Transcript Seed plants
Overview of Green Plants Chapter 30 Defining Plants The kingdom Viridiplantae includes land plants and green algae -Red and brown algae are excluded All green plants arose from a single species of freshwater algae The green algae split into two major clades -Chlorophytes – Never made it to land -Charophytes – Did! 2 Defining Plants 3 Defining Plants Land plants have two major features 1. Protected embryos 2. Multicellular haploid and diploid phases 4 Defining Plants Adaptations to terrestrial life 1. Protection from desiccation by a waxy cuticle and stomata 2. Evolution of leaves which increase photosynthetic surface area 3. Shift to a dominant vertical diploid generation 5 Plant Life Cycles Humans have a diplontic life cycle -Only the diploid stage is multicellular Plants have a haplodiplontic life cycle -Multicellular diploid stage = Sporophyte -Multicellular haploid stage = Gametophyte 6 Plant Life Cycles Sporophyte produces haploid spores by meiosis Spores divide mitotically, producing the gametophyte Gametophyte produces gametes by mitosis Gametes fuse to form the diploid sporophyte 7 8 Plant Life Cycles As more complex plants evolved: 1. Diploid stage became the dominant portion of the life cycle 2. Gametophyte became more limited in size 3. Sporophyte became nutritionally independent 9 Chlorophytes Green algae have two distinct lineages -Chlorophytes – Gave rise to aquatic algae -Streptophytes – Gave rise to land plants Chlamydomonas -Unicellular chlorophyte with two flagella -Have eyespots to direct swimming -Reproduces asexually as well as sexually 10 11 Chlorophytes Volvox -Colonial chlorophyte -Hollow sphere of a single layer of 500-60,000 cells -A few cells are specialized for reproduction 12 Chlorophytes Ulva -Multicellular chlorophyte -Haplodiplontic life cycle -Gametophyte and sporophyte have identical appearance 13 14 Charophytes Charophytes are green algae related to land plants There are two candidate clades 15 Charophytes Charales (300 species) -Macroscopic -Plant-like plasmodesmata -Sister clade to land plants Choleocaetales (30 species) -Microscopic -Plant-like mitosis -Next closest plant relatives 16 Bryophytes Bryophytes are the closest living descendants of the first land plants -Called nontracheophytes because they lack tracheids (specialized transport cells) Simple, but highly adapted to diverse terrestrial environments Non-photosynthetic sporophyte is nutritionally dependent on the gametophyte 17 Bryophytes Liverworts (phylum Hepaticophyta) -Have flattened gametophytes with liver-like lobes -Form gametangia in umbrellashaped structures -Also undergo asexual reproduction 18 Bryophytes Hornworts (phylum Anthocerotophyta) -Sporophyte has stomata -Sporophyte is photosynthetic -Cells have a single large chloroplast 19 Bryophytes Mosses (phylum Bryophyta) -Gametophytes consist of small, leaflike structures around a stemlike axis -Anchored to substrate by rhizoids -Multicellular gametangia form at the tips of gametophytes -Archegonia – Female gametangia -Antheridia – Male gametangia Mosses withstand drought, but not air pollution 20 21 Features of Tracheophyte Plants Cooksonia, the first vascular land plant, appeared about 420 MYA -Only a few centimeters tall -No roots or leaves -Homosporous 22 Features of Tracheophyte Plants Vascular tissues are of two types -Xylem – Conducts water and dissolved minerals upward from the roots -Phloem – Conducts sucrose and hormones throughout the plant -These enable enhanced height and size in the tracheophytes Tracheophytes are also characterized by the presence of a cuticle and stomata 23 Features of Tracheophyte Plants Vascular plants have gametophytes reduced in size and complexity relative to sporophytes Seeds -Highly-resistant structures that protect the plant embryo -Occur only in heterosporous plants Fruits in flowering plants add a layer of protection to seeds -Also attract animals that disperse seeds 24 Features of Tracheophyte Plants Vascular plants include seven extant phyla grouped in three clades 1. Lycophytes (club mosses) 2. Pterophytes (ferns and their relatives) 3. Seed plants 25 Lycophytes Club mosses are the earliest vascular plants -They lack seeds -Superficially resemble true mosses but they are not related -Homosporous or heterosporous 26 Pterophytes The phylogenetic relationships among ferns and their relatives is still being sorted out 27 Pterophytes Whisk ferns -Saprophyte consists of evenly forking green stems without leaves or roots -Some gametophytes develop elements of vascular tissue -Only one known to do so 28 Pterophytes Horsetails -All 15 living species are homosporous -Constitute a single species, Equisetum -Consist of ribbed, jointed photosynthetic stems that arise from branching rhizomes 29 Pterophytes Ferns -The most abundant group of seedless vascular plants with about 11,000 species -The conspicuous sporophyte and much smaller gametophyte are both photosynthetic 30 Pterophytes The fern life cycle differs from that of a moss -Much greater development, independence and dominance of the fern’s sporophyte Fern morphology -Sporophytes have rhizomes -Fronds (leaves) develop at the tip of the rhizome as tightly rolled-up coils -They unroll and expand 31 32 Pterophytes Fern reproduction -Most fern are homosporous -Produce distinctive sporangia in clusters