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Introduction to Plants What is a plant? Plants are multicellular eukaryotes that have cell walls made of cellulose. Plants develop from multicellular embryos and carry out photosynthesis using the green pigments chlorophyll a and b. Plants include trees, shrubs, and grasses, as well as other organisms, such as mosses and ferns. Most plants are autotrophs, although a few are parasites or saprobes that live on decaying materials. The Plant Life Cycle Plant life cycles have two alternating phases, a diploid (2N) phase and a haploid (N) phase, known as alternation of generations. Alternation of Generations During the two phases of the life cycle, mitosis and meiosis alternate to produce the two types of reproductive cells—gametes and spores. The diploid (2N) phase is called the sporophyte, or sporeproducing plant. The haploid (N) phase is called the gametophyte, or gamete-producing plant. Plant spores are haploid (N) reproductive cells formed in the sporophyte by meiosis. The spores can grow into new organisms called gametophytes. A gamete is a reproductive cell produced by mitosis, and it can fuse with another gamete to produce the sporophyte. What do plants need to survive? In order to survive, plants need: • sunlight • water and minerals • gas exchange • transport of water and nutrients throughout the plant body Sunlight Plants use energy from sunlight to carry out photosynthesis. Photosynthetic organs such as leaves are broad and flat to maximize light absorption. Water and Minerals All cells require a constant supply of water. Water is used up quickly when the sun is shining. As a result, plants have structures that limit water loss. As they absorb water, plants also absorb minerals. Minerals are nutrients in the soil needed for plant growth. Gas Exchange Plants require oxygen to support cellular respiration as well as carbon dioxide to carry out photosynthesis. They must exchange these gases with the atmosphere without losing excessive amounts of water through evaporation. Movement of Water and Nutrients Plants take up water and minerals through their roots, but they make food in their leaves. Most plants have specialized tissues that carry water and nutrients from the soil and distribute products of photosynthesis throughout the plant body. Simpler plants carry out these functions by diffusion. Early Plants When plants first appeared, life on Earth changed. As plants colonized the land, they changed the environment so other organisms could develop. New ecosystems arose, and organic matter began to form soil. The first plants evolved from an organism similar to the multicellular green algae living today. Multicellular green algae have the size, color, and appearance of plants. They have reproductive cycles similar to those of plants. Green algae also have cell walls and photosynthetic pigments that are identical to those of plants. The First Plants DNA sequences confirm that plants are closely related to certain groups of green algae, suggesting that the ancestors of the first plants were indeed algae. The oldest known plant fossils, about 450 million years old, are similar to today’s mosses. They had a simple structure and grew close to the ground. Fossils suggest that the first plants needed water to complete their life cycles. The demands of life on land favored the evolution of plants that were: • more resistant to the drying rays of the sun. • more capable of conserving water. • more capable of reproducing without water. From these plants, several major groups of plants evolved. • One group developed into the mosses and their relatives. • Another group gave rise to all other plants. All plants have evolved different adaptations for a variety of terrestrial environments. Overview of the Plant Kingdom Plants are divided into four groups based on these features: • water-conducting tissues • seeds • flowers Plants are also classified by other features, including reproductive structures and body plan. Evolutionary Relationships Among Plants Today, scientists can classify plants more precisely by comparing the DNA sequences of various species. Bryophytes Mosses and their relatives are called bryophytes, or nonvascular plants. They do not have vascular tissues, or specialized tissues that conduct water and nutrients. What adaptations of bryophytes enable them to live on land? Bryophytes have life cycles that depend on water for reproduction. Bryophytes draw up water by osmosis only a few centimeters above the ground. During one stage of their life cycle, bryophytes produce sperm that swim through water to reach eggs of other individuals. Therefore, bryophytes must live where there is rainfall or dew for part of the year. Groups of Bryophytes Bryophytes are low-growing plants found in moist, shaded areas. What are the three groups of bryophytes? The three groups of bryophytes are: • mosses • liverworts • hornworts The Structure of a Moss Each moss plant has a shoot that looks like a stem with leaves. These are not true stems or leaves, because they do not contain vascular tissue. When mosses reproduce, they produce thin stalks, each containing a capsule. This is the sporophyte stage. The “leaves” of mosses are one cell thick, so they lose water quickly if the surrounding air is dry. Mosses have rhizoids, which are long cells that anchor them in the ground and absorb water and minerals from the soil. Water moves through rhizoids and into the rest of the plant. Liverworts Liverworts’ gametophytes form broad, thin structures that draw up moisture from the soil surface. Mature gametophytes produce structures that look like