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16.4: Plant Adaptations and Responses

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Lesson Objectives

  • Explain how plants have adapted to a diversity of environments.
  • Identify types of plant responses to environmental stimuli.

Vocabulary

epiphyte
plant that is adapted to grow on other plants and obtain moisture from the air
tropism
turning by an organism or part of an organism toward or away from an environmental stimulus
xerophyte
plant that is adapted to a very dry environment

Introduction

Plants live just about everywhere on Earth. To live in so many different habitats, they have evolved adaptations that allow them to survive and reproduce under a diversity of conditions.

Plant Adaptations

All plants are adapted to live on land. Or are they? All living plants today have terrestrial ancestors, but some plants now live in the water. They have had to evolve new adaptations for their watery habitat.

Adaptations to Water

Aquatic plants are plants that live in water. Living in water has certain advantages for plants. One advantage is, well, the water. There’s plenty of it and it’s all around. Therefore, most aquatic plants do not need adaptations for absorbing, transporting, and conserving water. They can save energy and matter by not growing extensive root systems, vascular tissues, or thick cuticles on leaves. Support is also less of a problem because of the buoyancy of water. As a result, adaptations such as strong woody stems and deep anchoring roots are not necessary for most aquatic plants.

Living in water does present challenges to plants, however. For one thing, pollination by wind or animals isn’t feasible under water, so aquatic plants may have adaptations that help them keep their flowers above water. For instance, water lilies have bowl-shaped flowers and broad, flat leaves that float. This allows the lilies to collect the maximum amount of sunlight, which does not penetrate very deeply below the surface. Plants that live in moving water, such as streams and rivers, may have different adaptations. For example, cattails have narrow, strap-like leaves that reduce their resistance to the moving water (see Figure below).

Water lilies and cattails have different adaptations for life in the water. Compare the leaves of the two kinds of plants. How do the leaves help the plants adapt to their watery habitats?

Adaptations to Extreme Dryness

Plants that live in extremely dry environments have the opposite problem: how to get and keep water. Plants that are adapted to very dry environments are called xerophytes. Their adaptations may help them increase water intake, decrease water loss, or store water when it is available.

The saguaro cactus pictured in Figure below has adapted in all three ways. When it was still a very small plant, just a few inches high, its shallow roots already reached out as much as 2 meters (7 feet) from the base of the stem. By now, its root system is much more widespread. It allows the cactus to gather as much moisture as possible from rare rainfalls. The saguaro doesn’t have any leaves to lose water by transpiration. It also has a large, barrel-shaped stem that can store a lot of water. Thorns protect the stem from thirsty animals that might try to get at the water inside.

The saguaro cactus has many adaptations for extreme dryness. How does it store water?

Adaptations to Air

Plants called epiphytes grow on other plants. They obtain moisture from the air and make food by photosynthesis. Most epiphytes are ferns or orchids that live in tropical or temperate rainforests (see Figure below). Host trees provide support, allowing epiphyte plants to obtain air and sunlight high above the forest floor. Being elevated above the ground lets epiphytes get out of the shadows on the forest floor so they can get enough sunlight for photosynthesis. Being elevated may also reduce the risk of being eaten by herbivores and increase the chance of pollination by wind.

This elkhorn fern is growing on a rainforest tree as an epiphyte.

Epiphytes don’t grow in soil, so they may not have roots. However, they still need water for photosynthesis. Rainforests are humid, so the plants may be able to absorb the water they need from the air. However, many epiphytes have evolved modified leaves or other structures for collecting rainwater, fog, or dew. The leaves of the bromeliad shown in Figure below are rolled into funnel shapes to collect rainwater. The base of the leaves forms a tank that can hold more than 8 liters (2 gallons) of water. Some insects and amphibians may spend their whole life cycle in the pool of water in the tank, adding minerals to the water with their wastes. The tissues at the base of the leaf are absorbent, so they can take in both water and minerals from the tank.

The leaves of this bromeliad are specialized to collect, store, and absorb rainwater.

