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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

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A Describe the structures involved in seed germination.

Learning Outcomes

By the end of this chapter you will be able to:

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.


What other forces affect the growth of this plant?

Plant Growth and Development How would you design an

experiment to learn if this seedling is responding to a source of light?

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.


• Sexual reproduction begins with fertilization in both plants and animals.

Plant Development

11.1 The Embryo and the Seed

• The embryo that develops into a new plant forms from the zygote.

• Asexual reproduction also occurs in plants.


• About 95% of all plant species are flowering plants, while the rest are nonflowering seed plants.

11.1 The Embryo and the Seed (cont.)

• In both kinds of plants, mitotic cell divisions of the zygote form a spherical mass of cells that develops into the embryo.

Plant Development

The embryo develops from the zygote and includes one or more cotyledons, a shoot tip, and a root tip, x125.


• As the embryo develops, it is surrounded by a tissue called endosperm, which helps transfer nutrients from the mother plant to the developing embryo.


Plant Development

Endosperm, a food-storage tissue, surrounds and nourishes the developing embryo. x125


• Differentiation begins as small bumps form on the developing embryo, which become the cotyledons, or seed leaves, of the embryo.


Plant Development

Endosperm, a food-storage tissue, surrounds and nourishes the developing embryo. x125


• 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


• 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


• 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.


Plant Development

x125


• 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


• 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


• When the environment is suitable, germination, or sprouting of the seed, occurs.

11.2 Seed Germination

• During germination, the embryo resumes metabolism, growth, and development.

Plant 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


• 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 (cont.)

Plant Development


• During germination, the root and then the stem begin their primary growth—growth from the meristems present in the embryo.

11.3 Primary and Secondary Growth

• 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


• 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 (cont.)

Plant Development



Plant Development

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.


• 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.


Plant Development

Epidermal tissue in the upper surface of a lily (Clivia) leaf is shown, x100. Note the cuticle (stained red) on the surface.


• 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


• 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


• 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


• 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


• 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.


Plant Development

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.


• 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



Plant Development

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.


• 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


• 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


• 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



Plant Development

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.


• 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.


Plant Development

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.


• 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.


Plant Development


Location of major meristems in a typical plant


• 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.


Plant Development


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.


• Genes provide the primary control of when and how plant organs grow.


11.4 Factors Affecting Plant Growth

• 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 developmentTwo 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.


• 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 (cont.)



• Auxins, the first PGRs to be identified, are produced in apical meristems and move through the plant by active transport.

11.5 Auxins

• 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.



• At extremely low concentrations, auxins promote elongation of roots. However at higher concentrations, they inhibit elongation.

11.5 Auxins (cont.)


The application of auxin induced this cutting to form roots.


• 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 (cont.)



• Discovered in the 1920s, compounds called gibberellins are synthesized in the apical parts of stems and roots stimulate stem elongation.

11.6 Other Growth Stimulants: Gibberellins and Cytokinins

• Germinating embryos produce gibberellins that stimulate the transcription of genes that encode digestive enzymes in endosperm.



• Plants treated with gibberellins may produce flowers that develop into seedless fruits.

11.6 Other Growth Stimulants: Gibberellins and Cytokinins (cont.)

• 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.


• 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.


• 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.



• 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.



• Abscisic acid (C15H20O4) is a naturally occurring PGR that is synthesized in response to dry conditions.

11.7 Growth Inhibiters: Abscisic Acid and Ethylene

– Abscisic acid stimulates the closing of stomata, protecting plants against water loss.

– Buds and seeds become dormant when abscisic acid accumulates in them.



• Ethylene (C2H4), a PGR that is a simple gas, promotes aging of tissues, such as the ripening of fruits.

11.7 Growth Inhibiters: Abscisic Acid and Ethylene (cont.)

• Ethylene opposes many effects of auxins and cytokinins.



• 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.


• 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 survival depends, in part, on movements and changes in growth in response to a stimulus.

Plant Responses

11.8 Plant Movements and Growth Responses

• 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.


11.8 Plant Movements and Growth Responses (cont.)

Plant Responses

Leaflets of a mimosa plant, Mimosa pudica, (a) droop when they are touched (b) as a result of a loss of turgor pressure.


• Other plant movements are really changes in the type or direction of growth.

• Growth toward or away from a stimulus is called a tropism.


Plant Responses

• Tropisms result from differences in growth between parts of an organ.


• Phototropism is the tendency of most plants to grow toward light.


Plant Responses

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.


• Gravitropism is growth toward or away from Earth’s gravitational pull.

– Stems are negatively gravitropic, growing away from gravity.

– Roots are positively gravitropic, growing toward gravity.


Plant Responses


• 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


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


• Photoperiodism is a response to the relative length of light and darkness in a 24-hour period.

11.9 Photoperiodism

– 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


• 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.

11.9 Photoperiodism (cont.)

Plant Responses

– (Pr) absorbs red light

– (Pfr) absorbs far-red light (a wavelength in the farthest red part of the visible spectrum)


• 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



Plant Responses

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.


• 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.


Plant Responses


Summary

• 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.

• Plants grow when the number and size of their cells increase.


Summary (cont.)

• 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.

• Primary growth occurs in roots and stems at apical meristems; secondary growth occurs at the cambium and, in some plants, at the cork cambium.


Summary (cont.)

• Plants also exhibit photoperiodism, a response to relative length of the dark period that involves the pigment phytochrome.

• PGRs interact in response to environmental cues, to affect plant growth and development.


Reviewing Key TermsMatch the term on the left with the correct description.

___ cytokinins

___ germination

___ apical meristem

___ auxins

___ cotyledon

___ endosperm

a. PGRs that promote growth by enlarging or lengthening cells

b. embryonic plant tissue that is responsible for primary growth

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

c

f

b

a

e

d


Reviewing Ideas1. 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 Ideas2. 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 Concepts3. 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 Concepts4. 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.


Synthesize5. 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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