tiny green umbrellas. These carry the structures that produce eggs and sperm. Some liverworts can reproduce asexually by means of gemmae. Gemmae are small multicellular reproductive structures. In some species, gemmae form in gemma cups. When washed out of the cup, the gemmae can divide by mitosis to produce a new individual. Hornworts Hornworts are found only in soil that is damp nearly yearround. Their gametophytes look like those of liverworts. The hornwort sporophyte looks like a tiny green horn. How do bryophytes reproduce? Bryophytes reproduce and develop by alternation of generations. The gametophyte is the dominant stage of the life cycle and is the stage that carries out most of the plant's photosynthesis. Life Cycle of a Moss The life cycle of a moss illustrates how bryophytes reproduce and develop. Life Cycle of a Bryophyte When a spore lands in a moist place, it germinates and grows into a mass of tangled green filaments called a protonema. As the protonema grows, rhizoids grow into the ground and shoots grow into the air. These shoots grow into green moss plants, which are the gametophyte stage of its life cycle. Gametes form in structures at the tips of the gametophytes. Sperm are produced in antheridia, the male reproductive structure. Eggs are produced in archegonia, the female reproductive structure. Some species produce both sperm and eggs on the same plant. Fertilization produces a diploid zygote. The zygote grows directly from the gametophyte and depends on it for water and nutrients. The mature sporophyte is a long stalk ending in a capsule. Inside the capsule, haploid spores are produced by meiosis. When the capsule ripens, it opens and spores are scattered. Human Use of Mosses Sphagnum mosses thrive in the acidic water of bogs. Dried sphagnum acts as a natural sponge. It can accumulate to form peat deposits. Peat can be cut from the ground and used as fuel. Peat can be used to improve the soil’s ability to retain water and to increase soil acidity. Seedless Vascular Plants 420 million years ago, mosslike plants on land were joined by taller plants. Evidence shows that these plants had vascular tissue, which is specialized to conduct water and nutrients throughout the plant. Evolution of Vascular Tissue The first vascular plants contained tracheids which are cells specialized to conduct water. Tracheids make up xylem, a transport subsystem that carries water from the roots to every part of a plant. Tracheids are hollow with thick cell walls that resist pressure. They connect end to end to allow water to move efficiently. Vascular plants have a second transport subsystem composed of vascular tissue called phloem. Phloem transports solutions of nutrients and carbohydrates produced by photosynthesis. How is vascular tissue important to ferns and their relatives? Both xylem and phloem can move fluids through the plant body, even against the force of gravity. Together xylem and phloem move water, nutrients, and other materials throughout the plant. In many plants, xylem and lignin (a substance that makes cell walls rigid) enable them to grow upright and tall. What are the characteristics of the three phyla of seedless vascular plants? Seedless vascular plants include: • club mosses • horsetails • ferns Ferns and Their Relatives The most numerous phylum is the ferns. Ferns and their relatives have true roots, leaves, and stems. Roots are underground organs that absorb water and minerals. Leaves are photosynthetic organs that contain one or more bundles of vascular tissue. Tissue is gathered into veins made of xylem and phloem. Stems are supporting structures that connect roots and leaves, carrying water and nutrients between them. Club Mosses Ancient club mosses grew into trees and produced forests. Fossilized remains of these exist today as huge beds of coal. Today, club mosses are small plants that live in moist woodlands. Horsetails The only living genus of Arthrophyta is Equisetum. Equisetum has true leaves, stems, and roots. Equisetum is called horsetail, or scouring rush. Ferns Ferns probably evolved 350 million years ago, when club moss forests covered Earth. Ferns thrive in wet areas with little light. Life Cycle of Ferns Ferns have vascular tissues, strong roots, underground stems called rhizomes, and leaves called fronds. What are the stages in the life cycle of ferns? Life Cycle of Ferns Ferns and other vascular plants have a life cycle in which the diploid sporophyte is the dominant stage. Fern sporophytes develop haploid spores on the underside of their fronds in structures called sporangia. Sporangia are grouped into clusters called sori. The Underside of a Fern Frond Fern Life Cycle When the spores germinate, they develop into haploid gametophytes. The gametophyte first grows a set of rootlike rhizoids. It then flattens into a mature gametophyte. The gametophyte grows independently of the sporophyte. The antheridia and archegonia are found on the underside of the gametophyte. Fertilization requires water, which allows the sperm to swim to the eggs. The diploid zygote formed by fertilization, develops into a new sporophyte. As the sporophyte grows, the gametophyte withers away. Fern sporophytes often live for many years. In some species, fronds die in the fall, but rhizomes live through the winter and produce new leaves in the spring. Seed Plants Seed plants are the most dominant group of photosynthetic organisms on land. Seed plants are divided into two groups: • Gymnosperms bear seeds directly on the surfaces of cones. • Angiosperms, or flowering plants, bear seeds within a layer of tissue that protects the seed. Gymnosperms include