Chapter 29: Overview of Plant Diversity

Divya Raj, Katie Redinger, Allie Wrabel

The Evolutionary Origins of Plants

• Plant will refer to a group of organisms that share a freshwater algal ancestor and have evolved over a 470-million-year period

• The exact ancentral alga is still a mystery, but close relatives like the Charales exist today

• The shared history green algae has with other plants has led scientists to change the name of kingdom Plante to kingdom Viridiplante to include the algae

Land Plants

• All afford protection to their embryos

• All have multicellular haploid and diploid phases

Conductive Systems

• Nonvascular Plants- lack vascular tissue

• Vascular Plants- have water-conduction xylem and food-conducting phloem strands of tissues in their stems, roots, and leaves.

Four Major Groups of Land Plants

• Nonvascular plants- not monophyletic, include three phyla: mosses, liverworts, and hornworts

• Seedless vascular plants- (1) club mosses, and (2) ferns, whisk ferns, and horsetails.

• Gymnosperms- have seeds that protect their embryos

• Angiosperms- have flowers, which may attract pollinators, and fruits surrounding the seeds, protecting the embryos and aiding in seed dispersal

Adaptations to Land

• Many plants are protected from desiccation-the tendency of organisms to lose water to the air-by a cuticle that is secreted onto their exposed surfaces

• Gas diffusion into and out of the plant occurs though tiny openings called stomata

• Evolution of leaves increases the amount of photosynthetic surface area

Plant Life Cycles

• Humans have diplontic life cycles, while plants have haplodiplontic

• Diploid sporophytes produce haploid spores by meiosis

• Spores develop into haploid gametophytes by mitosis and produce haploid gametes

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2n2n

Egg

MitosisSpore

2n

Sporophyte(2n)

Sporangia

Spore mothercell

Sporesn

nn

n

Gametophyte(n)

Gamete fusion

Sperm

Embryo

Zygote

Meiosis

Haploid

Diploid

Mosses, Liverworts, and Hornworts

• Nonvascular plants are divided into three major phyla-Bryophyta, Hepaticophyta, and Anthocerophyta- with all members being relatively unspecialized but collectively able to inhabit diverse environments

Mosses

• The leaf like structures belong to the gametophyte. Each of the yellowish brown stalks with a capsule at is summit is a sporophyte

Liverworts

• The sporophytes are formed by fertilization within the tissues of the umbrella-shaped structures that arise from the surface of the flat green, creeping, gametophyte

Hornworts

• Unlike the sporophytes of other bryophytes, most hornworts sporophytes are photosynthetic

Seedless Vascular Plants

• Seedless vascular Seedless vascular plants have well-plants have well-developed conducting developed conducting tissues in their tissues in their sporophytessporophytes..

• The first vascular plants for which we The first vascular plants for which we have a relatively complete record of have a relatively complete record of belonged to the phylum belonged to the phylum RhyniophyaRhyniophya..

• They flourished some 410 million They flourished some 410 million years ago but are now extinct.years ago but are now extinct.

• These plants were These plants were homosporoushomosporous (only produce one type of spore for (only produce one type of spore for reproduction; asexual).reproduction; asexual).

• Seeds only occur in Seeds only occur in heterosporousheterosporous plants (two types of spores for plants (two types of spores for reproduction; sexual).reproduction; sexual).

• Eventually these ancient vascular Eventually these ancient vascular plants evolved more complex plants evolved more complex arrangements, leading to the formation arrangements, leading to the formation of real leaves.of real leaves.

• Early plants soon began to Early plants soon began to develop develop vascular tissues vascular tissues that acted as efficient water that acted as efficient water and food conducting and food conducting systems.systems.

• These consist of strands of These consist of strands of specialized cylindrical or specialized cylindrical or elongated cells that form a elongated cells that form a network throughout a plant, network throughout a plant, extending from near the tips extending from near the tips of the roots, through the of the roots, through the stems, and into true leaves.stems, and into true leaves.

• The seven living phyla of vascular plants The seven living phyla of vascular plants dominate terrestrial habitats everywhere, dominate terrestrial habitats everywhere, except for the highest of mountains and the except for the highest of mountains and the tundra.tundra.

• Most vascular plants have well-developed Most vascular plants have well-developed conducting tissues, specialized stems, leaves, conducting tissues, specialized stems, leaves, roots, cuticles, and stomata. Many have roots, cuticles, and stomata. Many have seeds, which protect embryos until conditions seeds, which protect embryos until conditions are suitable for further development.are suitable for further development.

• Earliest vascular plants Earliest vascular plants lacked seeds.lacked seeds.

• Two phyla of seedless Two phyla of seedless vascular plants:vascular plants:

• Lycophyta Lycophyta Club mosses Club mosses (relicts of an ancient past (relicts of an ancient past when vascular plants first when vascular plants first evolved.)evolved.)

• Pterophyta Pterophyta Whisk ferns, Whisk ferns, Horsetails, and Ferns.Horsetails, and Ferns.

– All reproduce using All reproduce using sporophytes (asexually).sporophytes (asexually).

• In modern vascular In modern vascular plants, the plants, the gametophytesgametophytes have have been reduced in size been reduced in size and complexity, and complexity, individuals have individuals have specialized tissues.specialized tissues.

• Gametophytes are the Gametophytes are the phases where gametes phases where gametes are produced for sexual are produced for sexual reproduction (two parts).reproduction (two parts).

