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 enzymes 3.Structure of flagellated sperm 4.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 Plants Four key traits appear in nearly all land plants but are absent in the charophyceans 1.Alternation of generations 2.Walled spores produced in sporangia 3.Multicellular gametangia 4.Apical meristems Additional 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 Embryos Life 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 Embryos Embryophytes 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 sporopollenin Sporophyte 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 Plants Nonvascular Plants bryophytes Vascular 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)

Bryophytes Not 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, seta 6.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 Leaves Roots 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  strobili Homosporous Only produce one type of spore that typically develops into a bisexual gametophyte Heterosporous 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 epiphytes Pterophyta Ferns, horsetails, whisk ferns

Vascular Seed Plants Gymnosperms Naked seed plants Angiosperms Flowering seed plants