conifers, cycads, ginkgoes, and gnetophytes. Angiosperms include grasses, flowering trees and shrubs, and all species of flowers. Reproduction Free From Water Seed plants have a life cycle that alternates between a gametophyte stage and a sporophyte stage. They do not need water for fertilization of gametes. Seed plants can live just about anywhere. What adaptations allow seed plants to reproduce without standing water? Adaptations that allow seed plants to reproduce without water include: • flowers or cones • the transfer of sperm by pollination • the protection of embryos in seeds Cones and Flowers Gametophytes grow within sporophytes called cones, which are the seed-bearing structures of gymnosperms, and flowers, which are the seed-bearing structures of angiosperms. Gametophyte generations live inside these structures. Pollen The male gametophyte is contained in a tiny structure called a pollen grain. Sperm do not need water to fertilize eggs; instead the pollen grain is carried to the female reproductive structure by wind, insects, or small animals. This transfer of pollen is called pollination. Seeds A seed is an embryo of a plant that is encased in a protective covering and surrounded by a food supply. An embryo is an organism in its early stage of development. The seed coat surrounds and protects the embryo and keeps contents of the seed from drying out. Internal Structures of a Seed Seeds may have special tissues or structures that aid in their dispersal to other habitats. • Some seed coats stick to the fur or feathers of animals. • Other seeds are within tissues eaten and dispersed by animals. After fertilization, the zygote grows into a plant—the embryo. The embryo can stop growing while it is within the seed, and it can remain this way for a long time. When it grows, it uses nutrients from the stored food supply. Seeds can survive extreme cold or heat, or even drought. Evolution of Seed Plants The fossil record indicates that ancestors of seed plants evolved adaptations enabling them to survive where most mosses and ferns could not. The most important of these adaptations was the seed itself. A seed can survive dry conditions and extreme temperatures. What are the four groups of gymnosperms? Gymnosperms—Cone Bearers The four groups of gymnosperms are: • gnetophytes • cycads • ginkgoes • conifers Gnetophytes About 70 present-day species of the phylum Gnetophyta are known, placed in just three genera. Reproductive scales of these plants are clustered into cones. Cycads Cycads are palmlike plants that reproduce with large cones. They first appeared during the Triassic, 225 million years ago. Today, only nine genera of cycads exist. They grow naturally in tropical and subtropical places. Ginkgoes Today the phylum Ginkgophyta contains only one species, Ginkgo biloba. The living Ginkgo species looks like its fossil ancestors. Ginkgo trees are planted in U.S. cities because of their resistance to air pollution. Conifers Conifers are the most common gymnosperms, with more than 500 known species. Conifers include pines, spruces, firs, cedars, sequoias, redwoods, junipers, and yews. Ecology of Conifers Conifer leaves have specific adaptations to dry conditions. Most developed long, thin leaves, which reduce evaporation. Their leaves have a thick, waxy layer. Most conifers are “evergreens” and retain leaves all year. Angiosperms—Flowering Plants Flowers and Fruits The majority of living plant species are flowering plants, or angiosperms. What are the characteristics of angiosperms? Flowers and Fruits Angiosperms develop unique reproductive organs known as flowers. Flowers are an evolutionary advantage because they attract animals, which then transport pollen from flower to flower. Flowers contain ovaries, which surround and protect the seeds. After pollination, the ovary develops into a fruit. A fruit is a wall of tissue that surrounds a seed. A fruit protects the seed and aids in its dispersal. Diversity of Angiosperms Angiosperms are categorized in many ways: • monocots and dicots • woody and herbaceous plants • annuals, biennials, and perennials An angiosperm can belong to more than one category. Diversity of Angiosperms Monocots and Dicots There are two classes within the angiosperms—monocots and dicots. What are monocots and dicots? Monocots and dicots are named for the number of seed leaves, or cotyledons, in the plant embryo. Monocots have one seed leaf, and dicots have two. A cotyledon is the first leaf or the first pair of leaves produced by the embryo of a seed plant. Monocots include corn, wheat, lilies, orchids, and palms. Dicots include roses, clover, tomatoes, oaks, and daisies. Woody and Herbaceous Plants Flowering plants are also categorized by the woodiness of the stem. Woody plants are made primarily of cells with thick cell walls that support the plant body. Woody plants include trees, shrubs, and vines. Plant stems that are smooth and nonwoody are characteristic of herbaceous plants. Herbaceous plants do not produce wood as they grow. Herbaceous plants include dandelions, zinnias, and petunias. Plant life span is determined by genetic and environmental factors. What are the three categories of plant life spans? Annuals, Biennials, and Perennials There are three categories of plant life spans: annual, biennial, and perennial. Annuals are plants that complete a life cycle in one growing season. Biennials complete their life cycle in two years. In the first year, they germinate and grow roots, short stems, and sometimes leaves. In the second year, they grow new stems and leaves, produce flowers and seeds, and die. Perennials live for more than two years.