Plants Challenges of terrestriality Key point: History of land plants is the increasing adaptation to terrestriality. Colonized land 475 mya (Ordovician), began growing taller 370 mya (Carboniferous). Plants form the basis for every terrestrial ecosystem. The motile sperm of Charophyta, the bryophytes, and Pterophytes need a water medium to find ova. Advantages of terrestriality Important adaptations to life on land Waxy cuticle Gametangia and protected embryo Roots (below) stems (above) Leaves, stomata: photosynthesis 1
Figure 29.2 Red algae Origins: Charophyte Green Algae EVIDENCE ANCESTRAL ALGA Chlorophytes Charophytes Embryophytes Plantae Streptophyta Viridiplantae Homologous chloroplasts Biochemical similarity: cellulose in cell wall Similarity in mitosis, cytokinesis Phragmoplast: charophytes & Plants Similarity in sperm ultrastructure Genetic relationship: some nuclear genes, ribosomal RNA Origins: Charophyte Green Algae ESSENTIAL DIFFERENCES Green Algae Medium: water (whole alga has access) Photosynthesis in most cells (light limited) Plants Medium: air, nonsupportive (roots, stiff stem, cuticle) Photosynthesis in aerial parts (leaves, stomata) Major events in plant evolution A. Changes in life history B. Vascular tissue C. Seeds D. Flowering plants Reproduction mostly asexual Reproduction sexual: gametangia, embryo 2
Major events in plant evolution A. Changes in life history Generalized Alternation of Generation in Plants The changes that affect an organism s schedule of reproduction and survival. 1. Alternation of generations Evolved independently in fungi, cellular slime molds, brown algae, red algae, and green algae. Major events in plant evolution A. Changes in life history Major events in plant evolution A. Changes in life history 2. Generations are heteromorphic. Gametophyte Sporophyte 2. Generations are heteromorphic. 3. Shift of dominance from gametophyte (n) to sporophyte (2n). Gametophyte Sporophyte 3
Major events in plant evolution A. Changes in life history 2. Generations are heteromorphic. 3. Shift of dominance from gametophyte (n) to sporophyte (2n). 4. Replacement of flagellated sperm by pollen. B. Vascular tissue Cells joined into conducting tubes. Xylem: dead, lignified tubes conduct water, mineral from roots upward. Phloem: living tubes conduct sugar, amino acids from photosynthetic parts downward. An embryo packed with food. First seeds unencapsulated Gymnosperms. C. Seeds D. Flowering Plants: Complex structure containing seeds within protective ovary. Most diverse (species) plant group. 4
Charophytes Bryophytes Lycophytes Pterophytes Gymnosperms Charophytes Bryophytes Lycophytes Pterophytes Gymnosperms Embryo: zygotes are retained within tissues of the female parent plant. All descendants known as Embryophyta Embryo Vascular tissue: Cells joined into tubes to transport water and soil nutrients upward (xylem) and carbohydrates downward (phloem). All descendants known as Tracheophytes Charophytes Bryophytes Lycophytes Pterophytes Gymnosperms Charophytes Bryophytes Lycophytes Pterophytes Gymnosperms Embryo Vascular tissue Seeds: Embryo packed with a supply of nutrients inside a protective coat. All descendants known as Spermatophytes Embryo Vascular tissue Seeds Flowers: Complex structure containing seeds within protective ovary. All descendants known as 5
Charophytes Bryophytes Lycophytes Pterophytes Embryo Vascular tissue Gymnosperms Seeds Flowers Increasing Terrestriality BRYOPHYTES Mosses, Liverworts, Hornworts KEY POINTS Gametophyte dominant No vascular tissue Ancient but persistent group. Key point: History of land plants is the increasing adaptation to terrestriality. BRYOPHYTES Mosses, Liverworts, Hornworts Not monophyletic (???) Date to > 475 mya Very successful, 24,000 species, but never dominated landscape. Non-vascular; therefore only found in moist environments. Need water for sperm transport and absorption. Fig. 29.5 from textbook indicates that bryophytes are paraphyletic, however Common Characteristics Gametophyte generation dominates Lack stiff, supporting structures Therefore low-growing Separate male and female gametophyte Male gametangium = antheridium (flagellated sperm cells) Female gametangium = archegonium (single egg) Fertilization within archegonium zygote embryo 6