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Archegonium

Egg

Antheridium

Sperm

Embryo

Leaf of youngsporophyte

GametophyteRhizome

Maturefrond

Maturesporangium

MEIOSIS

SporeRhizoids

Gametophyte

n

2n

Adultsporophyte

Mitosis

FERTILIZATION

Sorus (clusterof sporangia)

Seed Plants

• First appeared 425 million years ago

• Descended from spore-bearing plants called progymnospermsprogymnosperms

• This common ancestor gave way to gymnosperms and angiosperms

Formation

• Embryo is protected by extra tissue, creating an ovule

• Tissue hardens to form seed coat

• This protects the embryo from drought and helps disperse the seed

• Presence of seeds introduced dormant phase- allows embryo to survive until conditions are better

Reproduction

• 2 kinds of gametophytes- male and female

• In seed plants, the sperm moves to the egg through a pollen tube eliminating the need for external water

• Unlike seedless plants, whole male gametophyte moves to female

• Female gametophyte develops in ovule

Gymnosperms- “naked-seeded plants”

• Cone-bearing seed plants; ovules not completely enclosed at time of pollination

• 4 groups: conifers, cycads, gnetophytes, and Ginkgo

• Can range from 25g to 45 kg and mm’s to over a m

Conifers

• Ex. Pines, spruces, firs, cedars, hemlocks, yews, larches, cypresses, and others

• Found in colder temperate and drier regions

• Sources of timber, paper, resin, taxol, and other products

Pines

• Over 100 species exist• Tough, needlelike leaves

produced in clusters of 2 to 5

• Thick cuticle and recessed stomata- adaptation for preventing water loss

• Secrete resin used in turpentine and rosin

• Wood consists of xylem and is considered ‘soft’

Reproduction- Males

• Seed plants are heterosporus so spores give rise to two types of gametophytes

• Male gametophytes develop from microspores• A pair of microsporangia form sacs in which

numerous ‘mother’ cells undergo meiosis creating 4 microspores

• Develop into 4-celed pollen grains with air sacs• Single cluster of male pine cones can produce

over 1 million pollen grains

Reproduction- Female

• Two ovules develop near base and contain megasporangium called nucellus

• Nucellus is surrounded by cells called the integument with small opening called the micropyle

• Single megaspore mother within each megasporangium undergoes meiosis and becomes 4 megapsores

• Only one survives and develops into female gametophyte and can contain thousands of cells

Pollination

• Pollen is drawn into microphyle

• While female is developing, pollen makes its way through nucellus

• One of the pollen’s 4 cells divide through mitosis and one of these two divides again

• Final cells are sperm

• 15 months after pollinations, zygote is formed

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Longisection ofseed, showingembryo

FERTILIZATION(15 months afterpollination)

Ovulate (seed-bearing)cone

Microsporemother cell

Microspores

Pollen

Pollen-bearingcone

Pollentube

Pollination

n

2n

Megaspore

Sporophyte

Seedling

Pineseed

Mature seedcone (2nd year)

Scale

Embryo

Scale

Mitosis

Mitosis Mitosis

Mitosis

Megasporemother cell

MEIOSIS

Cycads (Cycadophyta)

• Slow-growing in tropical and subtropical regions

• Life cycle similar to pines• Sperm is formed within

pollen tube and released within ovule to archegonium

• Sperm cells are largest among living organisms

Gnetophytes (Gnetophyta)

• 3 genera, 65 species• Only gymnosperms

with vessels in xylem• Over half are in genus

Ephedra and plants are shrubby and have scalelike leaves

• Species of genus Gnetum are vineline with broad leaves

Ginkgo (Ginkgophyta)

• Only one living species remains

• Name comes from fan-shaped leaves

• Sperm have flagella and ginkgo is dioecious- male and female reproductive structure produced on different trees

• Resistant to air pollution

Angiosperms- “Vessel seed”

• Flowering plants; ovules completely enclosed in tissue

• 250,000 known species

• Vessel in name refers to carpal- modified leaf that encapsulates seeds

Origins

• Fossils as old as 125 million years old

• Known to be angiosperms because fossils have both male and female reproductive structures

• Lack petals and sepals that evolved later

Monocots and Eudicots

• Eudicots (175,000)- more primitive; 1/6 of species are annuals; mints, peas, sundlowers

• Monocots (65,000)- lilies, cattails, orchids; few are annual; have no ‘true’ wood

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Stigma

Style

Ovule

Ovary

Carpel

AntherFilament

Stamen

Petal

Sepal

Receptacle

Pedicel

Megasporemother cell

Nucellus

Integuments

MicropyleStalk ofOvule(funiculus)

Structure of flowers

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n2n

Generative cell

8-nucleateembryo sac

(megagametophyte) (n) Formationof pollentube (n)

MEIOSIS

MEIOSIS Tube nucleus

Tubenucleus

Sperm

Style

Sperm

Egg

Polarnuclei

Seed coatEmbryo (2n)

Cotyledons

Seed (2n)Endosperm (3n)

Youngsporophyte

(2n)

Adultsporophytewith flower

(2n)

Anther

Ovary

Stigma

Anther (2n)

Microsporemother cells (2n)

Microspores (n) Pollen grains(microgametophytes) (n)

Megaspore (n)

Megasporemother cell (2n) Pollen

tube

Ovule

DOUBLE FERTILIZATION

Germination

Egg

Mitosis

Mitosis

Life Cycle