Plant Evolution and Classification

Overview of Plants

Section 1

• Plants dominate land and water • Have lots of diversity • Some less than 1 mm in length • Some more than 100 m tall • 12 phyla (divisions) have more than 270,000

species • Some live a few weeks • Some live for thousands of years

Adapting to Land�Life in ocean more than 3 billion years

�Life on land only 430 million years ago when ozone formed

�Ozone protected living things from sun’s radiation

Preventing Water Loss�Move to land from water provided advantages �More exposure to sunlight (photosynthesis) �Increased carbon dioxide levels �Larger supply of inorganic nutrients

�Also presented challenges �Drying out through evaporation

�Early adaptation to land: cuticle (KYOOT-ih-kuhl) à waxy protective covering on plant surfaces that prevents water loss

�Keeps water in the plant �Also keeps out CO2 �Plants that had small openings

(stomata) survived �Stomata allowed exchange of gases

Reproducing by Spores and Seeds

�Land plants also developed structures that helped protect reproductive cells from drying out

�Spore à haploid reproductive cell surrounded by hard outer wall

�Allow widespread dispersal of plant species �Eventually most plants developed seeds �Seed à embryo surrounded by protective

coat

• Seeds also contain endosperm à tissue that provides nourishment to developing plant

• Seeds are more effective at dispersal than spores

Transporting Materials Through Plant• Certain species evolved vascular (VAS-kyu-luhr) tissue à type of tissue that transports water and dissolved substances from one part of the plant to another

• Two types: • Xylem (ZIE-luhm) à carries water and inorganic

nutrients in one direction, from roots to stems/leaves • Phloem (FLOH-uhm) à carries organic compounds

in any direction, depending on plant’s needs

�Vascular tissue also helps support plant �Aquatic plants supported by water around them

• Some plants developed woody tissue à formed from several layers of xylem, usually concentrated around center of stem • Allowed them to grow taller • Woody stems usually brown and firm

�Nonwoody plants are called herbaceous à have soft, usually green stems

�b/c vascular tissue not surrounded by rigid wood, stems are flexible

Classifying Plants• 12 divisions of plants divided into 2 groups

• Nonvascular plants à no true vascular tissue, no true roots, stems, leaves

• Vascular plants à have vascular tissue and true roots, stems, leaves

• Vascular divided into seed plants or seedless plants • Seeds plants divided as gymnosperms or

angiosperms

The Fossil Record of Plants�A lot of what we know comes from

studying fossil record �It’s incomplete, but scientists think

plants evolved from algal ancestors

• Strongest evidence is similarities between green algae and plants • Both green algae and plants have the same

photosynthetic pigments – chlorophyll a and b • Both store energy as starch • Both have cell walls made of cellulose

Alternating Life Cycles• All plants have life cycles that involves 2 phases • First phase consists of diploid (2n) sporophyte

(SPOR-uh-fiet) plant that makes spores • Second phase – haploid (1n) gametophyte

(guh-MEET-uh-fiet) plant that makes eggs and sperm • 2 phases named for the type of reproductive cells

they produce

� This type of life cycle à alternation of generations

�Gametophyte (1n) makes structures that form gametes – eggs and sperm – by mitosis

�Once egg is fertilized by sperm and makes zygote, plants starts diploid phase of life cycle

�Zygote divides by mitosis to form sporophyte plant

�Sporophyte (2n) makes structures that go through meiosis to form haploid spores

�Spores released by most seedless plants but kept by seed plants �Life cycle begins again when spores divide to make new gametophytes

�In nonvascular plants the gametophyte is the dominant phase

�In vascular plants, sporophyte is dominant phase

• In seedless vascular plants, gametophyte usually separate small organisms different from sporophyte

• In seed plants, gametophyte is small parasite of sporophyte

Nonvascular PlantsSection 2

�3 phyla �Collectively called bryophytes �Botanists identified 16,600 species �Don’t have vascular tissue �Do not form true roots, stems, leaves �Usually grow on land near streams and rivers

