Compound leaf. In acompound leaf, theblade consists of multiple leaflets.Notice that a leaflethas no axillary budat its base. Axillary bud Leaflet Petiole Axillary bud (c) Doubly compound leaf.In a doubly compound leaf, each leaflet is divided into smaller leaflets. Leaflet Petiole Figure 35.6a–c Axillary bud • In classifying angiosperms • Taxonomists may use leaf morphology as a criterion

16. (a) Tendrils. The tendrils by which thispea plant clings to a support are modified leaves. After it has “lassoed” a support, a tendril forms a coil that brings the plant closer to the support. Tendrils are typically modified leaves, but some tendrils are modified stems, as in grapevines. (b) Spines. The spines of cacti, such as this prickly pear, are actually leaves, and photosynthesis is carried out mainly by the fleshy green stems. (c) Storage leaves. Most succulents, such as this ice plant, have leaves modified for storing water. (d) Bracts. Red parts of the poinsettia are often mistaken for petals but are actually modified leaves called bracts that surround a group of flowers. Such brightly colored leaves attract pollinators. (e) Reproductive leaves. The leaves of some succulents, such as Kalanchoe daigremontiana, produce adventitious plantlets, which fall off the leaf and take root in the soil. Figure 35.6a–e • Some plant species • Have evolved modified leaves that serve various functions

17. Dermal tissue Ground tissue Vascular tissue Figure 35.8 The Three Tissue Systems: Dermal, Vascular, and Ground • Each plant organ • Has dermal, vascular, and ground tissues

18. The dermal tissue system • Consists of the epidermis and periderm

19. The vascular tissue system • Carries out long-distance transport of materials between roots and shoots • Consists of two tissues, xylem and phloem

20. Xylem • Conveys water and dissolved minerals upward from roots into the shoots • Phloem • Transports organic nutrients from where they are made to where they are needed

21. Ground tissue • Includes various cells specialized for functions such as storage, photosynthesis, and support

22. Common Types of Plant Cells • Like any multicellular organism • A plant is characterized by cellular differentiation, the specialization of cells in structure and function

23. Some of the major types of plant cells include • Parenchyma • Collenchyma • Sclerenchyma • Water-conducting cells of the xylem • Sugar-conducting cells of the phloem

24. Parenchyma • Parenchyma is the most common and versatile ground tissue. • It forms, for example, the cortex and pith of stems, the cortex of roots, the mesophyll of leaves, the pulp of fruits, and the endosperm of seeds. • Parenchyma cells are living cells and may remain meristematic at maturity, meaning that they are capable of cell division. • They have thin but flexible cellulosecell walls, and are generally polygonal when close-packed, but approximately spherical when isolated from their neighbours. • They have large central vacuoles, which allows the cells to store and regulate ions, waste products and water.

25. Parenchyma Functions • Parenchyma cells have a variety of functions: • In leaves, they form the mesophyll and are responsible for photosynthesis and the exchange of gases, parenchyma cells in the mesophyll of leaves are a specialized type of chlorenchyma (parenchyma with chloroplasts). • Storage of starch, protein, fats and oils and water in roots, tubers (e.g. potato), seed endosperm (e.g. cereals) and cotyledons (seed leaves) • Secretion (e.g. cells lining the inside of resin ducts – produce and secrete resin into duct) • Wound repair and the potential for renewed meristematic activity • Other specialized functions such as aeration (aerenchyma) and support

26. Collenchyma • Collenchyma tissue is composed of elongated cells with unevenly thickened walls. • They provide structural support, particularly in growing shoots and leaves. • Collenchyma tissue composes, for example, the resilient strands in stalks of celery. • Its growth is strongly affected by mechanical stress upon the plant. • The walls of collenchyma in shaken plants (to mimic the effects of wind etc), may be 40%-100% thicker than those not shaken. • The wall is made up of cellulose and pectin.

27. Sclerenchyma • Sclerenchyma is a supporting tissue in plants. • Two groups of sclerenchyma cells exist: fibres and sclereids. Their walls consist of cellulose, hemicellulose and lignin. • Sclerenchyma cells are the principal supporting cells in plant tissues that have ceased elongation. • Sclerenchyma fibres are of great economical importance, since they constitute the source material for many fabrics (flax, hemp, jute, ramie). • Unlike the collenchyma, mature sclerenchyma is composed of dead cells with extremely thick cell walls (secondary walls) that make up to 90% of the whole cell volume. • The term "sclerenchyma" is derived from the Greek ("sklē-rós"), meaning "hard". • It is the hard, thick walls that make sclerenchyma cells important strengthening and supporting elements in plant parts that have ceased elongation.

