Figure 10.31
A reduction in levels of abscisic acid allows these buds to break dormancy and put out leaves.
Auxins are hormones that influence many different processes in plants. Auxins produced at the tip of the plant are involved in apical dominance, preventing the growth of side branches. In apical dominance the main central stem of the plant is dominant over other side stems; the main stem grows more strongly than other stems and branches. When the tip of the plant is removed, the auxins are no longer present and the side branches begin to grow. This is why pruning generally will help produce a fuller plant with more branches. Auxins are also involved in tropisms, which will be discussed in the next section.
Tropisms
Figure 10.32
These seedlings bending toward the sun are displaying phototropism.
Plants may not be able to move, but they are able to change their growth in response to a stimulus. Growth toward or away from a stimulus is known as a tropism. The ability of a plant to curve its growth in one direction is achieved with the signaling of auxin. The auxin moves to one side of the stem, where it starts a chain of events that elongate the cells on just that one side of the stem. With one side of the stem growing faster than the other, the plant begins to bend.
You might have noticed that plants tend to bend towards the light. This is an example of a tropism where light is the stimulus, known as phototropism (Figure above). To obtain more light for photosynthesis, it’s advantageous for leaves and stems to grow towards the light. On the other hand, roots are either insensitive to light or actually grow away from light. This is advantageous for the roots since their purpose is to obtain water and nutrients from deep within the ground.
A seed often starts out underground in the dark, yet the roots always grow downwards into the earth and not toward the surface. How do the roots know which way is up? Gravitropism is a growth towards or away from the pull of gravity. Shoots also exhibit gravitropism, but in the opposite direction. If you place a plant on its side, the stem and new leaves will curve upwards. Again, the hormone auxin is involved in this response. Auxin builds up on the lower side of the stem, elongating this side of the stem and causing it to bend upwards over time.
Plants also have a touch response, called thigmotropism. If you have ever seen a morning glory or the tendrils of a bean plant twist around a pole, then you know that plants must be able to detect the pole. Thigmotropism works much like the other tropisms. The plant grows straight until it comes in contact with the pole. Then the side of the stem in contact with the pole grows slower than the opposite side of the stem. This causes the stem to bend around the pole.
Tropisms Type of Tropism Stimulus
Phototropism light
Gravitropism gravity
Thigmotropism touch
Seasonal Changes
Figure 10.33
Leaves changing color is a response to the shortened length of the day in autumn.
Along with detecting differences in light or gravity, plants also are able to detect the seasons. Leaves change color and drop each autumn in temperate climates (Figure above). Certain flowers, like poinsettias, only bloom during the winter. And in the spring, the winter buds on the trees break open and the leaves start to grow. How do plants detect time of year?
Although you might detect the change of seasons by the change in temperature, this is not the primary way by which plants detect the change of seasons. Plants determine the time of year by the length of the day. Because of the tilt of the Earth, during winter days there are less hours of light than during summer days. That’s why during the winter it may start getting dark very early during the evening and even stay dark while you’re getting ready for school the next morning. But in the summer it will be bright early in the morning and the sun may not set until late that night. Plants can detect the differences in day length and respond accordingly. For example, in the fall when the days start to get shorter, the trees sense it is time to begin the process of shedding their leaves.
Lesson Summary
Plant hormones are chemical signals that regulate a variety of processes in plants.
A plant tropism is growth towards or away from a stimulus such as light or gravity.
Many plants undergo seasonal changes after detecting differences in day length.
Review Questions
What is the term for dropping fruits, flowers, or leaves?
What hormone is involved with fruit ripening?
How are hormones involved in seed germination?
What hormone is involved in tropisms?
What hormones promote cell division?
What hormone causes stems to elongate?
What is phototropism?
How does a tendril wind around a pole?
How do plants detect the change in seasons?
What are some seasonal responses in plants?