Bryophytes Fig. 29-8-3 Raindrop Bryophytes Bryophyta: Mosses Sperm Best known of bryophytes Note that bryophytes is a term of convenience, where as Bryophyta is restricted to the mosses. Small individual plants in tight packages--form spongy, supporting mats. Anchor to substrate with rhizoids roots, like leaves not homologous with vascular plants Life cycle good example of alternating generations Key Haploid (n) Diploid (2n) Spore dispersal Peristome Spores Sporangium MEIOSIS Mature sporophytes 2 mm Capsule with peristome (SEM) Protonemata (n) Bud Bud Gametophore Seta Capsule (sporangium) Foot Young sporophyte (2n) Female gametophytes Antheridia Male gametophyte (n) Female Archegonia gametophyte (n) Rhizoid Zygote (2n) Embryo Egg FERTILIZATION (within archegonium) Archegonium Bryophytes Bryophytes Hepatophyta: Liverworts Very inconspicuous, lobed bodies hugging ground Life cycle like mosses Thalloid form & Leafy form Likely sister-group of remaining (all other) Plantae Anthocerophyta: Hornworts Resemble liverworts Name derives from sporophytes in hornlike capsules of matlike gametophyte Cells have a single large chloroplast 7
Charophytes Bryophytes Lycophytes Pterophytes Gymnosperms Seedless Vascular Plants Lycophytes & Pterophytes Embryo Vascular tissue: Cells joined into tubes to transport water and soil nutrients upward (xylem) and carbohydrates downward (phloem). All descendants known as Tracheophytes KEY POINTS Plants with xylem and phloem Branched sporophyte dominates Ancestral state retains flagellated sperm and thus inhabit moist environments Paraphyletic Vascular plant monophyletic: Tracheophyta. General Macroevolution Earliest are found in mid- Paleozoic, 425 mya (e.g. Cooksonia) Dominated landscape by end of paleozoic Branched sporophyte dominant generation All with flagellated sperm Fern with characteristic macrophylls Seedless Vascular Plants Lycopod with microphylls shown General Structure Roots & shoots Lignified vascular tissue: Xylem, Phloem Some heterosporous Megaspores (female) Microspores (male) As in seed plants Megaspores Seedless Vascular Plants Microspores Fern sperm with multiple flagellae Lycopod Selaginella 8
Seedless Vascular Plants Lycophyta: Lycopods Club mosses, quillworts, ground pines Charophytes Bryophytes Lycophytes Pterophytes Gymnosperms Relicts of flourishing past, two lineages: Giant, tree-like, woody Small herbaceous Only small forms extant E.g. Lycopodium, Selaginella Many are tropical epiphytes; temperate forms grow at ground level Sporangia borne on sporophylls: leaves specialized for reproduction Lycopodium Club moss or ground pine Embryo Vascular tissue Note Pterophyta are sister to Spermatophyta: Megaphyll leaves, roots that can branch Isoetes Quillwort Seedless Vascular Plants Seedless Vascular Plants Pterophyta I: Horsetails Pterophyta I: Horsetails Previously considered own phylum, Sphenophyta, now placed within Pterophyta. Late Paleozoic forms grew to 15m Today only the genus Equisetum 15 species mostly northern hemisphere Conspicuous horsetail sporophyte Homosporous bisexual gametophyte Underground rhizome from which stems arise Stems: hollow, jointed with whorls of small branches Cone-like sporangia at tip of stem. An exception: Equisetum giganteum from Chile 9
