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Unit 1:
Ch. 1
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Interlude A
Unit 2:
Ch. 4
Ch. 5
Ch. 6
Ch. 7
Ch. 8
Ch. 9
Interlude B
Unit 3:
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Interlude C
Unit 4:
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Interlude D
Unit 5:
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Ch. 23
Ch. 24
Ch. 25
Ch. 26
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Ch. 30
Interlude E
Unit 6:
Ch. 31
Ch. 32
Interlude F
Unit 7:
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Ch. 35
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Ch. 38
Interlude G

» Getting Started » A Guide to the Reading » Tying it all together

Getting Started

Below are a few questions to consider prior to reading Chapter 32. These questions will help guide your exploration and assist you in identifying some of the key concepts presented in this chapter.

  1. How does the pattern of growth seen in plants differ from that seen in most animals?
  2. What distinguishes primary growth from secondary growth?
  3. What are meristems?
  4. What do the terms wood, heartwood, and sapwood refer to?
  5. Why do some plants expend the energy needed to produce flowers?
  6. Where in the plant, and under what circumstances, does meiosis occur?
  7. How have plants been able to form cooperative relationships with birds and insects?
  8. How does a plant coordinate the pattern of its growth?
  9. How do plants defend themselves against herbivores and pathogens?

A Guide to the Reading

When exploring the content in Chapter 32 for the first time, the following concepts typically give students the most difficulty. For each concept, one or more references have been identified which may help you gain a better understanding of these potentially problematic areas.

How Plants Grow: Modularity

Other than beginning life as a fertilized egg, plants grow in ways that are completely different from the growth seen in animals. Plants lack the fixed developmental plan of animals; their growth is described as indeterminate, indicating that the exact shape of a mature plant is unpredictable. Other differences also exist; plants grow throughout their lives and add new body parts as needed. In Chapter 31 you learned that plants consist of a belowground root system and aboveground shoot system. Plant growth is essentially the repeated addition of shoots and the repeated formation of new lateral roots. This pattern allows the plant to control where growth occurs. Throughout the plant are clusters of undifferentiated cells called meristems. Cell division within a meristem produces new cells, which then differentiate to become the new shoot or root elements. This organization allows the plant to respond to events like grazing or favorable environmental opportunities, such as the death of a neighboring plant and new access to sunlight.

For more information on this concept, be sure to focus on:

  • Section 32.1, How Plants Grow: Modularity
  • Figure 32.1, How Plants Grow

How Plants Grow: Primary and Secondary Growth

Long-lived plants, like trees, typically become taller with time. The increase in length of a plant is called primary growth. It occurs when cells in an apical meristem at the tip of a shoot or root are stimulated to divide and grow.  The majority of new cells become one of the primary plant tissues. Other cells remain undifferentiated and become new apical meristems, either at the tip of the new shoot or root, preserving the potential for future increases in length, or within dormant buds, preserving the potential for the growth of branches. As plants grow, they also increase in thickness. This outward growth occurs when a second type of meristem tissue, a lateral meristem, begins to divide. Two types of lateral meristems occur: vascular cambium, which produces new vascular tissue, and cork cambium, which produces cork, the plant’s protective outer layer. All growth associated with lateral meristem tissue is called secondary growth. In Chapter 31 you learned that a vascular bundle is formed from xylem, oriented toward the center of the stem, and a phloem, oriented toward the outside of the stem. Vascular cambium is what separates the two tissues. Division within the vascular cambium produces new meristem, new xylem, and new phloem. Called secondary xylem and phloem, these new tissues displace the primary xylem and phloem further from the vascular cambium. Thus, the oldest vascular tissue in a stem is either closest to the center (primary xylem) or closest to the bark (primary phloem). In trees the growth of secondary xylem can be substantial, producing what is commonly called wood. The material to the outside of the vascular cambium, called bark, is organized into two layers. The inner bark is the secondary phloem. The outer bark contains the cork cambium, the second type of lateral meristem tissue. The addition of new secondary xylem eventually fractures the existing outer bark as the trunk expands. New outer bark is added only to the outside of the cork cambium. Thus, the oldest region of the outer bark is that directly in contact with the outside environment.

For more information on this concept, be sure to focus on:

  • Section 32.2, How Plants Grow: Primary and Secondary Growth
  • Figure 32.3, Grows Thicker Over Time
  • Figure 32.4, The Insides of a Tree