Further Reading / Supplemental Links
www.plantphysiol.org/cgi/reprint/116/1/329.pdf
http://plantphys.info/apical/apical.html
http://www.cals.ncsu.edu/nscort/outreach_exp_gravitrop.html
http://biology.kenyon.edu/edwards/project/steffan/b45sv.htm
http://www.bbc.co.uk/schools/gcsebitesize/biology/greenplantsasorganisms/2plantgrowthrev1.shtml
http://en.wikipedia.org/wiki
Vocabulary
abscisic acid
Plant hormone involved in maintaining dormancy and closing the stomata.
abscission
The shedding of leaves, fruits, or flowers.
apical dominance
Suppressing the growth of the side branches of a plant.
auxin
Plant hormone involved in tropisms and apical dominance.
cytokinins
Plant hormone involved in cell division.
ethylene
Plant hormone involved in fruit ripening and abscission.
gibberellins
Plant hormone involved in seed germination and stem elongation.
gravitropism
Plant growth towards or away from the pull of gravity.
hormones
Chemical messengers that signal responses to stimuli.
phototropism
Plant growth towards or away from light.
senescence
The programmed process of aging and eventual death.
thigmotropism
Differential plant growth in response to contact with an object.
tropism
Plant growth response towards or away from a stimulus.
Points to Consider
In the next chapter we will turn our attention to animals.
List some ways animals are different from plants.
What characteristics do you think define an animal?
Can you think of examples of animals that do not have hard skeletons?
Chapter 11: Introduction to Invertebrates
Lesson 11.1: Overview of Animals
Lesson Objectives
List the characteristics that define the animal kingdom.
Define and give examples of the invertebrates.
Check Your Understanding
What are the main differences between an animal cell and a plant cell?
How do animals get their energy?
Introduction
How are animals different from other forms of life? Recall that all animals are eukaryotic, meaning that they have cells with true nuclei and membrane-bound organelles. Another feature that distinguishes animals from animal-like protists is that animals are multicellular, while protists are often unicellular. Because animals are multicellular, animal cells can be organized into tissues, organs, and organ systems. Finally, animals are heterotrophic, meaning they must ingest some type of organic matter for nutrition and energy (Figure below).
Eukaryotic, multicellular, and heterotrophic are features shared by all the millions of diverse types of animals on earth, from tiny ants and snails to giant whales
and grizzly bears. In this chapter we will just focus on the invertebrates, the animals that do not have a backbone of bone or cartilage.
Figure 11.1
Animals are heterotrophs, meaning they must eat to get molecules necessary for their growth and energy.
Classification of Animals
Recall that each kingdom of life, including the animal kingdom, is divided into smaller groups called phyla based on their shared characteristics. For example the phylum Mollusca largely consists of animals with shells like snails and clams. Although modern classification is also based on looking at molecular data, such as DNA sequencing, animals have long been classified in their current phyla largely by their physical characteristics.
One example of a physical characteristic used to classify animals is body symmetry. In radially symmetrical organisms, such as sea stars, the body is organized like a circle (Figure below). Therefore, any cut through the center of the animal results in two identical halves. Other animals, such as humans and worms, are bilaterally symmetrical, meaning their left and right sides are mirror images.
Figure 11.2
Sea stars are radially symmetrical.
Animals are also often classified by their body structure. For example, segmentation, the repetition of body parts, defines one phylum of worms (Figure below). Animals that have a true body cavity, defined as a fluid-filled space, and internal organs are also classified in separate phyla from those animals that do not have a true body cavity. Finally, the structure of the digestive system of animals can also be used as a characteristic for classification. Animals with incomplete digestive tracts have only one opening in their digestive tracts, while animals with complete digestive tract have two openings, the mouth and anus.
Figure 11.3
A segmented body plan defines the phylum that includes the earthworms.
What Are Invertebrates?
Besides being classified into phyla, animals are also often characterized as being invertebrates or vertebrates. This is an informal classification term based on the skeletons of the animals. Vertebrates have a backbone of bone or cartilage, while invertebrates have no backbone. All vertebrate organisms are in the phylum Chordata, while invertebrates make up several diverse phyla. As seen in Figure below, the invertebrates include the insects, the earthworms, the jellyfish, the star fish, and a variety of other animals. In the next lessons we will discuss some of phyla within the animal kingdom that contain invertebrates.
Figure 11.4
Snails are an example of invertebrates, animals without a backbone.
Phylum Examples
Porifera Sponges
Cnidaria Jellyfish, corals
Platyhelminthes Flatworms, tapeworms
Nematoda Nematodes, heartworm
Mollusca Snails, clams
Annelida Earthworms, leeches
Arthropoda Insects, crabs
Echinodermata Sea stars, sea urchins
Lesson Summary
Animals are multicellular, eukaryotic heterotrophs.
Animals can be classified by both molecular data and physical characteristics such as symmetry.
Invertebrates are animals without a backbone.
Review Questions
What are some key features that define the animal kingdom?
What does heterotrophic mean?
What defines the invertebrates?
What are some examples of invertebrates?