called sori on the back of the fronds -Diploid spore mother cells in sporangia produce haploid spores by meiosis -At maturity, the spores are catapulted by snapping action 33 The Evolution of Seed Plants Seed plants first appeared 305-465 MYA -Evolved from spore-bearing plants known as progymnosperms The seed represents an important advance 1. Protects the embryo 2. Easily dispersed 3. Introduces a dormant phase in the life cycle 34 The Evolution of Seed Plants Seed plants produce 2 kinds of gametophytes -Male gametophytes -Pollen grains -Dispersed by wind or a pollinator -Female gametophytes -Develop within an ovule -Enclosed within diploid sporophyte tissue 35 Gymnosperms Gymnosperms are plants with “naked seeds” -Ovule is exposed on a scale at pollination There are four living groups -Coniferophytes -Cycadophytes -Gnetophytes -Ginkgophytes All lack flowers and fruits of angiosperms 36 Gymnosperms Conifers (phylum Coniferophyta) are the largest gymnosperm phylum -Include: -Pines, spruces, firs, cedars and others -Coastal redwood – Tallest tree -Bristlecone pine – Oldest living tree Conifers are sources of important products -Timber, paper, resin and taxol (anti-cancer) 37 Gymnosperms Pines -More than 100 species, all in the Northern hemisphere -Produce tough needlelike leaves in clusters -Leaves have: 1. Thick cuticle and recessed stomata 2. Canals into which cells secrete resin 38 Gymnosperms Pine reproduction -Male gametophytes (pollen grains) develop from microspores in male cones by meiosis -Female pine cones form on the upper branches of the same tree -Female cones are larger, and have woody scales -Two ovules develop on each scale 39 Gymnosperms Pine reproduction -Each ovule contains a megasporangium called the nucellus -Surrounded by the integument -Opening – Micropyle -One layer becomes the seed coat -While scales of female cone are open, pollen grains drift down between them -Are drawn to top of nucellus 40 Gymnosperms Pine reproduction -While female gametophyte is developing, a pollen tube emerges from the pollen grain -It digests its way to the archegonium -Fifteen months after pollination, pollen tube reaches archegonium and delivers its sperm 41 42 Gymnosperms Cycads (phylum Cycadophyta) -Slow-growing gymnosperms of tropical and subtropical regions -Sporophytes resemble palm trees -Have largest sperm cells of all organisms! 43 Gymnosperms Gnetophytes (phylum Gnetophyta) -Only gymnosperms with vessels in their xylem -Contain three (unusual) genera -Welwitschia -Ephedra -Gnetum 44 Gymnosperms Ginkgophytes (phylum Ginkgophyta) -Only one living species remains -Ginkgo biloba -Dioecious -Male and female reproductive structures form on different trees 45 Angiosperms Angiosperms are the flowering plants -Ovules are enclosed in diploid tissue at the time of pollination -The carpel, a modified leaf that covers seeds, develops into fruit 46 Angiosperms Angiosperm origins are a mystery -The oldest known angiosperm in the fossil record is Archaefructus -The closest living relative to the original angiosperm is Amborella 47 Angiosperms 48 Angiosperms 49 Angiosperms Flower morphology -Primordium develops into a bud at the end of a stalk called the pedicel -Pedicel expands at the tip to form a receptacle, to which other parts attach -Flower parts are organized in circles called whorls 50 Angiosperms Flower morphology -Outermost whorl = Sepals -Second whorl = Petals -Third whorl = Stamens (androecium) -Each stamen has a pollen-bearing anther and a filament (stalk) -Innermost whorl = Gynoecium -Consists of one or more carpels that house the female gametophyte 51 Angiosperms 52 Angiosperms Carpel structure -Three major regions -Ovary = Swollen base containing ovules -Later develops into a fruit -Stigma = Tip -Style = Neck or stalk 53 Angiosperm Life Cycle The female gametophyte (embryo sac) has 8 haploid nuclei arranged in two groups of four A nucleus from each group migrate toward the ovule’s center and become polar nuclei Cell walls form round remaining three nuclei -At the micropyle end, one cell functions as the egg, and the other two are synergids -At the other end, three cells are antipodals -They eventually break down 54 Angiosperm Life Cycle Pollen production occurs in the anthers -It is similar but less complex than female gametophyte formation -Diploid microspore mother cells undergo meiosis to produce four haploid microspores -Binucleate microspores become pollen grains 55 Angiosperm Life Cycle Pollination is the mechanical transfer of pollen from anther to stigma -Pollen grains develop a pollen tube that is guided to the embryo sac -One of the two pollen grain cells lags behind -This generative cell divides to produce two sperm cells 56 Angiosperm Life Cycle As the pollen tube enters the embryo sac, a double fertilization occurs -One sperm unites with egg to form the diploid zygote -Other sperm unites with the two polar nuclei to form the triploid endosperm -Provides nutrients to embryo When the seed germinates, a young sporophyte plant emerges 57 58 Angiosperm Life Cycle Angiosperms include: -Eudicots (about 175,000 species) -Trees, shrubs, snapdragons, peas, other -Use flowers to attract insect pollinators -Monocots (about 65,000 species) -Grasses, lilies, palms, irises, others -Some rely on wind for pollination Note: Self-pollination may also occur 59