13.7 Reproduction. Practice. Flowers. Seed Growth and Development

Transcription

1 Practice Understanding Concepts 1. State two similarities and two differences between the transport systems found in mammals and those found in plants. 2. Describe the bulk flow of water from the roots to the leaves. 3. Name three different mechanisms that help keep the water column intact. 4. Explain how carbohydrates produced in leaves are transported throughout the plant. 5. When pine and spruce trees are damaged, a thick, sticky gum seeps from the wound. Suggest reasons why this process benefits the trees. Making Connections 6. Maple syrup is produced from the sap moving in the xylem of sugar maple trees in early spring. Explain why it is important for syrup producers to take only a limited amount of sap from each tree Reproduction Flowers The variety of angiosperm flowers was discussed in Chapter 10. They exhibit a dramatic variety of colours, shapes, and sizes, and are probably the most complex and intricate of all plant structures. The role of the various intricate flower parts is to ensure successful pollination and fertilization. Indirectly, flowers are also responsible for protecting and adequately distributing the seeds because it is flower parts which form the seed coats and fruit. dormant: describes a state of extremely slow biological activity. A dormant seed contains a living embryo but it does not grow; it remains protected by a seed coat and sometimes the fruit as well. Seed Growth and Development In Chapter 10, it was shown that spermatophytes are divided into gymnosperms and angiosperms. Seed development in both these groups is very similar. However, the following text focuses on angiosperm seeds formed inside fruit. A seed consists of an embryo, tissue to provide nutrients for the embryo, and a protective coat. The growing embryo slowly forms a root and shoot structure. If the plant is a monocot, a single seed leaf () develops; if it is a dicot, two seed leaves form (Figure 1). The one or two s may contain all the nutrients for the embryo, or there may be an additional nutrient-rich material called the endosperm. Seeds with large s often have insignificant endosperm tissue. Seeds with a large quantity of endosperm often have insignificant s. Seeds, protected by the seed coat, may enter a dormant period, often lasting many years. When temperature and moisture conditions are optimum, seeds begin to germinate. Once the seed and seed coat absorb water, the embryo begins to grow rapidly. The seed coat ruptures and the root and shoot emerge. The water- and nutrient-absorbing root begins to grow downward, and the shoot grows upward. This upward and downward growth occurs because of the presence of certain chemicals and the effect of gravity. In some plants, the s are raised out 528 Chapter 13

2 13.7 side view cut open here seed coat side view pry open here endosperm embryo embryo seed coat (a) whole cut whole open of the soil and some of these actually photosynthesize for a short time. In other plants, the s remain underground. Nutrients stored in the endosperm and/or the s support development until the new plant has sufficient root surface area and chlorophyll to be self-sustaining. (b) Figure 1 (a) Corn has a typical monocot seed. (b) Beans have typical dicot seeds. When the seeds are opened as shown, the embryo as well as the nutritive tissue can be seen. Seed Adaptations Being stationary creates numerous challenges for plants, not the least of which is the dispersal of offspring. If all the seeds from a plant were to drop to the ground and germinate immediately below the parent plant, certain difficulties would arise. The developing seedlings would have to compete with the parent for light, water, and minerals, and the rate of survival would be low. Competition within a species is reduced through the evolution of various mechanisms for dispersing seeds. Plant species unable to disperse seeds adequately do not survive well. Also, individuals within a species that cannot disperse their seeds well are less likely to survive than individuals that can. Over time, all successful plants seem to have developed some means of ensuring survival of their young. This evolution is ongoing. Fruit development represents a significant investment in resources on the part of the plant. This fruit is the price the plant pays for improved seed dispersal. Some fruits have special features such as hooks or spines that will attach to the fur of mammals, the feathers of birds, or the clothes of humans. Fleshy fruits (e.g., apples, strawberries, and tomatoes) are both attractive to and nutritious for animals. Once eaten by an animal, seeds pass unharmed through the digestive tract and are deposited along with the animal s waste, which is an ideal fertilizer. Birds are capable of carrying seeds tremendous distances in their digestive tract. Some plant seeds will not germinate unless they have passed through the digestive tract of a specific animal. The dodo, a large flightless bird that lived on the island of Mauritius, went extinct in the 1600s. It ate the fruits of the tree Calvaria and was the only bird naturally capable of inducing these seeds to germinate. Following the dodo s extinction, not a single seed of this tree germinated. There are now fewer than 20 of these trees remaining. In the 1970s, scientists fed some of the large Calvaria seeds to turkeys. After passing through the complete digestive tract of the turkeys, some of these seeds germinated the first of their kind to do so in over 300 years! The common practice of squirrels or chipmunks hiding or burying acorns and other large seeds for their winter food supply is another example of seed dispersal (Figure 2). Rarely are all the acorns consumed; some of the remaining seeds grow into new oak trees. Figure 2 Many seed-eating rodents, such as this chipmunk, help plants reproduce by dispersing and burying seeds. Plants: Form and Function 529