Pterophyta II: True ferns Most diverse seedless vascular plants Today ~12,000 species Most diversity in tropics, also common in temperate regions. Fronds are large leaves with branched veins: megaphyll Compound leaflets grow from fiddlehead tip Leaves may sprout directly from prostrate stems (rhizomes); or as upright treeferns Seedless Vascular Plants LIFE CYCLE: sporophyte with specialized leaves, each with clustered sporangia below (sori) which launch spores, grow into gametophyte Pterophyta II: Whiskferns Previously considered own phylum, Psilophyta, now placed within Pterophyta. Simple plants. Diploid sporophyte has dichotomous branches (like primitive Cooksonia) True roots and leaves are absent Like Lycopods! However, these have been secondarily lost. The CARBONIFEROUS 360-300 mya Height of seedless vascular diversity and ecological dominance. Formed first forests. Eventually become COAL: Dead plants did not completely decay. Became peat bogs. Eventually covered by sea. Heat and pressure from sediments converted peat to coal. 4 square miles of Carboniferous coal forest in Pennsylvania (see http://www.mnh.si.edu/highlight/riola/ for highlights) 10
Gymnosperms and the Evolution of Seed Plants Seed Plants Key points Seed replaces spore Fertilization by pollen instead of sperm Two groups: gymnosperms (naked seeds) (protected seeds) Gymnosperms: Conifers & Allies 1. Success marked by change in life cycle 2. Evolution 3. Four divisions Changes in Life Cycle 1. Gametophyte highly reduced Retained within reproductive tissue of sporophyte and not cast out as independent generation. Shift toward diploidy. 11
Changes in Life Cycle 2. Fertilization by pollen rather than swimming sperm. There is no longer a reliance on water! Changes in Life Cycle 3. Seed. Zygote not independent Zygote to embryo packaged with food in a seed coat. Seed is naked born on scales of cones. Protects from desiccation. Increases dispersal capabilities: replaces spore as dispersal agent. Fig. 30-6-4 Key Haploid (n) Diploid (2n) Mature sporophyte (2n) Embryo (2n) Microsporangia Seedling Seeds Ovulate cone Integument Pollen cone Microsporocytes (2n) Pollen Pollen grain grains (n) MEIOSIS Microsporangium (2n) Food reserves (n) Seed coat (2n) Pollen tube FERTILIZATION Ovule Archegonium Female gametophyte Sperm nucleus (n) Megasporocyte (2n) MEIOSIS Egg nucleus (n) Megasporangium (2n) Surviving megaspore (n) Appear much earlier than, in Devonian. Modern gymnosperms by early-mid Mesozoic Permian marks end of Paleozoic. Mesozoic: Age of Dinosaurs (zoologists); Age of Gymnosperms (botanists) Evolution Permian harshness (formation of Pangaea) Interior aridity Demise of Carboniferous forests Mass Extinctions Rise of Gymnosperms 12
Cycadophyta: Cycads Cycadophyta Cycads Ginkgophyta Ginkgo Gnetophyta Gnetales Coniferophyta Four Divisions Cycads or Sago palm Currently ~130 species Slow-growing, tropical & subtropical Flourished in Mesozoic Massive cone-shaped structures bearing pollen or ovules Conifers Cycadophyta: Cycads Ginkgophyta: Gingko Primarily insect pollinated, some wind pollinated. Seeds eaten in Asia after removing Alkaloids Until 1990s, pollination was assumed to be by wind, with insect pollination limited to. Now, insect pollination is known for most species. Diverse in Mesozoic, single species today Known only from fossils until discovered growing in Chinese Buddhist temples Fan-shaped, deciduous leaves Male trees widely planted landscape tree Resistant to drought, pollution, pests Seeds of female produce stench when crushed Maiden-hair tree Hot males, nice females 13
Cycads & Ginkgos Have flagellated sperm: Differ from all other Gymnosperms, and. What are the implications if Gymnosperms are monophyletic? What are the implications if sperm with no flagellae is homologous? Gnetophyta Gnetum: tropical tree/vine. Ephedra: Mormon or Mexican tea of American deserts, jointed stem. Welwitschia: Largest known leaves, deserts of SW Africa, deep root, exposed leaves. Fossils from Permian, but peak diversity in Cretaceous. Have vessel elements: Differ from all other Gymnosperms, share this with. What are the implications if Gymnosperms are monophyletic? What are the implications if vessel elements are homologous? Gnetophyta Coniferophyta (or Pinophyta): Conifers Pines, firs, spruce, larches, cedars, hemlocks, (all Pinaceae), yews, cypress, redwoods, hoop pines, umbrella pines, yellowwoods, plum-yews. ~600 species, dominate vast regions of taiga: northern and southern evergreen forests. 14