Classifying Bryophytes

• Most primitive type of plants • Characteristics more closely related to plants than

algae • Mostly terrestrial • Have alternation-of-generations life cycle • Seedless • Produce spores • b/c no vascular tissue, usually small (1-2 cm)

�Bryophytes need water to reproduce sexually b/c sperm must swim to egg

�In dry areas, reproduce sexually only when enough moisture is available

�Asexual reproduction of haploid spores does not require water

Phylum Bryophyta“brie-AHF-uh-tuh”

�Thick green carpet of moss you see is actually thousands of tiny moss gametophytes

�Each gametophyte attached to soil by rootlike structures called rhizoids (RIE-zoidz)

�Rhizoids do not have vascular tissue (unlike roots)

�Do function like roots by anchoring moss and absorbing water and inorganic nutrients

�Moss gametophytes usually less than 3 cm tall

�Moss sporophyte attached to and dependent on gametophyte

�Gametophytes may be male, female, or both

�Mosses are pioneer plants b/c they are often the 1st species to inhabit an area

�Important environmental function

�Mosses gradually build up inorganic and organic matter on surface of rocks

�Creates a layer of soil for other plants to grow �Also prevent soil erosion by covering soil surface and

absorbing water

• Sphagnum (SFAG-nuhm) à ‘peat moss’ à genus of moss that is major part of peat bogs in northern parts of world • Peat moss consists mainly of decomposed

plant matter • Many parts of world mine and dry for use as

fuel

• Also makes acid that slows down decomposition in bogs • Widely used to enhance water-retaining ability of

potting and gardening soils • Also used by florists to pack bulbs and flowers for

shipping

Phylum Hepatophyta“hep-uh-TAHF-uh-tuh”

�Includes liverworts �Grow in moist, shady areas �Most have thin, transparent leaflike

structure arranged along stemlike axis

Some have thalloid (THAL-oid) à flat body with distinguishable upper and lower surfaces

�Thalloid is adaptation that allows them to absorb water readily

�In some species, gametophyte topped by umbrella-shaped structure that holds reproductive cells

Phylum Anthocerophyta“an-thoh-suh-RAHF-uh-tuh”

�Includes hornworts �Look like liverworts �Also live in moist, shady

areas �Share characteristic with

algae: each cell has single large chloroplast instead of many small ones

Vascular Plants

Section 3

�Vascular plants have specialized conducting tissues (xylem and phloem) that transport water and substances

�Can grow larger �Live in more environments than

nonvascular plants �Strong stems allow them to grow tall �Rise above other plants – more sunlight

Seedless Vascular Plants

�Dominated earth until about 200 million years ago

�4 phyla �First 3 phyla called “fern allies” �Last phylum are ferns

�Spores are mobile sexual reproductive parts of all seedless plants

Phylum Psilotophyta“sie-lah-TAHF-uh-tuh”

�Whisk ferns �Not really ferns at all �No roots or leaves �Make spores at ends of short

branches �This suggest they were early

land plants �Whisk ferns are epiphytes à plants that grow on other plants

�NOT considered parasites

Phylum Lycophyta“lie-KAHF-uh-tuh”

�Club mosses �Look like miniature

pine trees (also called ground pines)

�They make strobilus (STROH-bi-luhs) (cone) à cluster of sporangia-bearing modified leaves

�One member of Lycophyta is spike moss native to American Southwest

�Turns brown and curls up into a ball at night �When moistened, it uncurls and turns green

again

Phylum Sphenophyta“sfee-NAHF-uh-tuh”

�Horsetails �Have jointed

photosynthetic stems that contain silica

�Have scalelike leaves at each joint

�American pioneers used to scrub pots and pans

Phylum Pterophyta“tuhr-AHF-uh-tuh”�Diverse group of ferns �Some floating plant less than 1 cm across �Some reach 25 meters �Most have underground stem called rhizome �Some used as growing medium for orchids �Tightly coiled new leaves called fiddleheads �Fiddleheads uncoil to become fronds (leaves)

Vascular Seed Plants

�Mobile sexual reproductive part is multicellular seed

�Plants with seeds have greater chance of reproductive success than seedless plants

�Inside tough, protective outer coat is embryo and nutrient supply (endosperm)