What is the difference between radially and bilaterally symmetrical animals?
What’s an example of a bilaterally symmetrical animal?
What are some examples of a radially symmetrical animal?
What is a body cavity?
What is the difference between an incomplete and complete digestive system?
What is segmentation?
Further Reading
http://animaldiversity.ummz.umich.edu/site/index.html
http://doe.sd.gov/octa/ddn4learning/themeunits/animals
http://animals.nationalgeographic.com/animals/invertebrates.html
http://en.wikipedia.com
Vocabulary
bilaterally symmetrical
Body plan in which the left and right side are mirror images.
complete digestive tract
A digestive tract that has two openings, the mouth and the anus.
heterotroph
Organism that cannot make its own food, so it must ingest some type of organic matter.
invertebrates
Animals without a backbone.
incomplete digestive tract
A digestive tract that has only one opening.
radially symmetrical
A body plan in which any cut through the center results in two identical halves.
segmentation
Repetition of body parts or segments.
Points to Consider
What do you think that jellyfishes and corals have in common?
Think of some examples of animals that are bilaterally symmetrical, where the left side is a mirror image of the right?
Lesson 11.2: Sponges and Cnidarians
Lesson Objectives
Describe the key features of the Sponges.
Describe the key features of the Cnidarians.
List examples of the Cnidarians.
Check Your Understanding
How are animals classified?
What is an invertebrate?
Introduction
The ocean is home to a variety of organisms. Phytoplankton, tiny photosynthetic organisms that float in the water, make their own food from the energy of the sun. Small aquatic animals, known as zooplankton, and larger animals, such as fish, use phytoplankton as a food source. These animals can in turn be eaten by larger aquatic animals, such as larger fish and sharks.
Among the various types of animals that live in the ocean, the sponges and cnidarians are important invertebrates. The Sponges are believed to be one of the most ancient forms of animal life on earth. The cnidarians, which include the jellyfish, also are among the oldest and most unusual animals on earth. In this lesson we will discuss the features that make these two types of invertebrates unique from other types of animals.
Sponges
Sponges are classified in the phylum Porifera, which derives its name from Latin words meaning “pore bearing.” These pores allow the movement of water into the sponges’ sac-like bodies (Figure below). Sponges pump water through their bodies because they are sessile filter feeders, meaning they cannot move and must filter organic matter and tiny organisms out of the water to obtain food.
Figure 11.5
Sponges have tube-like bodies with many pores.
Sponges are relatively primitive animals and do not have brains, stomachs, or other organs. In fact, sponges do not even have true tissues. Instead, their bodies are made up of specialized cells that each has specific functions. For example, the collar cells are flagellated and encourage water movement, while other types of cells regulate the water flow by increasing or decreasing the size of the pores.
Cnidarians
The cnidarians, in the phylum Cnidaria, include organisms such as the jellyfish (Figure below) and sea anemones (Figures below and below) that are found in shallow ocean water. You might recognize that these animals can give you a painful sting if you step on them. That’s because cnidarians have stinging cells known as nematocysts. When touched, the nematocysts unleash long, hollow threads that are intended to trap prey, and sometimes toxins are also injected through these threads to paralyze the prey.
The body plan of cnidarians is unique because these organisms are radially symmetrical, meaning that they have a circular body plan so that any cut through the center of the animal leaves two equal halves. The cnidarians have two basic body forms, polyp and medusa. The polyp is a cup-shaped body with the mouth directed upward, such as a sea anemone (Figure below). The medusa is a bell-shaped body with the mouth and tentacles directed downward, such as a jellyfish (Figure below).
> Unlike the sponges, the cnidarians are made up of true tissues. The inner tissue layer secretes digestive enzymes into the gastrovascular cavity, a large cavity that has both digestive and circulatory functions. The cnidarians also have nerve tissue organized into a net-like structure. Cnidarians do not have true organs, however.
Figure 11.6
Jellyfish have bell-shaped bodies with tenticles.
Figure 11.7
Sea anemones can sting and trap fish with their tentacles.
Figure 11.8
One type of sea anemone is home to the clownfish.
Cnidarian Colonies
Some types of cnidarians are also known to form colonies. For example, the Portuguese man-of-war looks like a single organism but is actually a colony of polyps (Figure below). One polyp is filled with air to help the colony float, while several feeding polyps hang below with tentacles full of nematocysts. Consequently, the Portuguese man-of-war is known to cause extremely painful stings to swimmers and surfers who accidentally brush up against these creatures in the water.
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