Coniferophyta (or Pinophyta): Conifers Evergreen, even perform limited photosynthesis year round. Needles are leaves Commercially important as timber Tallest plant: coastal redwood (>110m) Heaviest plant: giant sequoia (2500 metric tons) Oldest plant: Bristlecone pine >4600 years old Summary: Gymnosperms Monophyletic sister group to angiosperms. Peak diversity in the Mesozoic, still ecologically dominant in some ecosystems. Evolution of seed and pollen key transitions (shared with ) Charophytes Bryophytes Lycophytes Pterophytes Gymnosperms Structure & Classification Embryo Vascular tissue Seeds Flowers: Complex structure containing seeds within protective ovary. All descendants known as 15
Flower and fruit as defining reproductive strategy KEY POINT General Three Tissues Two Systems Lecture Outline Lots of terminology Monocots & Dicots Life Forms General Aspects Terminology Sets the stage for understanding the next 4 lectures! Angiosperm life cycle Plant tissues and growth Transport in plants Plant control systems Anthophyta: flower plant 270,000 described species. Compare this to ~900 species of Gymnosperms! 16
General Aspects General Aspects Refined vascular tissue Vessel elements present in xylem shorter, wider cells placed end to end perforation plates at end of each cell line up end-to-end to create vessels Specialized for transport, less for support Vessel elements Also present in Gnetales and absent from many basal angiosperm groups.??? Possibilities??? General Aspects Of course Characterized by flowers Characterized by fleshy ovary protecting seed Two Systems Roots: below-ground non-photosynthetic Anchor Absorption Storage Aerial shoots: aboveground photosynthetic and/or transport Leaves Stems These are interdependent 17
Roots Anchor Shoot System Tap root Single large vertical root with secondary rootlets Firm anchor Food storage used by plant when producing flower, fruit (harvest before flowering) Fibrous root Mat-like and spreading, shallow, wide coverage Grasses (good erosion control) Roots Anchor Shoot System Root hairs: For absorption at root tip Adventitious roots: above ground roots help support stem Adventitious = Name for any plant part growing in abnormal place Prop roots Aerial roots Nodes Internodes Axillary buds Apex = terminal bud Apical dominance Modified stems Stolons Horizontal above-ground runners (e.g. strawberries) 18
Modified stems Rhizomes Horizontal below-ground stems (potatoes, iris) Bulbs Vertical below ground with leaves modified for storage. Primary photosynthetic part of plant (usually) Part 1: Blade Part 2: Petiole Absent in many grasses and relatives (monocots) Highly variable Within individuals Between species Between deeper clades Shape Arrangement Margins Venation Simple vs Compound Highly variable Within individuals Between species Between deeper clades Shape Arrangement Margins Venation Simple vs Compound 19
Highly variable Within individuals Between species Between deeper clades Shape Arrangement Margins Venation Simple vs Compound Highly variable Within individuals Between species Between deeper clades Shape Arrangement Margins Venation Simple vs Compound Highly variable Within individuals Between species Between deeper clades Shape Arrangement Margins Venation Simple vs Compound Some can be highly modified: Tendrils Spines Storage Asexual reproduction Bracts Insectivory 20
