Plant Diversity Chapter 22 What Is A Plant

Plant Diversity Chapter 22

What Is A Plant? • Multicellular eukaryotes • Cell walls made of cellulose • Multicellular embryos • Carry out photosynthesis using green pigments chlorophyll a and b • “Stationary animals that eat sunlight”

The Plant Life Cycle • Alternation of generations • Haploid gametophyte – Gamete-producing phase of an organism • Diploid sporophyte – Spore-producing phase of an organism • To be terrestrial, plants must be able to reproduce in dry environments where there is no water through which gametes can move from plant to plant

What Plants Need To Survive • Sunlight • Water and minerals • Gas exchange • Movement of water and nutrients

Early Plants • For most of Earth’s history plants did not exist • Life was concentrated in the oceans, lakes and streams • When plants appeared, much of the existing life on Earth changed • Green algae: the first plants evolved from an organism much like the multicellular green algae living today

The First Plants • Depended on water to complete their life cycles • Life on land favored the evolution of plants: – More resistant to the drying rays of the sun – More capable at conserving water – More capable of reproducing on dry land

The First Plants • Like today’s mosses: simple in structure and close to the ground • Gave rise to the different lineages of plants we have today!

Overview of the Plant Kingdom • Bryophytes • Seedless Vascular Plants • Gymnosperms • Angiosperms

Overview of the Plant Kingdom • Four groups based on: – Water-conducting tissues – Seeds – Flowers • Today scientists can classify plants more precisely by comparing DNA sequences of various species

BRYOPHYTES

Bryophytes • Mosses, liverworts and hornworts

Bryophytes • Life cycle depends on water for reproduction • Sperm swim through water to reach eggs of other plants • Lack vascular tissue • Can draw up water by osmosis only a few cm above the ground

Mosses • The most common bryophytes • Grow most abundantly in areas with water-swamps and bogs, near streams and in rainforests • Well adapted to wet habitats and nutrient-poor soils • Can tolerate low temperatures…they are the most abundant plants in polar regions

Mosses • Moss plants you see are gametophytes growing close together • When mosses reproduce, they produce thin stalks with a sporophyte • They do not have true roots; they have rhizoids • Rhizoids: long, thin cells that anchor them in the ground absorb water and minerals from the surrounding soil

Liverworts • Look like flat leaves attached to the ground • Some resemble the shape of a liver • Gametophyte stage: broad and thin structures that draw up moisture from the surface of the soil • Mature gametophyte stage: produces “umbrellas” that carry the structures to produce eggs and sperm

Liverworts • Sexual reproduction by means of gemmae • Gemmae: small, multicellular sphere that contain haploid cells • Produced in cuplike structures called gemma cups • When washed out of the gemma, these cells divide by mitosis and produce a new individual

Hornworts • Generally found in soil that is damp nearly year round • Gametophyte looks like those of liverworts • The sporophyte looks like a tiny green horn

Life Cycle of Bryophytes • Gametophyte = Dominant • The gametophyte carries out most of the plant’s photosynthesis • Dependence on water • Sperm of the bryophyte must swim to an egg for fertilization to occur

Example: Life Cycle of a Moss • Protonema: mass of tangles green filaments formed when a moss spore lands in a moist place, germinates and grow • Rhizoids grow into the ground and shoot grow into the air • The shoots become the familiar green moss plants, the gametophyte stage of the life cycle

Life Cycle of A Moss • Gametes are formed in the tips of the gametophyte • Antheridia: male reproductive structure in which sperm are produced • Archegonia: female reproductive structure in which eggs are produced

Life Cycle of A Moss • Some species have both sperm and egg on the same plant; other do not • A diploid zygote results from fertilization • The zygote = the sporophyte • Grows out of the gametophyte and depends on it for water and nutrients

Human Use of Mosses • Sphagnum mosses are a group of mosses that thrive in the acidic water bogs • Dried sphagnum moss absorbs many times its own weight in water and acts as a natural sponge • In certain environments the dead remains of sphagnum accumulate to form thick deposits of peat

Human Use of Mosses • Peat can be cut from the ground and burned as fuel • Peat moss has low p. H and increases soil acidity • Azaleas grow well only if planted in acidic soil

