Lesson Overview 22.1 What is a Plant?

Characteristics of Plants What do plants need to survive?   Plants need: Sunlight Gas exchange Water Minerals.

The Plant Kingdom Plants are classified as members of the kingdom Plantae.   Plants are eukaryotes that have cell walls containing cellulose and carry out photosynthesis using chlorophyll a and b.

What Plants Need All plants have the same basic needs: sunlight, a way to exchange gases with the surrounding air, water, and minerals.

Sunlight Plants use the energy from sunlight to carry out photosynthesis.   Leaves are typically broad and flat and are arranged on the stem so as to maximize light absorption.

Gas Exchange Plants require oxygen to support cellular respiration, as well as carbon dioxide to carry out photosynthesis.   Plants must exchange these gases with the atmosphere and the soil without losing excessive amounts of water through evaporation.

Water and Minerals Land plants have evolved structures that limit water loss and speed the uptake of water from the ground.   Minerals are nutrients in the soil that are needed for plant growth.

Many plants have specialized tissues that carry water and nutrients upward from the soil and distribute the products of photosynthesis throughout the plant body.

The History and Evolution of Plants How did plants adapt to life on land?   Over time, the demands of life on land favored the evolution of plants more resistant to the drying rays of the sun, more capable of conserving water, and more capable of reproducing without water.

Origins in the Water The ancestors of today’s land plants were water-dwelling organisms similar to today’s green algae.   Although not as large and complex as many plants, green algae have cell walls and photosynthetic pigments that are identical to those of plants. Green algae also have reproductive cycles that are similar to plants. Studies of the genomes of green algae suggest that they are so closely related to other plants that they should be considered part of the plant kingdom.

The First Land Plants The greatest challenge that early land plants faced was obtaining water. They met this challenge by growing close to the ground in damp locations.   Fossils suggest the first true plants were still dependent on water to complete their life cycles. One of the earliest fossil vascular plants was Cooksonia, shown here.

The First Land Plants Several groups of plants evolved from the first land plants.   One group developed into mosses. Another lineage gave rise to ferns, cone-bearing plants, and flowering plants.

An Overview of the Plant Kingdom Botanists divide the plant kingdom into five major groups based on four important features: embryo formation specialized water-conducting tissues Seeds flowers.

An Overview of the Plant Kingdom The relationship of plant groups is shown below

The Plant Life Cycle What feature defines most plant life cycles?   The life cycle of land plants has two alternating phases, a diploid (2N) phase and a haploid (N) phase.

The Plant Life Cycle The life cycle of land plants has two alternating phases, a diploid (2N) phase and a haploid (N) phase.   The shift between the haploid phase and the diploid phase is known as the alternation of generations, as shown in the figure.

The Plant Life Cycle   The multicellular diploid phase is known as the sporophyte, or spore-producing plant.

The Plant Life Cycle The multicellular haploid phase is known as the gametophyte, or gamete-producing plant.

The Plant Life Cycle A sporophyte produces haploid spores through meiosis. These spores grow into multicellular structures called gametophytes.

The Plant Life Cycle Each gametophyte produces reproductive cells called gametes—sperm and egg cells.   During fertilization, a sperm and egg fuse with each other, producing a diploid zygote that develops into a new sporophyte.

Trends in Plant Evolution An important trend in plant evolution is the reduction in size of the gametophyte and the increasing size of the sporophyte.

Lesson Overview 22.2 Seedless Plants

Green Algae What are the characteristics of green algae?   Green algae are mostly aquatic. They are found in fresh and salt water, and in some moist areas on land. Algae are not a single group of organisms. “Algae” applies to any photosynthetic eukaryote other than a land plant. “Green algae” are classified with plants.

The First Plants Green algae are mostly aquatic. They are found in fresh and salt water, and in some moist areas on land.   Ancient green algae shared the ocean floor with corals and sponges. Green algae absorb moisture and nutrients directly from their surroundings and do not contain the specialized tissues found in other plants.

Life Cycle Many green algae switch back and forth between haploid and diploid phases. However, some may not alternate with each and every generation.   In the life cycle of Chlamydomonas, as long as living conditions are suitable, the haploid cell reproduces asexually by mitosis.

Life Cycle If conditions become unfavorable, Chlamydomonas can switch to a stage that reproduces sexually. Its cells release gametes that fuse into a diploid zygote (a sporophyte). The zygote has a thick protective wall, permitting survival in harsh conditions.  

Life Cycle The zygote begins to grow once conditions become favorable. It divides by meiosis to produce four haploid cells that swim away, mature, and reproduce asexually.

