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Chapter Menu 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 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. 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? 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. This photo shows a seedling emerging from the soil. 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. 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. 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.) • As the embryo develops, it is surrounded by a tissue called endosperm, which helps transfer nutrients from the mother plant to the developing embryo. Endosperm, a foodstorage 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. Endosperm, a foodstorage tissue, surrounds and nourishes the developing embryo. 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. 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.) • 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. 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. x125 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. 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. 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. 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. Seed germination 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. 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. 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. 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. 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 epidermis that covers the plant. 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.) • Phloem and xylem consist of vascular tissue. 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.) • 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. 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.) • 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. 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. 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. 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.) • 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. Plant Development 11.3 Primary and Secondary Growth (cont.) 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.) • 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. 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. 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. Plant Development 11.3 Primary and Secondary Growth (cont.) The newly formed cambium in a threeyear-old basswood sapling has begun to produce layers of cells that differentiate into xylem toward the inside and phloem toward the outside, x40. 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. 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.) • 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. Location of major meristems in a typical plant 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. 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. 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. 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. 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. 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. 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. The application of auxin induced this cutting to form roots. 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. 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. 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. 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.) • Treatment with gibberellins can cause some dwarf plants to grow to the height of normal varieties. 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.) • 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. 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. Control of Growth and Development 11.7 Growth Inhibiters: Abscisic Acid and Ethylene • Abscisic acid (C15H20O4) is a naturally occurring PGR that is synthesized in response to dry conditions. – Abscisic acid stimulates the closing of stomata, protecting plants against water loss. – Buds and seeds become dormant when abscisic acid accumulates in them. Control of Growth and Development 11.7 Growth Inhibiters: Abscisic Acid and Ethylene (cont.) • Ethylene (C2H4), a PGR that is a simple gas, promotes aging of tissues, such as the ripening of fruits. • Ethylene opposes many effects of auxins and cytokinins. Control of Growth and Development 11.7 Growth Inhibiters: Abscisic Acid and Ethylene (cont.) • Ethylene makes leaves, flowers, and fruits drop from an aging plant. The circled vertical band in the micrograph is the abscission layer, which forms at the attachment of a leaf or fruit to the stem before the leaf or fruit falls. The abscission layer seals off the vascular tissue connecting the organ to the stem. An increase in the ratio of ethylene to auxin in this tissue triggers the process. Control of Growth and Development 11.7 Growth Inhibiters: Abscisic Acid and Ethylene (cont.) • Many farmers pick delicate fruits, such as tomatoes, when they are green and less susceptible to damage. • The tomatoes are shipped in an atmosphere of carbon dioxide, which blocks the action of ethylene. • When the tomatoes arrive at their destination, they are treated with ethylene to speed their ripening so they can be sold. Some effects and interactions of PGRs in plant organs Plant Responses 11.8 Plant Movements and Growth Responses • Plant survival depends, in part, on movements and changes in growth in response to a stimulus. • Most plant movements are responses to changes in the environment. • When any part of the leaf of the sensitive plant, Mimosa pudica, is touched, the leaflets droop together suddenly as a result of changes at the cellular level. Plant Responses 11.8 Plant Movements and Growth Responses (cont.) Leaflets of a mimosa plant, Mimosa pudica, (a) droop when they are touched (b) as a result of a loss of turgor pressure. Plant Responses 11.8 Plant Movements and Growth Responses • Other plant movements are really changes in the type or direction of growth. (cont.) • Growth toward or away from a stimulus is called a tropism. • Tropisms result from differences in growth between parts of an organ. Plant Responses 11.8 Plant Movements and Growth Responses • Phototropism is the tendency of most plants to grow toward light. In these seedlings growing toward light, auxin transport away from light reduces growth on the lit side and promotes growth on the shaded side of each stem. (cont.) Plant Responses 11.8 Plant Movements and Growth Responses • Gravitropism is growth toward or away from Earth’s gravitational pull. (cont.) – Stems are negatively gravitropic, growing away from gravity. – Roots are positively gravitropic, growing toward gravity. Plant Responses 11.8 Plant Movements and Growth Responses (cont.) • Evidence indicates