Plant Responses

Like all organisms, plants detect and respond to stimuli in their environment. Unlike animals, plants can’t run, fly, or swim toward food or away from danger. They are usually rooted to the soil. Instead, a plant’s primary means of response is to change how it is growing. Plants also don’t have a nervous system to control their responses. Instead, their responses are generally controlled by hormones, which are chemical messenger molecules.

Plant Tropisms

As you read earlier in this chapter, plant roots always grow downward because specialized cells in root caps detect and respond to gravity. This is an example of a tropism. A tropism is a turning toward or away from a stimulus in the environment. Growing toward gravity is called geotropism. Plants also exhibit phototropism, or growing toward a light source. This response is controlled by a plant growth hormone called auxin. As shown in Figure below, auxin stimulates cells on the dark side of a plant to grow longer. This causes the plant to bend toward the light.

Phototropism is controlled by the growth hormone auxin.

Daily and Seasonal Responses

Plants also detect and respond to the daily cycle of light and darkness. For example, some plants open their leaves during the day to collect sunlight and then close their leaves at night to prevent water loss. Environmental stimuli that indicate changing seasons trigger other responses. Many plants respond to the days growing shorter in the fall by going dormant. They suspend growth and development in order to survive the extreme cold and dryness of winter. Dormancy ensures that seeds will germinate and plants will grow only when conditions are favorable.

Responses to Disease

Plants don’t have immune systems, but they do respond to disease. Typically, their first line of defense is the death of cells surrounding infected tissue. This prevents the infection from spreading. Many plants also produce hormones and toxins to fight pathogens. For example, willow trees produce salicylic acid to kill bacteria. The same compound is used in many acne products for the same reason. Exciting new research suggests that plants may even produce chemicals that warn other plants of threats to their health, allowing the plants to prepare for their own defense. As these and other responses show, plants may be rooted in place, but they are far from helpless.

KQED: Plant Plague: Sudden Oak Death

Devastating over one million oak trees across Northern California in the past ten years, Sudden Oak Death is a killer with no cure. But biologists now are looking to the trees' genetics for a solution. See http://www.kqed.org/quest/television/plant-plague-sudden-oak-death for more information.

Lesson Summary

  • Plants live just about everywhere on Earth, so they have evolved adaptations that allow them to survive and reproduce under a diversity of conditions. Various plants have evolved adaptations to live in the water, in very dry environments, or in the air as epiphytes.
  • Like all organisms, plants detect and respond to stimuli in their environment. Their main response is to change how they grow. Their responses are controlled by hormones. Some plant responses are tropisms. Plants also respond to daily and seasonal cycles and to disease.

Lesson Review Questions

Recall

1. List special challenges that aquatic plants face.

2. What are xerophytes? Give an example.

3. Identify three general ways that plants can adapt to extreme dryness.

4. Describe how epiphytes can absorb moisture without growing roots in soil.

5. What is the primary way that plants respond to environmental stimuli? What controls their responses?

6. Define tropism. Name one example in plants.

7. State ways that plants respond to disease.

Apply Concepts

8. Apply the concept of symbiosis to epiphytes and their host plants. Do you think they have a symbiotic relationship? If so, which type of symbiotic relationship do you think they have? Explain your answer.

Think Critically

9. Why are epiphytes found mainly in rainforest ecosystems?

10. Why is it adaptive for plants to detect and respond to daily and seasonal changes?

EOL Research

11. Bromeliads are some of the most common epiphytes. Research bromeliads at EOL and discuss the evolution of these species. See the Communities and Populations chapter for information about EOL.

Points to Consider

In this chapter you read about the cells, tissues, and organs that make up plants. You also read about plant life cycles. Like plants, animals are complex organisms with tissues and organs. Animals also have life cycles.

  • How do the cells of animals differ from those of plants? What tissues and organs might be found in animals?
  • What is the general animal life cycle? How does it differ from the general life cycle of plants?

Opening image copyright Dudarev Mikhail, 2010. Used under license from Shutterstock.com.

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