Multicellularity Green algae can form colonies. Spirogyra forms long threadlike colonies called filaments. Volvox colonies consist of as few as 500 to as many as 50,000 cells arranged to form hollow spheres.   Volvox shows some cell specialization and straddles the fence between colonial and multicellular life.

Mosses and Other Bryophytes What factor limits the size of bryophytes?   Bryophytes are small because they lack vascular tissue.

Mosses and Other Bryophytes Mosses have a waxy, protective coating that makes it possible for them to resist drying, and thin filaments known as rhizoids that anchor them to the soil. Rhizoids also absorb water and minerals from the soil.

Mosses and Other Bryophytes Mosses, hornworts, and liverworts all belong to a group of plants known as bryophytes.   Bryophytes have specialized reproductive organs enclosed by other, non-reproductive cells. Bryophytes show a higher degree of cell specialization than do the green algae and were among the first plants to become established on land.

Why Bryophytes Are Small Bryophytes do not make lignin, a substance that hardens cell walls, and do not contain true vascular tissue. Because of this, bryophytes cannot support a tall plant body against the pull of gravity.

Life Cycle Bryophytes display alternation of generations.   The gametophyte is the dominant, recognizable stage of the life cycle and the stage that carries out most of the photosynthesis. The sporophyte is dependent on the gametophyte for its supply of water and nutrients.

Life Cycle Bryophytes produce sperm cells that swim using flagella.   For fertilization to occur, the sperm must swim to an egg. Because of this, bryophytes must live in habitats where open water is available at least part of the year.

Gametophyte When a moss spore lands in a moist place, it sprouts and grows into a young gametophyte. The gametophyte forms rhizoids that grow into the ground and shoots that grow into the air.

Gametophyte Sperm and egg cells fuse to produce a diploid zygote.

Sporophyte The zygote marks the beginning of the sporophyte stage of the life cycle.

Sporophyte A sporophyte grows within the body of the gametophyte, depending on it for water and nutrients.

Sporophyte Eventually the sporophyte grows out of the gametophyte and develops a long stalk ending in a capsule called the sporangium.

Haploid spores are produced inside the capsule by meiosis and are released when the capsule ripens and opens.

Vascular Plants How is vascular tissue important?   Vascular tissue—xylem and phloem—make it possible for vascular plants to move fluids through their bodies against the force of gravity.

Vascular Plants About 420 million years ago, plants for the first time were able to grow high above the ground.   Fossil evidence shows these plants were the first to have a transport system with true vascular tissue. Vascular tissue carries water and nutrients much more efficiently than does any tissue found in bryophytes.

Evolution of a Transport System Vascular plants are known as tracheophytes, after a specialized type of water-conducting cell they contain. These cells, called tracheids, are hollow tubelike cells with thick cell walls strengthened by lignin.   Tracheids are found in xylem, a tissue that carries water upward from the roots to every part of a plant.

Evolution of a Transport System Vascular plants also have a second transport tissue called phloem that transports solutions of nutrients and carbohydrates produced by photosynthesis.

Seedless Vascular Plants Among the seedless vascular plants alive today are three phyla commonly known as club mosses, horsetails, and ferns. The most numerous of these are the ferns.

Life Cycle In the life cycle of a fern, spores produced by the sporophyte grow into thin, heart-shaped haploid gametophytes. The gametophytes grow independently of the sporophyte.

Life Cycle Sperm and eggs are produced on the gametophytes in antheridia and archegonia.   Fertilization requires at least a thin film of water, so that the sperm can swim to the eggs.

Life Cycle The diploid zygote produced by fertilization develops into a new sporophyte plant. This is the dominant stage of the fern life cycle.   Haploid spores develop on the undersides of the fronds in sporangia, and the cycle begins again

Lesson Overview 22.3 Seed Plants

The Importance of Seeds What adaptations allow seed plants to reproduce without open water?   Adaptations that allow seed plants to reproduce without open water include a reproductive process that takes place in cones or flowers, the transfer of sperm by pollination, and the protection of embryos in seeds.

The Importance of Seeds A seed is a plant embryo and a food supply, encased in a protective covering. The living plant within a seed is diploid and represents the early developmental stage of the sporophyte phase of the plant life cycle.  

Cones and Flowers In seed plants, the male gametophytes and the female gametophytes grow and mature directly within the sporophyte. The gametophytes usually develop in reproductive structures known as cones or flowers.   Nearly all gymnosperms bear their seeds directly on the scales of cones. The word gymnosperm means “naked seed.” Gymnosperms produce seeds that are exposed on the scales within cones. Flowering plants, or angiosperms, bear their seeds in flowers inside a layer of tissue that protects the seed.

Pollen In seed plants, the entire male gametophyte is contained in a tiny structure called a pollen grain.   Pollen grains are carried to the female reproductive structure by wind or animals such as insects. The transfer of pollen from the male reproductive structure to the female reproductive structure is called pollination.