SEEDLESS VASCULAR PLANTS

Seedless Vascular Plants • Club mosses, horsetails, and ferns • Vascular tissue: a type of tissue that is specialized to conduct water and nutrients through the body of the plant • Tracheid: hollow plant cell in xylem tissue with thick cell walls that resist pressure

Seedless Vascular Plants • The xylem and phloem can move fluids throughout the plant body, even against the force of gravity • Lignin: substance made by vascular plants that makes cell walls rigid • Allows vascular plants to grow upright and reach great heights

Structure of Xylem: vascular tissue that carries water upward from the roots to every part of the plant

Structure of Phloem: vascular tissue responsible for the transport of nutrients and the carbohydrates produced by photosynthesis

Seedless Vascular Plants • Roots: absorb water and minerals • Leaves: photosynthetic organisms that contain one or more bundles of vascular tissue • Stems: supporting structures that connect roots and leaves, carrying water and nutrients between them

Club Mosses • Once ancient club mosses grew into huge trees-up to 35 meters tall • Some produced the Earth’s first forests • Today club mosses are small plants • Live in moist woodlands and near streambeds and marshes • “Ground Pine” …some look like miniature pine trees

Horsetails • True leaves, stems and roots • Leaves arranged in whorls at joints along the stem • Stem looks similar to horses’ tails • Stem contains crystals of abrasive silica • Horsetails were commonly used to scour pots and pans during Colonial times

Ferns • Probably evolved about 350 mya when great club moss forests covered the ancient Earth • Ferns have survived Earth’s long history in numbers greater than any other group of sporebearing vascular plants • More than 11, 000 species live today

Ferns • Have truly vascular tissues • Strong roots • Rhizomes: creeping or underground stems • Fronds: large leaves • Can thrive in areas with little light • Most abundant in wet or seasonably wet habitats

Example: Life Cycle of Ferns • Sporophyte = Dominant • Fern sporophytes produce haploid spores on the underside of their fronds in sporangia • Spores released from sporangia may be carried by wind and water over long distances

Life Cycle of Ferns • When the spores germinate they develop into haploid gametophytes • The diploid zygote is produced from fertilization • This sporophyte grows and the gametophyte withers away • Sporophytes often live for many years

SEED PLANTS

Seed Plants • Seeds include: – Acorns, pine nuts, dandelion seeds, kernels of corn, etc. • Seed plants are divided into two groups: – Gymnosperms – Angiosperms

Seed Plants • Reproduction free from water • As a result, seed plants can live just about anywhere • Adaptations to reproduce without water: – Flowers or cones – Transfer of sperm by pollination – Protection of embryos in seeds

Cones and Flowers • Gametophyte of seed plants grow and mature within sporophyte structures called cones (gymnosperms) and flowers (angiosperms)

Pollen • The entire male gametophyte is contained in a tiny structure called a pollen grain • The wind, insects, birds and small mammals may carries pollen grains • Pollination: the transfer of the pollen grain to the female gametophyte

Seeds • Embryo of a plant encased in a protective covering and surrounded by a food supply

Evolution of Seed Plants • The seed was very important evolutionarily • Mosses and ferns underwent major adaptive radiation 300 -400 mya • Land environments were much wetter than they are today

Evolution of Seed Plants • As the land became drier, it became harder for seedless plants to survive • Fossils of seed-bearing plants exist from almost 360 mya • The original seed plants resembled ferns

GYMNOSPERMS… CONE BEARERS

Gymnosperms…Cone Bearers • The most ancient surviving seed plants • Include gnetophytes, cycads, ginkgoes, and conifers • “Naked Seed”: these plants all reproduce with seeds that are exposed

Gymnosperms…Cone Bearers • Gnetophytes • Cycads • Ginkgoes • Conifers

Gnetophytes • The reproductive scales are clustered into cones • 70 present-day species are known

Cycads • Palm-like plants that reproduce with large cones • First appeared during the Triassic Period, 225 mya • Huge forests of cycads thrived when the dinosaurs roamed Earth • Only nine genera exist today

Ginkgoes • Were common when dinosaurs were alive • Today there is only one species, Ginkgo biloba • May be one of the oldest seed plant species alive today • Ginkgo trees were carefully cultivated in China where they were planted around temples • They are tough and resistant to air pollution-thus they are popular shade trees