98 Unit 1: Diversity of Living Things

VOCABULARYseed p. 98embryo p. 98germination p. 99pollen p. 100gymnosperm p. 102

BEFORE, you learned

• Plant species evolved fromalgaelike ancestors

• Mosses are nonvascular plantsthat reproduce with spores

• Ferns are vascular plants thatreproduce with spores

NOW, you will learn

• How some plants reproducewith pollen and seeds

• About the advantages ofpollen and seeds

KEY CONCEPT

Seeds and pollen arereproductive adaptations.

THINK ABOUT

Is a seed alive?

A lotus is a type of pond lily that iscommonly found in water gardens.The plants take root in the bottomof a pond. A plant scientist inCalifornia experimented with lotusseeds from China that were over1000 years old. The scientist made a small opening in thehard covering of each seed and planted the seeds in wet soil. Some ofthe seeds sprouted and grew. What made it possible for these seeds tosurvive for such a long time? Is a seed alive?

Seeds are an important adaptation.Spores are one adaptation that made it possible for plants to reproduceon land. Seeds are another. A is a young plant that is enclosed in aprotective coating. Within the coating are enough nutrients to enablethe plant to grow. Seeds and spores can both withstand harsh conditions.Seed plants, however, have several survival advantages over seedlessplants. These advantages make it possible for seed plants to spreadinto environments where seedless plants are less likely to survive.

In all plants, fertilization brings about the growth of the next generation of the plant, beginning with an embryo. An(EHM-bree-OH) is the immature form of an organism that has thepotential to grow and develop. In seed plants, the embryo is protected.The seed coat protects the immature plant until conditions are rightfor it to grow.

embryo

seed

VOCABULARYRemember to make wordtriangles for seed andembryo in your notebook.

Chapter 3: Plants 99

An embryo can remain inside a seed for a long timewithout growing. When moisture, temperature, and otherconditions are right, a seed will start to grow.(JUR-muh-NAY-shuhn) is the beginning of growth of a newplant from a spore or a seed. If you’ve ever planted a seedthat sprouted, you’ve observed germination.

Check Your Reading What is germination?

When a seed germinates, it takes in water from its surroundings. As the embryo begins to grow, it uses thestored nutrients in the seed for energy and materials.The nutrients need to last until the new plant’s roots andshoots can start to function.

Some plants reproduce with seeds.In most places, plants that reproduce with seeds are common and easyto see. Trees, bushes, flowers, and grasses are all seed plants. It’s a bitharder to find plants that reproduce with spores, such as mosses orferns. Why are there so many more seed plants in the world? The diagram below shows some of the differences and similarities betweenseeds and spores.

One important difference between a seed and a spore is that theseed contains a multicellular organism. If you look closely at the photograph above you can see the tiny leaves at the top of the embryo,with the root below. Spores are just a single cell. Seeds can be spreadby wind, animals, or water. Spores are mostly carried by the wind.The sperm of seed plants, unlike the sperm of mosses and ferns, donot need water to reach the egg. One important similarity between aseed and a spore is that both can grow into a new plant.

Check Your Reading Name three ways seeds are different from spores.

Germination

RESOURCE CENTERCLASSZONE.COM

Learn more about seeds.

Inside a Seed

plantembryo

protectivecoating

storednutrients

seeds spores• have multicellular

embryo inside

• contain supply of nutrients

• made up of a single cell

• do not contain supply of nutrients

• have protective coating

• can survive dry, harsh conditions

• contain parent plants’genetic material

reading tip

As you read the numberedparagraphs on this page,follow the numbers on thelabels of the diagram onpage 101.

100 Unit 1: Diversity of Living Things

Pine trees reproduce with pollen and seeds.Seed plants, such as pine trees, do not have swimming sperm. Instead,they have pollen. A grain is a small multicellular structure thatholds a sperm cell. It has a hard outer covering to keep the spermfrom drying out. Pollen grains can be carried from one plant toanother by wind, water, or by animals such as insects, bats, or birds.The process of pollination is completed when a pollen grain attachesto the part of a plant that contains the egg and releases the sperm.

The life cycle of a pine tree provides an example of how seedplants reproduce.

The reproductive structures ofa pine tree are the pinecones.Meiosis occurs in the pinecones, producing sperm and eggcells. Each tree has separatemale and female cones.

In male cones, the sperm cellsare contained in pollen grains,which are released into the air.In female cones, the egg cells areenclosed in protective compart-ments within the cone scales.

The female cone produces a sticky substance. When a pollen grainlands, it sticks. A pollen tube begins to grow from the pollen grainthrough the scale to the egg. Fertilization occurs when the pollentube reaches the protective compartment and sperm travel throughit to one of the eggs.

The fertilized egg grows into an embryo. The compartment, withits protective covering and supply of nutrients, becomes the seed.The pinecone eventually releases its seeds. The winged seeds canfloat through the air and may be carried long distances by thewind. If the seed lands on the ground and germinates, it canbecome a new pine tree.