Seeds After fertilization, the zygote contained within a seed grows into a tiny plant—the sporophyte embryo.   A tough seed coat surrounds and protects the embryo and keeps the contents of the seed from drying out. The embryo begins to grow when conditions are right. It does this by using nutrients from the stored food supply until it can carry out photosynthesis on its own.

The Life Cycle of a Gymnosperm How does fertilization take place in gymnosperms in the absence of water?   In gymnosperms, the direct transfer of pollen to the female cone allows fertilization to take place without the need for open water.

Pollen Seeds and Seed Cones Reproduction in conifers takes place in cones, which are produced by the mature sporophyte plant.

Pollen Seeds and Seed Cones Pollen cones produce pollen grains, which make up the entire male gametophyte stage.

Pollen Seeds and Seed Cones One of the haploid nuclei in the pollen grain will divide later to produce two sperm nuclei.

Pollen Seeds and Seed Cones Seed cones produce female gametophytes and are generally much larger than pollen cones.

Pollen Seeds and Seed Cones   Near the base of each scale are two ovules, where the female gametophytes develop.

Pollen Seeds and Seed Cones Within the ovules, meiosis occurs producing haploid cells that grow and divide into female gametophytes. Each gametophyte contains a few large egg cells.

Pollination and Fertilization In the spring, pollen cones release pollen grains that are carried away by the wind,.   Some pollen grains are caught in a sticky secretion on the scales of female cones and are pulled inside towards the ovule. This direct transfer of pollen to the female cone allows fertilization to take place without the need for open water.

Development Inside Seeds If a pollen grain lands near an ovule, the grain splits open and begins to grow a structure called a pollen tube, which contains two haploid sperm nuclei. Once the pollen tube reaches the female gametophyte, one sperm nucleus disintegrates; the other fertilizes the egg.

Development Inside Seeds Fertilization produces a diploid zygote, which grows into an embryo.

Development Inside Seeds A seed is an encased embryo that is ready to be scattered by the wind and grow into a plant.

Lesson Overview 22.4 Flowering Plants

THINK ABOUT IT Flowering plants are by far the most abundant organisms in the plant kingdom, and yet they evolved much more recently than did other seed plants.   What features of these plants enabled them to take Earth by storm? What are the secrets of their success?

Flowers and Fruits What are the key features of angiosperm reproduction?

Flowers and Fruits What are the key features of angiosperm reproduction?   Angiosperms reproduce sexually by means of flowers. After fertilization, ovaries within flowers develop into fruits that surround, protect, and help disperse the seeds.

Flowers and Fruits The origin of flowering plants is the most recent among the origins of all plant phyla.   Flowering plants originated on land and soon came to dominate Earth’s plant life.

Flowers and Fruits Angiosperms develop unique reproductive organs known as flowers, shown in the figure.   Flowers contain ovaries, which surround and protect seeds.

Advantages of Flowers Flowers are an evolutionary advantage to plants because they attract animals that carry pollen with them to the next flower they visit. This means of pollination is much more efficient than the wind pollination of most gymnosperms.

Advantages of Fruits After pollination, the ovary develops into a fruit, a structure containing one or more matured ovaries. The wall of the fruit helps disperse the seeds contained inside it. The development of the multiple ovaries of a blackberry flower into the cluster of fruits that make up one berry is shown.

Advantages of Fruits   When an animal eats a fleshy fruit, seeds from the fruit enter the animal’s digestive system. By the time the seeds leave the digestive system, the animal may have traveled many kilometers. By using fruit, flowering plants increase the ranges they inhabit.

Angiosperm Classification For many years, flowering plants were classified according to the number of seed leaves, or cotyledons, in their embryos. Those with one seed leaf were called monocots. Those with two seed leaves were called dicots.

Angiosperm Classification Scientific classification places the monocots into a single group but places the dicots in different categories.

Angiosperm Diversity How are different angiosperms conveniently categorized?

Angiosperm Diversity  How are different angiosperms conveniently categorized?   Angiosperms are often grouped according to the number of their seed leaves, the strength and composition of their stems, and the number of growing seasons they live.

Monocots and Dicots The differences between monocots and dicots include the distribution of vascular tissue in stems, roots, and leaves, and the number of petals per flower.

Monocots and Dicots The characteristics of monocots and dicots are compared in the table below.

Woody and Herbaceous Plants Flowering plants can be subdivided into groups according to the characteristics of their stems.   Woody plants are made primarily of cells with thick cell walls that support the plant body. Herbaceous plants have stems that are smooth and nonwoody.

Annuals, Biennials, and Perennials: Characteristics and Examples