Some can be highly modified: Tendrils Spines Storage Asexual reproduction Bracts Insectivory Some can be highly modified: Tendrils Spines Storage Asexual reproduction Bracts Insectivory Venus flytrap Sundew Pitcher plant Angiosperm structure specialized for sexual reproduction. Specialized shoot made up of (usually) four rings of modified leaves (floral organs): Sepals Petals Carpels: Female Anthers: Male Carpels are the female organs Ovules contain megasporangium Stigma receives pollen Style leads from stigma to ovary Ovary contains ovules 21
Stamens are the male organs Filament is the stalk Anther houses microsporangia and produces pollen. Pollen will contain male gametophyte Perfect flowers contain both carpels and stamens Imperfect flowers contain either carpels or stamens Monoecious: having separate male and female flowers on the same plant. Dioecious: having separate male and female plants. Evolutionary Trends 1. Reduction in number of floral parts 2. Floral parts fused, compound carpels to single and large 3. From radial to bilateral symmetry 4. Ovary drops below petals and sepals ( inferior ovary) Water lily, Nymphaceae, showing plesiomorphic state Orchid, showing derived state Evolutionary Trends 1. Reduction in number of floral parts 2. Floral parts fused, compound carpels to single and large 3. From radial to bilateral symmetry 4. Ovary drops below petals and sepals ( inferior ovary) Water lily, Nymphaceae, showing plesiomorphic state Morning glory, showing derived state 22
Evolutionary Trends 1. Reduction in number of floral parts 2. Floral parts fused, compound carpels to single and large 3. From radial to bilateral symmetry 4. Ovary drops below petals and sepals ( inferior ovary) Water lily, Nymphaceae, showing plesiomorphic state Pea flower, showing derived state Evolutionary Trends 1. Reduction in number of floral parts 2. Floral parts fused, compound carpels to single and large 3. From radial to bilateral symmetry 4. Ovary drops below petals and sepals ( inferior ovary) Major Angiosperm Dichotomy Monocots and Dicots Refers to numbers of seed leaves or cotyledons. Monocots are monophyletic. Dicots are NOT. Monocots: Grasses, lilies, palms, etc. Dicots: Everything else Major Angiosperm Dichotomy Monocots and Dicots Refers to numbers of seed leaves or cotyledons. Monocots are monophyletic. Dicots are NOT. 23
Major Angiosperm Dichotomy Monocots and Dicots Refers to numbers of seed leaves or cotyledons. Monocots are monophyletic. Dicots are NOT. Dicot is a term of convenience. Dicots & Monocots But it is a useful distinction Dicots & Monocots But it is a useful distinction Dicots & Monocots Discussion question: For these traits, which are plesiomorphic and which are apomorphic? 24
Plant forms have evolved to fill numerous ecological roles or niches. Developmental plasticity and Indeterminate growth allow individuals to fit particular ecological conditions. Why is this so important to a plant? Life Forms Grasses Monocots No petiole Limited branching No woody tissue (herbaceous) Forbs Shrubs Trees Epiphytes Aquatic Herbaceous Woody Life Forms Grasses Forbs Dicots Generally wildflowers Herbaceous Shrubs Trees Epiphytes Aquatic Herbaceous Woody Life Forms Grasses Forbs Shrubs Woody tissue No distinct single trunk Trees Epiphytes Aquatic Herbaceous Woody Life Forms 25
Grasses Forbs Shrubs Trees Woody tissue Single main trunk Apical dominance pronounced Epiphytes Aquatic Herbaceous Woody Life Forms Grasses Forbs Shrubs Trees Epiphytes Plant that grows on another plant Aquatic Herbaceous Woody Life Forms Grasses Forbs Shrubs Trees Epiphytes Aquatic Numerous adaptations for living in water Herbaceous Woody Life Forms Life Forms Grasses Forbs Shrubs Trees Epiphytes Aquatic Herbaceous Die down each year. Can be annual to perennial. No structural lignin Woody Perennial and persistent Structural lignin (wood) Dichotomy used in above distinctions. 26
Summary Flowering plants have been very successful: innovation of flower & fruit. Diverse morphology structured around the root system and the shoot system. Developmental plasticity and evolution of different life forms important in colonization of novel ecological niches. 27