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Splash. Chapter Introduction Plant Development 11.1 The Embryo and the Seed 11.2 Seed Germination 11.3 Primary and Secondary Growth Control of Growth and Development 11.4 Factors Affecting Plant Growth 11.5 Auxins 11.6 Other Growth Stimulants: Gibberellins and Cytokinins

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  1. Splash

  2. Chapter Introduction Plant Development 11.1 The Embryo and the Seed 11.2 Seed Germination 11.3 Primary and Secondary Growth Control of Growth and Development 11.4 Factors Affecting Plant Growth 11.5 Auxins 11.6 Other Growth Stimulants: Gibberellins and Cytokinins 11.7 Growth Inhibitors: Abscisic Acid and Ethylene Plant Responses 11.8 Plant Movements and Growth Responses 11.9 Photoperiodism Chapter Highlights Chapter Animations Chapter Menu Contents

  3. Learning Outcomes By the end of this chapter you will be able to: A Describe the structures involved in seed germination. B Explain primary and secondary growth in plants. C Discuss the factors that affect plant germination and growth. D Discuss the actions of various plant hormones. E Describe how plants respond to light, gravity, and day length. Learning Outcomes

  4. This photo shows a seedling emerging from the soil. Plant Growth and Development • How would you design an experiment to learn if this seedling is responding to a source of light? • What other forces affect the growth of this plant? Chapter Introduction 1

  5. This photo shows a seedling emerging from the soil. Plant Growth and Development • Certain plant tissues develop and grow throughout the life of the plant. • Growth is an increase in size. • Development is the process by which the cells of a new organism become specialized to perform different functions such as photosynthesis, nutrient transport, and other necessary tasks. Chapter Introduction 2

  6. End of the Introduction

  7. Plant Development 11.1 The Embryo and the Seed • Sexual reproduction begins with fertilization in both plants and animals. • The embryo that develops into a new plant forms from the zygote. • Asexual reproduction also occurs in plants. 11.1 The Embryo and the Seed 1

  8. The embryo develops from the zygote and includes one or more cotyledons, a shoot tip, and a root tip, x125. Plant Development 11.1 The Embryo and the Seed (cont.) • About 95% of all plant species are flowering plants, while the rest are nonflowering seed plants. • In both kinds of plants, mitotic cell divisions of the zygote form a spherical mass of cells that develops into the embryo. 11.1 The Embryo and the Seed 2

  9. Endosperm, a food-storage tissue, surrounds and nourishes the developing embryo. x125 Plant Development 11.1 The Embryo and the Seed (cont.) • As the embryo develops, it is surrounded by a tissue called endosperm, which helps transfer nutrients from the mother plant to the developing embryo. 11.1 The Embryo and the Seed 3

  10. Endosperm, a food-storage tissue, surrounds and nourishes the developing embryo. x125 Plant Development 11.1 The Embryo and the Seed (cont.) • Differentiation begins as small bumps form on the developing embryo, which become the cotyledons, or seed leaves, of the embryo. 11.1 The Embryo and the Seed 4

  11. The core of densely stained cells in the center of the embryo is beginning to differentiate into future vascular tissue (xylem and phloem). x125 Plant Development 11.1 The Embryo and the Seed (cont.) • Cells in the embryo divide rapidly and begin to differentiate into specialized structures. • Cells between the cotyledons become the embryonic shoot, which will later produce the stem and leaves. • At the opposite end of the embryo, the embryonic root develops. 11.1 The Embryo and the Seed 5

  12. Plant Development 11.1 The Embryo and the Seed (cont.) • A zone of undifferentiated cells remains at the tip, or apex, of both shoot and root, even in mature plants, forming the apical meristems. • Meristem cells divide and produce new cells that differentiate into all the specialized tissues of a mature plant. 11.1 The Embryo and the Seed 6

  13. x125 Plant Development 11.1 The Embryo and the Seed (cont.) • Maternal flower tissues form a tough seed coat, enclosing the endosperm and the embryo. The embryo stops growing and remains dormant until the seed sprouts. 11.1 The Embryo and the Seed 7

