528 Chapter 13

Practice

Understanding Concepts

1. State two similarities and two differences between the transport sys-tems 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 columnintact.

4. Explain how carbohydrates produced in leaves are transportedthroughout the plant.

5. When pine and spruce trees are damaged, a thick, sticky gum seepsfrom 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 sugarmaple trees in early spring. Explain why it is important for syrup pro-ducers to take only a limited amount of sap from each tree.

13.7 ReproductionFlowers

The variety of angiosperm flowers was discussed in Chapter 10. They exhibit adramatic variety of colours, shapes, and sizes, and are probably the most com-plex and intricate of all plant structures. The role of the various intricate flowerparts is to ensure successful pollination and fertilization. Indirectly, flowers arealso responsible for protecting and adequately distributing the seeds because it isflower parts which form the seed coats and fruit.

Seed Growth and Development

In Chapter 10, it was shown that spermatophytes are divided into gymnospermsand angiosperms. Seed development in both these groups is very similar.However, the following text focuses on angiosperm seeds formed inside fruit. Aseed consists of an embryo, tissue to provide nutrients for the embryo, and aprotective coat. The growing embryo slowly forms a root and shoot structure. Ifthe plant is a monocot, a single seed leaf (cotyledon) develops; if it is a dicot,two seed leaves form (Figure 1). The one or two cotyledons may contain all thenutrients for the embryo, or there may be an additional nutrient-rich materialcalled the endosperm. Seeds with large cotyledons often have insignificantendosperm tissue. Seeds with a large quantity of endosperm often have insignif-icant cotyledons. Seeds, protected by the seed coat, may enter a dormant period,often lasting many years.

When temperature and moisture conditions are optimum, seeds begin togerminate. Once the seed and seed coat absorb water, the embryo begins to growrapidly. The seed coat ruptures and the root and shoot emerge. The water- andnutrient-absorbing root begins to grow downward, and the shoot grows upward.This upward and downward growth occurs because of the presence of certainchemicals and the effect of gravity. In some plants, the cotyledons are raised out

dormant: describes a state of extremelyslow biological activity. A dormant seedcontains a living embryo but it does notgrow; it remains protected by a seed coatand sometimes the fruit as well.

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13.7

of the soil and some of these actually photosynthesize for a short time. In otherplants, the cotyledons remain underground. Nutrients stored in the endospermand/or the cotyledons support development until the new plant has sufficientroot surface area and chlorophyll to be self-sustaining.

Seed Adaptations

Being stationary creates numerous challenges for plants, not the least of which isthe dispersal of offspring. If all the seeds from a plant were to drop to the groundand 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 speciesis reduced through the evolution of various mechanisms for dispersing seeds. Plantspecies unable to disperse seeds adequately do not survive well. Also, individualswithin a species that cannot disperse their seeds well are less likely to survive thanindividuals that can. Over time, all successful plants seem to have developed somemeans of ensuring survival of their young. This evolution is ongoing.

Fruit development represents a significant investment in resources on the partof 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 furof 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 ani-mals. Once eaten by an animal, seeds pass unharmed through the digestive tractand are deposited along with the animal’s waste, which is an ideal fertilizer. Birdsare capable of carrying seeds tremendous distances in their digestive tract.

Some plant seeds will not germinate unless they have passed through thedigestive tract of a specific animal. The dodo, a large flightless bird that lived onthe island of Mauritius, went extinct in the 1600s. It ate the fruits of the treeCalvaria and was the only bird naturally capable of inducing these seeds to ger-minate. 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 fedsome of the large Calvaria seeds to turkeys. After passing through the completedigestive tract of the turkeys, some of these seeds germinated—the first of theirkind to do so in over 300 years!

The common practice of squirrels or chipmunks hiding or burying acornsand other large seeds for their winter food supply is another example of seed dis-persal (Figure 2). Rarely are all the acorns consumed; some of the remainingseeds grow into new oak trees.

seed coat

endosperm

cotyledon

embryo

pry open here

seed coat

cotyledon

embryo

side view side viewcut open here

whole cut whole open

Figure 1

(a) Corn has a typical monocot seed.(b) Beans have typical dicot seeds. When the

seeds are opened as shown, the embryoas well as the nutritive tissue can be seen.

Figure 2

Many seed-eating rodents, such as this chip-munk, help plants reproduce by dispersingand burying seeds.

