Lesson Overview Seedless Plants CHAPTER 22 Lesson Overview

Lesson Overview Seedless Plants CHAPTER 22 Lesson Overview 22. 2 Plant Diversity

Lesson Overview Seedless Plants

Lesson Overview Seedless Plants An Overview of the Plant Kingdom • Botanists divide the plant kingdom into the following major groups based on four important features: • • Embryo formation Specialized water-conducting tissues Seeds Flowers

Lesson Overview Seedless Plants The First Plants • 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. • 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.

Lesson Overview Seedless Plants 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.

Lesson Overview Seedless Plants 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.

Lesson Overview Seedless Plants 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.

Lesson Overview Seedless Plants 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. • Bryophytes produce sperm cells that swim using flagella (a slender threadlike/whiplike appendage that enables swimming). • 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.

Lesson Overview Seedless Plants Gametophyte Phase • 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’ (stems) that grow into the air. • Gametes are formed in reproductive structures at the tips of the gametophytes. *Eggs are produced in archegonia *Sperm are produced in antheridia • Sperm and egg cells fuse to produce a diploid zygote. Sporophyte Phase • The zygote marks the beginning of the sporophyte stage of the life cycle. • A sporophyte grows within the body of the gametophyte, depending on it for water and nutrients. • 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.

Lesson Overview Seedless Plants

Lesson Overview Seedless Plants 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.

Lesson Overview Seedless Plants Evolution of a Transport System • Vascular plants are known as tracheophytes, after a specialized type of waterconducting 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. • Tracheids are connected end to end like a series of tin cans, as shown in the figure. Openings between tracheids, known as pits, allow water to move through a plant more efficiently than by diffusion alone. • Vascular plants also have a second transport tissue called phloem that transports solutions of nutrients and carbohydrates produced by photosynthesis.

Lesson Overview Seedless Plants

Lesson Overview Seedless Plants 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. • Ferns have true vascular tissues, strong roots, creeping or underground stems called rhizomes, and large leaves called fronds. • Ferns can thrive in areas with little light and are most abundant in wet habitats.

Lesson Overview Seedless Plants 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.

Lesson Overview Seedless Plants 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 Seedless Plants 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. • Today’s seed plants are all descended from common ancestors. • The fossil record indicates that ancestors of seed plants evolved new adaptations that enabled them to survive on dry land.

Lesson Overview Seedless Plants 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. • Flowering plants, or angiosperms, bear their seeds in flowers inside a layer of tissue that protects the seed.

Lesson Overview Seedless Plants 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.

Lesson Overview Seedless Plants 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.

Lesson Overview Seedless Plants The Life Cycle of a Gymnosperm • The word gymnosperm means “naked seed. ” • Gymnosperms produce seeds that are exposed on the scales within cones.

Lesson Overview Seedless Plants Pollen Seeds and Seed Cones • Reproduction in conifers (cone-bearing seed plants) takes place in cones, which are produced by the mature sporophyte plant.

Lesson Overview Seedless Plants Pollen Seeds and Seed Cones • Pollen cones produce pollen grains, which make up the entire male gametophyte stage. • One of the haploid nuclei in the pollen grain will divide later to produce two sperm nuclei.

Lesson Overview Seedless Plants Pollen Seeds and Seed Cones • Seed cones produce female gametophytes and are generally much larger than pollen cones. • Near the base of each scale are two ovules, where the female gametophytes develop. *Within the ovules, meiosis occurs producing haploid cells that grow and divide into female gametophytes. Each gametophyte contains a few large egg cells.

Lesson Overview Seedless Plants Pollination and Fertilization • In the spring, pollen cones release pollen grains that are carried away by the wind, as shown in the figure. • 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.

Lesson Overview Seedless Plants 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. • Fertilization produces a diploid zygote, which grows into an embryo. • A seed is an encased embryo that is ready to be scattered by the wind and grow into a plant.