  14. Plant Development 11.1 The Embryo and the Seed (cont.) • The cell walls of neighboring plant cells are connected. This means that plant cells must differentiate where they are formed. • The position of an embryonic cell determines its future development. 11.1 The Embryo and the Seed 8

  15. Plant Development 11.1 The Embryo and the Seed (cont.) • Organelles and molecules that were unevenly distributed in the cytoplasm of a plant cell are divided unequally among the embryonic cells as the zygote divides. • The resulting differences in the cytoplasm of the embryonic cells can signal the genes, helping determine how each cell will develop. 11.1 The Embryo and the Seed 9

  16. In this germinating wheat seedling, the endosperm and cotyledon are still in the seed coat. Typical monocots, wheat seedlings push their green, photosynthetic embryonic leaves out of the soil. Plant Development 11.2 Seed Germination • When the environment is suitable, germination,or sprouting of the seed, occurs. • During germination, the embryo resumes metabolism, growth, and development. 11.2 Seed Germination 1

  17. Seed germination 11.2 Seed Germination 2

  18. Plant Development 11.2 Seed Germination (cont.) • Intricate mechanisms have evolved that favor germination only when survival of the seedlings is most likely. • For example, some seeds are genetically programmed to remain dormant until they experience several weeks of cold followed by warmer temperatures. 11.2 Seed Germination 3

  19. Plant Development 11.3 Primary and Secondary Growth • During germination, the root and then the stem begin their primary growth—growth from the meristems present in the embryo. • Cell divisions in the apical meristems provide a steady supply of new cells which expand mostly in the direction of the root and stem. 11.3 Primary and Secondary Growth 1

  20. Plant Development 11.3 Primary and Secondary Growth (cont.) • Meristems at each node, or point at which a leaf emerges, also contribute cells to stem growth. • A tough tissue mass, called the root cap covers and protects the apical meristem as the root grows through the soil. 11.3 Primary and Secondary Growth 2

  21. Plant Development 11.3 Primary and Secondary Growth (cont.) In shoots, apical and nodal meristems (in yellow) provide new cells. Expansion of these cells elongates the internodes (stem segments between nodes). In roots, elongation of cells produced in the root apical meristem (in yellow) lengthens the root, pushing the root tip through the soil. 11.3 Primary and Secondary Growth 3

  22. Epidermal tissue in the upper surface of a lily (Clivia) leaf is shown, x100. Note the cuticle (stained red) on the surface. Plant Development 11.3 Primary and Secondary Growth (cont.) • Growth and development go hand in hand. New stem or root segment is completing its growth, and its cells are beginning to differentiate into three major tissue types. • Surface cells make up the protective epidermisthat covers the plant. 11.3 Primary and Secondary Growth 4

  23. Vascular tissue as it appears in a cross section of a bundle of xylem and phloem in the stem of sunflower, Helianthus annuus, is shown, x400. Plant Development 11.3 Primary and Secondary Growth (cont.) • Phloem and xylem consist of vascular tissue. 11.3 Primary and Secondary Growth 5

  24. Ground tissue in a developing root of a buttercup, Ranunculus, is shown, x90. Note the many plastids containing starch granules, which are stained purple. Plant Development 11.3 Primary and Secondary Growth (cont.) • The other tissues that fill up the plant body, giving it shape and internal support, are called ground tissue. • Ground tissues contribute to nutrient production and storage, mechanical support, or other functions. 11.3 Primary and Secondary Growth 6

  25. Plant Development 11.3 Primary and Secondary Growth (cont.) • Two important factors in plant development are the growth of the cell wall and the rate and orientation of cell division. • As cell becomes thicker and stronger with time, it resists cell expansion causing growth to slow down. • The final size of a plant organ is the result of a race between cell growth and cell-wall hardening. • During primary growth, most cell divisions in stems and roots are horizontal, producing vertical columns of cylindrical cells. 11.3 Primary and Secondary Growth 7