�When conditions too hot, cold, wet, dry…the seed stays inactive

�When conditions are good for growth, the seed sprouts, or germinates à embryo begins to grow into a young plant (called seedling)

� 2 main groups �Gymnosperms à cone-bearing �Angiosperms à flower-bearing

� 4 phyla of gymnosperms make naked seeds � Seeds not enclosed and protected in fruits � Most gymnosperms are evergreen and bear seeds in cones �Cone à reproductive structure composed of hard scales � Seeds lie on surface of scales

�1 phylum of angiosperms

�Make seeds enclosed and protected in fruits

�Commonly called flowering plants

Phylum Cycadophyta“sie-kad-AHF-uh-tuh”

�Cycads (SIE-kadz) are gymnosperms in this phylum �Flourished during age of dinosaurs �Only about 100 species left today �Most native to tropics and grow slowly

�Many endangered b/c of habitat loss, overcollection and slow growth �Have fernlike leaves at top of short, thick trunk �Either male or female �Bear large cones

Phylum Ginkgophyta“gink-AHF-uh-tuh”

�Ginkgoes (GINK-ohz) flourished during dinosaur times

�ONLY living species today is Ginkgo biloba

�Native to China �Called living fossil –

looks like fossil ginkgoes that are 125 million years old

� Has fan-shaped leaves that fall from tree at end of each growing season

�Unusual for gymnosperm - Most gymnosperms are evergreens �Trees that lose their leaves at end of growing season à deciduous

� Tolerant of air pollution – good urban plants � Seeds considered delicacy in china and Japan � Plum-shaped fleshy seeds often mistaken for berries or fruit

Phylum Coniferophyta“kon-if-uhr-AHF-uh-tuh”

�Conifers (KOHN-uh-fuhrz) are gymnosperms of phyla Coniferophyta

�Include pine, cedar, redwood, fir, spruce, juniper, cypress

�Very important sources of wood, paper, turpentine, resin, Christmas trees

�Amber – fossilized resin from ancient conifers �Prehistoric insects preserved in amber (Jurassic Park)

�Gin – flavored with juniper seeds

Pine  Tree

Cedar

Redwood

Douglas  Fir

�Conifers are woody plants �Most have needle or scalelike leaves �Usually has both male and female cones �Small male cones grow in clusters �Release clouds of pollen �Then cone falls from branch �Pollen falls or blows into larger female

cones

�Egg cells attached to scales of female cone

�After pollination, cone closes up

�Protects developing seeds �Seeds mature after 1,2 years �Released when cone opens

Phylum Gnetophyta“nee-TAHF-uh-tuh”

�Gnetophytes (NEE-tuh-fiets) have vascular systems more like angiosperms

�Ephedra is genus of desert shrubs �Source of drug ephedrine (used as decongestant)

Phylum Anthophyta“an-THAHF-uh-tuh”

�Largest phylum of plants �Over 240,000 species of flowering plants

(angiosperm) �Fruit – ripened ovary that surrounds seeds

�Angiosperms grow differently �Some herbaceous plants with

showy flowers �Some are shrubs �Some are vines �Grasses are also angiosperms �Largest flower – Rafflesia – 1 m

in diameter

Evolution of Angiosperms�First appeared in fossil record about 135 million

years ago

�90 million years ago, started to be more than gymnosperms

�Successful because of several reasons

Life Cycle �In many angiosperms, seeds germinate and

make mature plants which make new seeds in 1 growing season

�Gymnosperms often take 10 or more years to be able to reproduce

�Fruits of angiosperms protect seeds and help in dispersal

�Angiosperms also have more efficient vascular system

�Can also use animal pollination (gymnosperms use wind)

�Angiosperms more diverse – live in more areas

Monocots and Dicots�Flowering plants classified under phylum Anthophyta divided into 2 groups 1. Monocotyledones (monocots) 2. Dicotyledones (dicots)

� Feature that distinguishes them is the cotyledones (kaht-e-LEE-duhnz) (seed leaves)

�Monocot leaves have several main veins running parallel à parallel venation

�Dicots have veins that branch out à net venation