Plant Diversity I: How Plants Colonized

Land (Ch.29)

• There are four main groups of land plants: bryophytes, pterophytes, gymnosperms, and angiosperms.

Evolutionary adaptations to terrestrial living characterize the four main groups of land

plants

• The angiosperms are the flowering plants.

• The gymnosperms include pines and other conifers.

• The pterophytes include ferns.

• The most common bryophytes are mosses.

Plant Evolution

Morphological and Molecular Evidence

Land plants evolved from charophytes (green algae)

1.Rings of cellulose-synthesizing proteins

2.Peroxisome enzymes3.Structure of flagellated sperm4.Formation of phragmoplast

– Group of microtubules that forms between the daughter nuclei of a dividing cell

– Cell plates develops in the middle

Adaptations Enabling the Move to Land

Charophytes inhabit shallow waters near the edges of ponds and lakes

Natural selection favored algae that could survive periods of dryness

• Sporopollenin– Durable polymer that prevents zygotes from

drying out

Benefits of a terrestrial habitat• Unfiltered sunlight• Increase in carbon dioxide availability• Mineral rich soil• Initially, few herbivores and pathogens

Derived Traits of PlantsFour key traits appear in nearly all land plants but are absent in the

charophyceans1. Alternation of generations2. Walled spores produced in sporangia3. Multicellular gametangia4. Apical meristemsAdditional Derived Traits Waxy cuticle• Waterproofing

Mycorhizzae• Mutualism between fungi and plants

Secondary compounds for protection• Bad tasting, protection from UV, deter pathogens

Alternation of Generations and Multicellular, Dependent

EmbryosLife cycle that alternates

between gametophytes and sporophytes

Gametophyte• Multicellular haploid organism• Mitosis produces gametes

Sporophyte• Multicellular diploid organism• Meiosis produces spores

Alternation of Generations and Multicellular, Dependent

EmbryosEmbryophytes• Multicellular plant embryos develop from zygotes

that are retained within the female gametophytes• Parental tissues provide developing embryo with

nutrients• Embryo has placental transfer cells

– Enhance the transfer of nutrients between parent and embryo

Walled Spores Produced in Sporangia

Plant Spores• Haploid reproductive cells• Grow into multicellular

gametophytes by mitosis• Contain sporopolleninSporophyte• Sporangia produce the spores• Sporocytes

– Diploid cells that undergo meiosis to produce the spores

Multicellular Gametangia

Gametangia• Multicellular organs that

produce gametes• Archegonia

– Pear shaped organ that producesa single non motile egg

– Site of fertilization• Antheridia

– Produce sperm that are released into the environment

Apical Meristems• Localized regions of cell division at the tips of

roots and shoots• Cells become part of the outer epidermis• Shoot apical meristems also generate leaves

Origin and Diversification of PlantsNonvascular Plants

• bryophytesVascular Plants• Vascular tissue is present to

transport water (xylem) and sugar (phloem)

Seedless Vascular Plants• Lycophytes and Pterophytes• Considered a grade

– collection of organisms that share a key biological feature but not ancestry

Seed Vascular Plants• Seed

– embryo packed with a supply of nutrients within a protective coat

• Gymnosperms and Angiosperms

Bryophytes• Nonvascular plants• Gametophyte is the dominant

generation• Division Bryophyta (mosses)• Division Hepatophyta (liverworts)• Division Anthocerophyta (hornworts)

BryophytesNot totally free from ancestral aquatic

habitat• Need water to reproduce (sperm

swim)• No vascular tissue to carry water

from soil to aerial parts (imbibe water instead)

• No woody tissue• Cannot support tall land plants• Bryophytes are anchored by tubular cells

or filaments of cells, called rhizoids.

Bryophytes

Bryophyte Life Cycle

1. Spores germinate and develop into threadlike protonema

2. Haploid protonema produce buds and produce gametophores by mitosis

3. Sperm swim through moisture to reach egg; fertilization occurs in the archegonium

4. Zygote develops into a sporophyte embryo

5. Sporophyte grows a long stalk, seta6. Attached by its foot, the sporophyte

remain nutritionally dependent on the gametophyte

7. Meiosis occurs and haploid spores develop in the capsule

8. When the capsule is mature, the lid pops off and spores are released – Upper part of capsule has a

peristome that open under dry conditions to release spores

Bryophyte Life Cycle

Ecological and Economic Importance of Mosses

• Moist forests and wetlands• Aid in increasing availability of nitrogen in the soil• Peat as a fuel source, stabilize carbon dioxide

concentrations

Vascular vs. Nonvascular Plants

Origins and Traits of Vascular Plants

• Branched sporophytes independent from gametophytes

• Life cycles with a dominant sporophyte stage

• Transport in vascular tissues (xylem and phloem)

• Well developed roots and leaves• Spore bearing leaves called

sporophylls

Life Cycles with Dominant Sporophytes

1. Sporangia release spores that develop into the gametophyte

2. Gametophyte develops antheridia and archegonia

3. Sperm swims to egg in archegonia; fertilization

4. Zygote develops into new sporophyte

5. Sori develop on the underside of the leaves

6. Sporangia within the sori undergo meiosis producing the spores

Life Cycles with Dominant Sporophytes

Fern Life Cycle

Transport in Xylem and Phloem

Xylem • conducts water & minerals from roots to rest of

the plant• Tracheids

– Tube-shaped cells that carry water and minerals from roots

• Cell walls strengthened by lignin– Allowed plants to grow tall

Phloem • Cells arranged in tubes that distribute sugars,

amino acids, and other organic products

Evolution of Roots and LeavesRoots

• Organs that absorb water and nutrients from the soil

• Anchor the plant allowing shoots to grow taller

Leaves• Increase the surface area of

the plant body• Primary photosynthetic organ• Microphylls

– Small, spine shaped leaves– Single strand of vascular

tissue• Megaphylls

– Leaves with a highly branched vascular system

Sporophylls and Spore Variations

Sporophylls• Modified leaves that bear sporangia

– Ferns sori; Conifers and lycophytes strobiliHomosporous• Only produce one type of spore that typically

develops into a bisexual gametophyteHeterosporous• Produce 2 types of spores• Megaspores (female gametophytes) and Microspores

(male gametophytes)

Two Distinct Reproductive Cycles for

Vascular Plants

Seedless Vascular Plants

Classification of Seedless Vascular Plants

Lycophyta• Club mosses, spike

mosses and quillworts

• Grow as epiphytesPterophyta• Ferns, horsetails,

whisk ferns

Vascular Seed PlantsGymnosperms• Naked seed plants

Angiosperms• Flowering seed plants