  26. Plant Development 11.3 Primary and Secondary Growth (cont.) • A leaf begins to form as randomly oriented cell divisions produce a bump on the side of a shoot apex. • The cells in the center of each bump divide, producing small, fingerlike growths. 11.3 Primary and Secondary Growth 8

  27. New leaves are shown as they begin to form on a shoot tip of a sugar maple, Acer saccharum, x135. Repeated divisions perpendicular to the surface of each young leaf will be followed by horizontal cell expansion. Plant Development 11.3 Primary and Secondary Growth (cont.) • Then meristem cells on the sides of the bud begin to divide at right angles to the leaf surface. • The new cells expand, and the leaf becomes flatter because their cells divide only perpendicular to the surface. 11.3 Primary and Secondary Growth 9

  28. Plant Development 11.3 Primary and Secondary Growth (cont.) • Growth and development of leaves reflect both genetic and environmental influences. • Genetic factors strongly influence the shape of the leaf bud, the distribution and orientation of cell divisions, and the amount and distribution of cell enlargement. 11.3 Primary and Secondary Growth 10

  29. Uniform growth of ground tissue produces an elm (Ulmus rubra) leaf with a simple, rounded shape. Rapid growth of ground tissue near veins produces a lobed maple (Acer saccharum) leaf. Water lily (Nymphaea odorata) leaves growing in air are relatively compact. Submerged leaves are thick and spongy with additional internal air spaces, x100. Plant Development 11.3 Primary and Secondary Growth (cont.) 11.3 Primary and Secondary Growth 11

  30. Plant Development 11.3 Primary and Secondary Growth (cont.) • Complete development of the leaf requires exposure to light so that chlorophyll and other photosynthetic pigments can be synthesized. • The hormonelike plant-growth regulators also appear to play an important role in leaf development. 11.3 Primary and Secondary Growth 12

  31. Plant Development 11.3 Primary and Secondary Growth (cont.) • In some plants, secondary growth occurs as older parts of stems and roots that have completed primary growth continue to increase in diameter. • Secondary growth comes from the vascular cambium, another type of meristem. 11.3 Primary and Secondary Growth 13

  32. Plant Development 11.3 Primary and Secondary Growth (cont.) • The cambium is a cylindrical layer near the outer surface of roots and stems that produces cells that differentiate into two types of transport tissue. • The inner surface of the vascular cambium provides cells that differentiate into xylem. • Cells produced on the outer surface of the vascular cambium develop into phloem. 11.3 Primary and Secondary Growth 14

  33. Plant Development 11.3 Primary and Secondary Growth (cont.) The newly formed cambium in a three-year-old basswood sapling has begun to produce layers of cells that differentiate into xylem toward the inside and phloem toward the outside, x40. 11.3 Primary and Secondary Growth 15

  34. The nonfunctional xylem in the center of the Chimney Tree of California has burned out. The tree survives because the cambium, phloem, and water-carrying xylem in the outer part of the trunk are intact. Plant Development 11.3 Primary and Secondary Growth (cont.) • Trees and other woody plants develop a meristem called a cork cambium that produces their bark which protects the internal plant tissues. • The xylem cells at the center of a large tree no longer carry water, but their thick, tough cell walls help support the tree. 11.3 Primary and Secondary Growth 16

  35. Plant Development 11.3 Primary and Secondary Growth (cont.) • The apical meristems in buds on the sides of stems grow into branches, leaves, and flowers. • Root branches, or secondary roots, arise from the pericycle, a cylinder of meristem tissue that surrounds the xylem and phloem in the root. 11.3 Primary and Secondary Growth 17

  36. Location of major meristems in a typical plant 11.3 Primary and Secondary Growth 18

  37. Plant Development 11.3 Primary and Secondary Growth (cont.) • As most plants mature, they begin to produce reproductive structures such as flowers and, eventually, seeds and fruits. • The timing of flowering is strongly affected by environmental factors, such as the length of the night. 11.3 Primary and Secondary Growth 19

  38. Stages in plant-tissue differentiation. Each organ contains examples of all three types of tissues. Note the similarities in development between the tissues of the root and the shoot. 11.3 Primary and Secondary Growth 20