In the life cycle of a pine tree, meiosis and fertilization occur completely within the tissue of the mature plant. Fertilization doesn’trequire an outside source of water. The sperm cells in a pollen grainand the egg cells in a cone scale represent the first generation of theplant. The seed and the tree that grows from the seed represent thesecond generation.

Check Your Reading What do pollen grains and cone scales contain?

4

3

2

1

pollen

Pinecones release lots of pollen into the air.

Life Cycle of a Pine Tree

Male Cones

Use the definition of a cycle to predict what happensafter seeds are released from pinecones.

Sperm move through the pollentube to fertilize one of the egg cells at the base of the female cone scale.

Fertilization3

pollen tube

sperm eggscales

openfemalecone

pollengrain

Male cones release polleninto the air. A pollen grainsticks to the scales of afemale cone, and a pollentube begins to grow.

Pollination2

Each fertilized egg becomesan embryo in a seed. A female cone can contain many seeds. The cone opens and the seeds are released.

Seeds4

Female Cone

A mature pine tree has bothmale and female cones.Meiosis occurs inside thecones, producing sperm and egg cells.

Meiosis1

seed

Chapter 3: Plants 101

102 Unit 1: Diversity of Living Things

Gymnosperms are seed plants.Pollen and seeds are reproductive adaptations. They did not appear inplants until millions of years after seedless plants such as mosses andferns had already begun to live on land. Today, however, most of theplant species on Earth reproduce with seeds, and many species ofseedless plants have become extinct. Some scientists think this isbecause over time Earth’s climate has become drier and cooler. Seedplants are generally better at reproducing in dry, cool environmentsthan seedless plants are.

Fossil evidence shows that species of seed plants in the(JIHM-nuh-SPURM) group have existed on Earth for

more than 250 million years. Plants classified as gymnosperms pro-duce seeds, but the seeds are not enclosed in fruit. The wordgymnosperm comes from the Greek words for “naked seed.” There arefour types of gymnosperms living on Earth today.

Check Your Reading What is distinctive about gymnosperm seeds?

ConifersThe conifers, or cone-bearing trees, are the type of gymnosperm you areprobably most familiar with. The conifers include pine, fir, spruce, hem-lock, cypress, and redwood trees. Many conifers are adapted for living incold climates, where there is relatively little water available. Their leavesare needle-shaped and have a thick cuticle. This prevents the plant fromlosing much water to transpiration. Conifers can also keep their needlesfor several years, which means the plants can produce sugars all year.

gymnosperm

What conditions make a pinecone open?PROCEDURE

Place your pinecone in the beaker of water. Observe any changes that takeplace. Leave the cone in the water until the changes stop.

Remove the cone from the water and place it on a paper towel to dry. Observe any changes that take place as the cone dries.

WHAT DO YOU THINK?• What did you observe when the cone was in the water?

• What happened when the cone dried out?

CHALLENGE Try this procedure on cones fromdifferent plant species.

2

1

PineconesPineconesSKILL FOCUSObserving

MATERIALS• dried, open

pinecones• beaker of water • paper towels

TIME20 minutes

MIND MAPMake a mind map for themain idea that gym-nosperms are seed plants.Don’t forget to include thedefinition of a gymnospermin your mind map.

Other Gymnosperms The other types of living gymnosperms are cycads, gnetophytes (NEE-toh-fyts), and ginkgoes. These three types of gymnospermsappear to be quite different from one another. Cycads are palmlikeplants that are found in tropical areas. They produce cones for seeds.Many cycads produce poisonous compounds. Gnetophytes are another type of tropical gymnosperm that produces cones. Chemicalstaken from certain gnetophyte plants have been used to treat coldsymptoms for thousands of years.

Ginkgoes are gymnosperms with fleshy seeds that hang from theirbranches. Ginkgoes are often grown in parks and along streets, butyou will not often see them with seeds. People avoid putting male andfemale ginkgo trees together because the seed coat of a ginkgo pro-duces a particularly foul smell.

KEY CONCEPTS1. How are seeds different from

spores?

2. How does fertilization in seedplants differ from fertilizationin seedless plants?

3. Seed plants are found in envi-ronments where seedlessplants are not. List at least tworeasons why.

CRITICAL THINKING4. Compare and Contrast

What is the differencebetween a spore and a pollengrain? In what ways are sporesand pollen grains similar?

5. Hypothesize Gymnospermsproduce a lot of pollen, andmost of it blows away, neverfertilizing an egg. Why mightthis characteristic help a plantspecies survive?

CHALLENGE6. Analyze Like all plants, a pine

tree has a two-part life cycle.Look again at the diagram onpage 101, and then make anew version of it in your note-book. Your version shouldshow where in the life cycleeach generation begins andends. Hint: reread the text onpage 100.

Chapter 3: Plants 103

Ginkgo trees like this oneare gymnosperms that donot produce cones. Theirseeds are exposed to theenvironment.