(a) (b)

cotyledoncotyledon

530 Chapter 13

Plants accomplish seed dispersal in a variety of ways (Figure 3). Many plantshave capsule fruits or pods that explode when mature. The resulting explosionthrows the seeds some distance away from the parent plant. Some plants, such asthe orchid and poppy, produce small, lightweight seeds that can be carried greatdistances 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 seedsthat are dispersed by water, such as those of water lilies and coconuts, air istrapped in the seeds and fruits to enable them to float. Often, seeds of waterplants are enclosed in a waxy, waterproof coating that protects the seed duringtravel over long distances in water. Later, as the coating wears away, water canpenetrate 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

Plants: Form and Function 531

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? Howwould 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 newroot?

6. What are the possible locations of the cotyledons 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 ortwo cotyledons and/or endosperm tissue. Seeds can remain dormant foryears.

4. When conditions are ideal, the seed coat absorbs water and the embryogrows rapidly. The root and shoot grow quickly and break open the seedcoat to allow germination.

5. Nutrients stored in the endosperm or cotyledon support development inthe early stages.

6. Seeds must be dispersed away from the parent to reduce competition withthe parent. Some seed dispersal adaptations are wings, parachutes, hooks,and fleshy edible fruit.

Activity 13.7.1

Monocot and Dicot Seeds

You will observe and compare the structure and germination of monocot anddicot 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 devel-opment?Where will the starch be found in opened corn and bean seeds?

13.7

532 Chapter 13

Materials

masking tapefour 100-mL beakers or jars paper towelsingle-edged razor blade probehand lens (magnifying glass) Lugol’s solution

For Part 1:bean and corn seeds—soaked in water for 2 htwo other types of seeds such as grass, radish, carrot, poppy, etc.—soaked inwater for 2 h

For Part 2:bean and corn seeds—soaked in water for 24 hdry bean seedsfour 250-mL beakers

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 havebeen soaked for 2 h before use. Use beakers C and D for the 2 additionalselected seeds.

2. Line each beaker with a double layer of wet paper towel and leave 2 cm ofwater in the beaker. It is important that the towels be kept wet throughoutthe activity.

3. Position 2 bean seeds so they are wedged between the wall of the beaker andthe 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 thedepth is restored to 2 cm. Examine the germinating seeds daily with a handlens until the root and shoot are established. Allow 2 weeks to complete theactivity. Record your observations in a daily log. Once changes to the seedsoccur, add labelled drawings to your written observations. You will need 1set of notes and drawings for each beaker.

Part 2: Examining Embryos5. 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 thesoaked bean seed. Use a probe to pry the 2 sections of the seed apart. Referto Figure 1, page 529 as a guide. Find the embryo and examine it with ahand lens. Draw a diagram of the cotyledon and the attached embryo.

7. Use one drop of Lugol’s solution to test various parts of the bean seed for thepresence 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 theembryo. Lay the corn seed down with the embryo facing upward. Describethe appearance of the embryo. Refer to Figure 1, page 529 as a guide.

Figure 4

Lugol’s solution is toxic andcan cause an itchy rash. Avoidskin and eye contact. Wash allsplashes off your skin andclothing thoroughly. If you getany chemical in your eyes,rinse for at least 15 min andinform your teacher.

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13.7

Always be careful whenusing a sharp instrument.

Understanding Concepts

1. What function does the starch serve in the seed?

2. Eventually, seedlings no longer need cotyledons. Why not?

3. Why do seeds need to be dispersed so that they do not germi-nate directly underneath the parent plant?

4. Why might it also be undesireable for a seed to be dispersed veryfar away to an area where no other individuals of the samespecies exist?

5. The production of fruit represents a very high “cost” to plants interms of nutrient energy. How does fruit improve chances ofspecies 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-by-step experimental design which could be used to test theeffect of temperature on seed germination. Specify how youwould set up your control. State your independent anddependent variables.

(b) Exchange your answer to part (a) with another person orgroup. Record your questions or comments about their pro-cedure and make suggestions to improve the design.

Making Connections

8. Most plants do not produce fruit unless their flowers are polli-nated. How does this information influence decisions concerningif and when to spray insecticides near an orchard?

Section 13.7 Questions

9. 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 tech-nique you used to test the bean seed. Make a drawing to show which areasof the corn seed have the greatest amount of starch.

Analysis

(a) What are the observed differences in the germination process of the beanseed 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?