3 Plants accomplish seed dispersal in a variety of ways (Figure 3). Many plants have capsule fruits or pods that explode when mature. The resulting explosion throws the seeds some distance away from the parent plant. Some plants, such as the orchid and poppy, produce small, lightweight seeds that can be carried great distances by the winds. Others, such as dandelions and milkweed, have fluffy, parachutelike structures attached to their seeds. Still others, such as the maple, sycamore, elm, and ash, have winglike structures attached to the seeds. For seeds that are dispersed by water, such as those of water lilies and coconuts, air is trapped in the seeds and fruits to enable them to float. Often, seeds of water plants are enclosed in a waxy, waterproof coating that protects the seed during travel over long distances in water. Later, as the coating wears away, water can penetrate the seed and trigger germination. (a) (b) (c) (d) (e) (f) (g) (h) Figure 3 Variety of mechanisms for seed dispersal (a) Milkweed pods releasing seeds with their own parachutes (b) Wild cucumber fruit open at the bottom to release the seeds (c) Thick squash fruit protecting the seeds (d) A cluster of Jack-in-the-pulpit fruit, each containing one seed (e) Kiwi fruit with many tiny black seeds (f) Clusters of staghorn sumac fruit, each with its own seed (g) A common burdock flower showing the form of its future fruit with many barbed seeds (h) Fruit of white baneberry 530 Chapter 13

4 13.7 Practice Understanding Concepts 1. What is the functional role of flower parts? 2. What is a seed? 3. Where, specifically, would you expect to find starch in a seed? How would you prove your answer to be correct? 4. What basic condition is required for seed germination? 5. What influences the direction of growth of the new shoot and new root? 6. What are the possible locations of the s after germination? 7. Describe four specific and different methods of seed dispersal. Reproduction 1. Spermatophytes include gymnosperms and angiosperms. 2. Flower parts of angiosperms are responsible for fruit and seed production. 3. A seed contains an embryo plus nutrient-rich material which is in one or two s and/or endosperm tissue. Seeds can remain dormant for years. 4. When conditions are ideal, the seed coat absorbs water and the embryo grows rapidly. The root and shoot grow quickly and break open the seed coat to allow germination. 5. Nutrients stored in the endosperm or support development in the early stages. 6. Seeds must be dispersed away from the parent to reduce competition with the parent. Some seed dispersal adaptations are wings, parachutes, hooks, and fleshy edible fruit. Activity Monocot and Dicot Seeds You will observe and compare the structure and germination of monocot and dicot seeds. Questions Do all seeds take the same time to germinate? How do monocot and dicot seeds compare in structure? What function is played by seed structures during germination and shoot development? Where will the starch be found in opened corn and bean seeds? Plants: Form and Function 531