28. SCLERENCHYMA CELLS PARENCHYMA CELLS COLLENCHYMA CELLS 5 m 80 m Cortical parenchyma cells Sclereid cellsin pear 25 m Cell wall Parenchyma cells 60 m Collenchyma cells Fiber cells Figure 35.9 • Parenchyma, collenchyma, and sclerenchyma cells

29. WATER-CONDUCTING CELLS OF THE XYLEM SUGAR-CONDUCTING CELLS OF THE PHLOEM Sieve-tube members: longitudinal view Tracheids 100 m Vessel Companion cell Pits Sieve-tube member Sieve plate Tracheids and vessels Nucleus Vessel element Vessel elements with partially perforated end walls 30 m 15 m Tracheids Companion cell Cytoplasm Figure. 35.9 • Water-conducting cells of the xylem and sugar-conducting cells of the phloem

30. Monocots and dicots • Differ in the arrangement of veins, the vascular tissue of leaves • Summary of Monocot/Dicot differences • http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPLANTANATII.html • Most monocots • Have parallel veins • Most dicots • Have branching veins

31. Concept 35.2: Meristems are regions where cells are actively dividing (growth areas) • Apical meristems • Are located at the tips of roots and in the buds of shoots • Elongate shoots and roots through primary growth

32. Lateral meristems • Add thickness to woody plants through secondary growth • Also referred to as “cambium” • Ex: Vascular cambium or cork cambium

33. Primary growth in stems Shoot apical meristems (in buds) Epidermis Cortex In woody plants, there are lateral meristems that add secondary growth, increasing the girth of roots and stems. Primary phloem Primary xylem Vascular cambium Lateral meristems Pith Cork cambium Secondary growth in stems Apical meristems add primary growth, or growth in length. Periderm Cork cambium Pith The cork cambium adds secondary dermal tissue. Primary xylem Cortex Primary phloem Root apical meristems Secondary xylem The vascular cambium adds secondary xylem and phloem. Secondary phloem Vascular cambium Figure. 35.10 • An overview of primary and secondary growth

34. 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

35. Cortex Vascular cylinder Epidermis Key Zone of maturation Root hair Dermal Ground Vascular Zone of elongation Apical meristem Zone of cell division Root cap Figure 35.12 100 m Primary Growth of Roots • The root tip is covered by a root cap, which protects the delicate apical meristem as the root pushes through soil during primary growth

36. The primary growth of roots • Produces the epidermis, ground tissue, and vascular tissue

37. Epidermis Cortex Vascular cylinder Endodermis Pericycle Core of parenchyma cells Xylem Phloem 100 m 100 m (a) 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. (b) 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 Figure 35.13a, b 50 m • Organization of primary tissues in young roots Dicot Monocot

38. Apical meristem Leaf primordia Developing vascular strand Axillary bud meristems Figure. 35.15 0.25 mm Primary Growth of Shoots • A shoot apical meristem • Is a dome-shaped mass of dividing cells at the tip of the terminal bud • Gives rise to a repetition of internodes and leaf-bearing nodes

39. Phloem Xylem Sclerenchyma(fiber cells) Ground tissueconnecting pith to cortex Pith Key Cortex Epidermis Dermal Vascularbundle Ground Vascular 1 mm Figure 35.16a (a) A eudicot stem. A eudicot stem (sunflower), withvascular bundles forming a ring. Ground tissue towardthe inside is called pith, and ground tissue toward theoutside is called cortex. (LM of transverse section) Tissue Organization of Stems • In most dicots • The vascular tissue consists of vascular bundles arranged in a ring

40. Groundtissue Epidermis Vascularbundles 1 mm Figure 35.16b (b) A monocot stem. A monocot stem (maize) with vascularbundles scattered throughout the ground tissue. In such anarrangement, ground tissue is not partitioned into pith andcortex. (LM of transverse section) • In most monocot stems • The vascular bundles are scattered throughout the ground tissue, rather than forming a ring

41. Tissue Organization of Leaves • The epidermal barrier in leaves • Is interrupted by stomata, which allow CO2 exchange between the surrounding air and the photosynthetic cells within 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

42. Guard cells Key to labels Dermal Stomatal pore Ground Vascular Epidermal cell Sclerenchyma fibers 50 µm Cuticle (b) 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 Guard cells Phloem Figure 35.17a–c 100 µm Transverse section of a lilac (Syringa) leaf (LM) (c) (a) Cutaway drawing of leaf tissues • Leaf anatomy

43. 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

44. The Vascular Cambium and Secondary Vascular Tissue • The vascular cambium • Is a cylinder of meristematic cells one cell thick • Develops from parenchyma cells

45. Secondary phloem Cork cambium Vascular cambium Late wood Secondary xylem Periderm Early wood Cork (b) Transverse sectionof a three-year-old stem (LM) Xylem ray Bark 0.5 mm 0.5 mm Figure 35.18b

46. 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

47. Growth ring Vascular ray Heartwood Secondary xylem Sapwood Vascular cambium Secondary phloem Bark Layers of periderm Figure 35.20

48. Cork Cambia and the Production of Periderm • The cork cambium • Gives rise to the secondary plant body’s protective covering, or periderm

49. 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