  39. End of Section 1

  40. Control of Growth and Development 11.4 Factors Affecting Plant Growth • Genes provide the primary control of when and how plant organs grow. • Factors that act as cues for expression of different genes at different times include temperature, night length, nutrition, chemical signals from other parts of the plant, and activities of neighboring cells. 11.4 Factors Affecting Plant Growth 1

  41. Interaction of factors affecting plant cell development Two important cell processes in plant development are the synthesis of cell wall components and their export from the cytoplasm via the Golgi apparatus. Another is the regulation of genes that affect the development of plastids into chloroplasts or more specialized types of plastids. 11.4 Factors Affecting Plant Growth 2

  42. Control of Growth and Development 11.4 Factors Affecting Plant Growth (cont.) • Many of the effects of these factors are signaled by substances called plant growth regulators (PGRs) that function somewhat like hormones in animals. • Botanists have identified five major classes of interacting PGRs that influence growth and development. • These compounds may cause different effects in different parts of the plant, at different times, or in different concentrations. 11.4 Factors Affecting Plant Growth 3

  43. Control of Growth and Development 11.5 Auxins • Auxins, the first PGRs to be identified, are produced in apical meristems and move through the plant by active transport. • Auxins can stimulate receptive cells in the growing regions of the plant to elongate, but the effects depend on a number of factors, especially their concentration. 11.5 Auxins 1

  44. The application of auxin induced this cutting to form roots. Control of Growth and Development 11.5 Auxins (cont.) • At extremely low concentrations, auxins promote elongation of roots. However at higher concentrations, they inhibit elongation. 11.5 Auxins 2

  45. Control of Growth and Development 11.5 Auxins (cont.) • Auxins also promote the development of fruits from flowers. • A synthetic auxin called 2,4-D is a herbicide used to kill dicot weeds. 11.5 Auxins 3

  46. Control of Growth and Development 11.6 Other Growth Stimulants: Gibberellins and Cytokinins • Discovered in the 1920s, compounds called gibberellins are synthesized in the apical parts of stems and roots stimulate stem elongation. • Germinating embryos produce gibberellins that stimulate the transcription of genes that encode digestive enzymes in endosperm. 11.6 Other Growth Stimulants: Gibberellins and Cytokinins 1

  47. The grapes on the left served as the control in this experiment; they were not treated with gibberellin. The grapes on the right were sprayed with a gibberellin solution early in their growth to increase their mature size. Control of Growth and Development 11.6 Other Growth Stimulants: Gibberellins and Cytokinins (cont.) • Plants treated with gibberellins may produce flowers that develop into seedless fruits. • Gibberellins also cause fruits to grow and may counter the effects of herbicides. 11.6 Other Growth Stimulants: Gibberellins and Cytokinins 2

  48. Each dwarf corn plant shown here was treated with the indicated dosage of gibberellin (GA3) and allowed to continue growing for 7 days. Note the increase in height of the plants with increased dosage. The plants treated with 10 or 100 µg GA3 have grown to the height of normal corn plants. Control of Growth and Development 11.6 Other Growth Stimulants: Gibberellins and Cytokinins (cont.) • Treatment with gibberellins can cause some dwarf plants to grow to the height of normal varieties. 11.6 Other Growth Stimulants: Gibberellins and Cytokinins 3

  49. Control of Growth and Development 11.6 Other Growth Stimulants: Gibberellins and Cytokinins (cont.) • The cytokinins are a third group of naturally occurring PGRs that promote cell division and organ development. • Cytokinins usually work in combination with auxins and other hormones to regulate the total growth pattern of the plant. 11.6 Other Growth Stimulants: Gibberellins and Cytokinins 4

  50. Control of Growth and Development 11.6 Other Growth Stimulants: Gibberellins and Cytokinins (cont.) • Cytokinins are produced mainly in the roots and then transported throughout the rest of the plant. • Cytokinins are necessary for stem and root growth, as well as chloroplast development. • Cytokinins stimulate the growth of lateral branches and inhibit the formation of lateral roots. 11.6 Other Growth Stimulants: Gibberellins and Cytokinins 5

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