5 Materials masking tape four 100-mL beakers or jars single-edged razor blade hand lens (magnifying glass) paper towel probe Lugol s solution For Part 1: bean and corn seeds soaked in water for 2 h two other types of seeds such as grass, radish, carrot, poppy, etc. soaked in water for 2 h For Part 2: bean and corn seeds soaked in water for 24 h dry bean seeds four 250-mL beakers Figure 4 Lugol s solution is toxic and can cause an itchy rash. Avoid skin and eye contact. Wash all splashes off your skin and clothing thoroughly. If you get any chemical in your eyes, rinse for at least 15 min and inform your teacher. Procedure Part 1: Germination and Shoot Development 1. Use masking tape to label 4 beakers A, B, C, and D. Use beaker A for bean seeds and beaker B for corn seeds. Both these bean and corn seeds have been soaked for 2 h before use. Use beakers C and D for the 2 additional selected seeds. 2. Line each beaker with a double layer of wet paper towel and leave 2 cm of water in the beaker. It is important that the towels be kept wet throughout the activity. 3. Position 2 bean seeds so they are wedged between the wall of the beaker and the wet towels (Figure 4). Repeat for the other seeds in beakers B, C, and D. 4. Every day check that the towel remains moist and add water so that the depth is restored to 2 cm. Examine the germinating seeds daily with a hand lens until the root and shoot are established. Allow 2 weeks to complete the activity. Record your observations in a daily log. Once changes to the seeds occur, add labelled drawings to your written observations. You will need 1 set of notes and drawings for each beaker. Part 2: Examining Embryos 5. Obtain 1 dry bean seed and 1 bean seed that has been soaked in water for 24 h. Compare the appearance of the soaked and dry bean seeds. 6. Use your fingers to gently rub the surface to remove the seed coat from the soaked bean seed. Use a probe to pry the 2 sections of the seed apart. Refer to Figure 1, page 529 as a guide. Find the embryo and examine it with a hand lens. Draw a diagram of the and the attached embryo. 7. Use one drop of Lugol s solution to test various parts of the bean seed for the presence of starch. On your diagram indicate the regions containing starch. 8. Obtain a corn seed that has been soaked for 24 h. Carefully examine the seed. Note that one side of the corn seed is lighter. This indicates the location of the embryo. Lay the corn seed down with the embryo facing upward. Describe the appearance of the embryo. Refer to Figure 1, page 529 as a guide. 532 Chapter 13

6 Using a razor blade, cut the seed in half lengthwise as shown in Figure 1, page 529. Test various areas of the corn seed for starch using the same technique you used to test the bean seed. Make a drawing to show which areas of the corn seed have the greatest amount of starch. Always be careful when using a sharp instrument. Analysis (a) What are the observed differences in the germination process of the bean seed and that of the corn grain? (b) Explain why two similar seeds might not germinate at the same time. (c) Suggest a reason for at least one difference you observed in step 5. (d) In what ways does the corn kernel differ from the bean seed? Evaluation (e) Why was it important to use soaked seeds in Part 1? (f) Why was it important to keep the paper towel wet in Part 1? (g) Why is it necessary to soak the seeds in Part 2 for a much greater length of time? Section 13.7 Questions Understanding Concepts 1. What function does the starch serve in the seed? 2. Eventually, seedlings no longer need s. Why not? 3. Why do seeds need to be dispersed so that they do not germinate directly underneath the parent plant? 4. Why might it also be undesireable for a seed to be dispersed very far away to an area where no other individuals of the same species exist? 5. The production of fruit represents a very high cost to plants in terms of nutrient energy. How does fruit improve chances of species survival? 6. Describe some specific mechanisms for seed dispersal. Applying Inquiry Skills 7. (a) Alone or in a group of two or three, outline a simple step-bystep experimental design which could be used to test the effect of temperature on seed germination. Specify how you would set up your control. State your independent and dependent variables. (b) Exchange your answer to part (a) with another person or group. Record your questions or comments about their procedure and make suggestions to improve the design. Making Connections 8. Most plants do not produce fruit unless their flowers are pollinated. How does this information influence decisions concerning if and when to spray insecticides near an orchard? Plants: Form and Function 533