that root sensitivity to gravity occurs in the root cap where plastids filled with dense starch grains fall to the bottom of certain cells. • Contact between the plastids and plasma membranes signals the direction of gravity. Plant Responses 11.8 Plant Movements and Growth Responses (cont.) Detection of gravity in root cap cells of thale cress (Arabidopsis thaliana) is shown here. When the root is vertical (a), starch-containing plastids (dark circles) reside at the bottom of the cells. When the root is turned on its side (b), gravity makes the plastids settle quickly to the new bottom of the cells. Plant Responses 11.9 Photoperiodism • Photoperiodism is a response to the relative length of light and darkness in a 24-hour period. – Long-day plants, or spring flowering plants, bloom only when day length exceeds a certain number of hours. – Short-day plants, or fall-flowering plants, reproduce only when day length is shorter than a certain number of hours. – Day-neutral plants flower whenever they become mature, regardless of the day length. Plant Responses 11.9 Photoperiodism (cont.) • In the 1940s, biologists learned that the length of the night, rather than the length of the day, controls photoperiodism. • Plants contain a pigment known as phytochrome that has two slightly different chemical structures. – (Pr) absorbs red light – (Pfr) absorbs far-red light (a wavelength in the farthest red part of the visible spectrum) Plant Responses 11.9 Photoperiodism (cont.) • Phytochrome is synthesized as Pr and remains in that form as long as the plant is in the dark. • In sunlight, which is richer in red light than in far-red light, the Pr absorbs red light and is converted to Pfr. • After sunset, Pfr gradually converts back to Pr. Plant Responses 11.9 Photoperiodism (cont.) Phytochrome occurs in two forms: Pr (red absorbing) and Pfr (far-red absorbing). Absorption of red light converts Pr to Pfr; absorption of far-red light converts Pfr to Pr. Plant Responses 11.9 Photoperiodism (cont.) • The conversion of phytochrome from one form to the other marks the beginning and end of the dark segment of the photoperiod. • This conversion also acts as a switch that controls many events, such as flowering, germination, and bolting. Summary • Plants grow when the number and size of their cells increase. • Development is the process by which the cells of a new organism form the differentiated tissues and organs of a complete individual. • A seed contains the embryo with its cotyledons and a reserve energy supply. • When conditions are appropriate, the seed germinates. The initial growth of the root tip and then the stem tip uses the energy reserves inside the seed. • Once the leaves emerge from the buds on the stem, the plant uses photosynthesis to harvest the energy it needs. • Growth occurs at specific areas in the plant called meristems. Summary (cont.) • Primary growth occurs in roots and stems at apical meristems; secondary growth occurs at the cambium and, in some plants, at the cork cambium. • Additional leaves and branches arise from the meristem tissue in the buds. • Five major groups of plant growth regulators (PGRs) have been identified—auxins, gibberellins, cytokinins, abscisic acid, and ethylene. • PGRs act directly on various enzymes and metabolic pathways and indirectly by influencing the activity of various genes that are involved in growth and development. • Protein kinases are an important part of the signaling pathways through which PGRs produce their effects. Summary (cont.) • PGRs interact in response to environmental cues, to affect plant growth and development. • Plants also exhibit photoperiodism, a response to relative length of the dark period that involves the pigment phytochrome. Reviewing Key Terms Match the term on the left with the correct description. ___ cytokinins c ___ germination f ___ b a. PGRs that promote growth by enlarging or lengthening cells b. embryonic plant tissue that is apical meristem responsible for primary growth ___ auxins a ___ cotyledon e ___ endosperm d c. PGRs that promotes growth through cell division d. tissue that provides nourishment to a developing embryo in seeds of flowering plants e. the single or double seed leaf of a flowering plant embryo f. the sprouting of a seed Reviewing Ideas 1. How can auxins be used as a growth stimulant and also as an herbicide? At extremely low concentrations, auxins promote elongation of roots. However, at higher concentrations, they inhibit elongation. A synthetic auxin called 2,4-D is a herbicide used to kill dicot weeds. At the concentrations generally used, 2,4-D provides a deadly overdose of auxin to dicots but does not affect the less-sensitive monocots, such as members of the grass family. Therefore, it can be used to control weeds such as dandelions in lawns and in grain fields. Reviewing Ideas 2. What causes a root to generally grow in a downward direction? Why? Roots are positively gravitropic, meaning that they grow toward gravity. In the root cap, plastids filled with dense starch grains fall to the bottom of certain cells. Contact between the plastids and plasma membranes signals the direction of gravity. Auxins appear to be involved, along with abscisic acid and perhaps other PGRs, in stimulating downward growth. Using Concepts 3. How does ethylene affect what you can buy in the produce section of a grocery store? Ethylene causes fruit to ripen. Taking advantage of this property of ethylene, many farmers pick delicate fruits when they are green and less susceptible to damage. The fruits are shipped in an atmosphere of carbon dioxide, which blocks the action of ethylene. When the fruit arrives at its destination, it is treated with ethylene to speed ripening so that it can be sold. This allows fruits to be shipped long distances without spoiling or damage. Using Concepts 4. What determines how large a plant organ will grow? As plant cells mature, they add material to their walls. The thicker, stronger wall resists cell expansion, so growth slows down. The final size of a plant organ is the result of a race between cell growth and cell-wall hardening. Synthesize 5. How could you create a year-round supply of flowers from a plant that naturally only flowers in the spring? Many spring-flowering plants, such as daffodils, bloom only when day length exceeds a certain number of hours. By increasing the amount of light that the plant receives, you could “force” long-day plants such as daffodils to bloom year-round. 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Chapter Animations Seed germination Location of major meristems in a typical plant Some effects and interactions of PGRs in plant organs Seed germination Location of major meristems in a typical plant Some effects and interactions of PGRs in plant organs End of Custom Shows This slide is intentionally blank.