Producing the Next Generation

The fundamental requirement of sexual reproduction is the transfer of the sperm of one individual to the egg of another. Lacking the mobility of animals, many plants depend on the environment to transfer sperm, a haphazard process at best. Angiosperms have overcome this limitation by using flowers, a structure entirely devoted to reproduction. Flowers both produce gametes and greatly improve the efficiency of sperm transfer. The central concept in understanding gamete production is the distinction between diploid (a species chromosome number) and haploid (half the species’ chromosome number). In sexual reproduction, haploid gametes fuse to produce a diploid zygote, a process that blends genetic material from two individuals and restores the chromosome number.  In animals, gametes are the only haploid cells. Plants, however, produce a multicellular haploid structure that in some species becomes an independent individual. In flowering plants the haploid stage is reduced to a small group of cells retained within the flower: male pollen grains or the female ovule. It’s within these structures that sperm and egg are produced.  Once a pollen grain and its two sperm cells are transferred to a stigma, a pathway to the ovule forms, allowing one sperm cell to fertilize the egg. The second sperm cell fuses with a cell of the ovule, producing a tissue called endosperm that will eventually nourish the developing embryo. Flowers also facilitate a highly efficient transfer of pollen by enticing animals to carry pollen grains from one flower to another. Termed pollinators, insects, birds, and bats play the primary roles in animal-assisted pollination. By providing a small amount of food at a certain time of the day or season, pollinators are encouraged to move between flowers of the same species, moving pollen directly from flower to flower. When fertilization occurs a single-celled diploid zygote is formed. With its first division the zygote becomes an embryo. Following a period of growth of both the embryo and endosperm, the outer portion of the ovule hardens to form the seed coat and the embryo becomes dormant.   The seed falls from the plant and is dispersed by wind or carried by an animal. Should environmental conditions become favorable the seed will germinate, a process that activates the embryo and initiates its growth as a seedling.

For more information on this concept, be sure to focus on:

  • Section 32.3, Producing the Next Generation: Flower Form and Function
  • Figure 32.6, From Generation to Generation; The Life Cycle of a Flowering Plant
  • Figure 32.7, Bribing Animals to Do the Work
  • Section 32.4, Producing the Next Generation: From Zygote to Seedling
  • Figure 32.9, From Zygote to Seedling

Controlling Plant Growth

One of the great challenges of a multicellular body plan is how to coordinate the actions of distant tissues and organs. Animals depend on both nervous and endocrine systems for this coordination. Plants lack the nervous system of animals, but do have hormones that control their growth, reproduction, and responses to the environment. One of the most thoroughly studied plant hormones is auxin. Produced in the apical meristem, auxin inhibits the growth of lateral buds while promoting elongation in the shoot tip. Clipping the shoot tip will temporarily remove the source of auxin, which will allow the lateral buds to grow.  A more compact, but bushier, plant will result. Plants also use hormones to coordinate their reproduction.  In the temperate regions of the Earth, plants must coordinate flowering with the seasons to assure that pollinators will be available or that seeds will be released when growth conditions are favorable. The most reliable indicator of seasonal change is day length, which plants appear to monitor with hormones whose production is sensitive in some way to light. 

For more information on this concept, be sure to focus on:

  • Section 32.6, Controlling Plant Growth and Reproduction: Hormones
  • Figure 32.11, Removing the Tip Lets the Branches Grow
  • Figure 32.12, Plants Can Detect Day Length

Plant Defense

Plants, like all forms of life, are biologically successful only if they reproduce. Since reproduction requires some level of maturity, plants must live long enough to reach reproductive age. Not surprisingly, they possess a variety of defenses. One key to understanding plant defense is recognizing that attacks on plants fall into two broad categories, attacks by herbivores and attacks by pathogens. Plants rely on several strategies to discourage herbivores. Physical defenses include modified body parts like thorns, thistles, and hairs. Less obvious, but often more effective, are toxic chemicals like nicotine that either kill herbivores outright, or make the plant unpalatable. Plants must also protect themselves from fungal, bacterial, and viral pathogens. This begins with a protective cuticle that limits entry. If a pathogen does enter the plant, it encounters specific and nonspecific immune responses. While not as sophisticated as an animal’s immune system, plants also inherit genes for specific resistance proteins. Each gene has many alleles, increasing the chances that the plant will be able to produce a protein that will bind to the pathogen.  Even if it lacks the specific gene for an attacker, the plant may be able to produce one or more non-specific chemicals to assist in the defense. These chemicals may have antimicrobial properties, limit the spread of the pathogen, prompt healthy tissues to prepare for attack, or alert the entire plant body to the presence of the pathogen.

For more information on this concept, be sure to focus on:

  • Section 32.7, How Plants Defend Themselves

Tying it all together

Several concepts presented in this chapter build upon concepts presented in previous chapters and are also revisited and discussed in greater detail in subsequent chapters, including:

How Plants Grow: Modularity

  • Chapter 3 – Section 3.4, The Plantae: Pioneers of Life on Land
  • Chapter 31 – Section 31.1, An Overview of the Plant Body

How Plants Grow: Primary and Secondary Growth

  • Chapter 31 – Section 31.2, The Three Plant Tissue Types

Producing the Next Generation

  • Chapter 3 – in Section 3.4, Angiosperms produced the world’s first flowers
  • Chapter 9 – Section 9.4, Meiosis: Halving The Chromosome Number
  • Chapter 29 – Section 29.1, Human Reproduction and Development: A Brief Overview
  • Chapter 35 – Section 35.1, Mutualisms

Controlling Plant Growth

  • Chapter 24 – Section 24.1, How Hormones Work

Plant Defense

  • Chapter 28 – in Section 28.4, The immune system depends on specialized cells, proteins, and the lymphatic system
  • Chapter 31 – Section 31.2, The Three Plant Tissue Types

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