Presentation on theme: "Plants and the Colonization of Land"— Presentation transcript:
1 Plants and the Colonization of Land AP Biology – Chapter 29
2 Evolution of PlantsThere were NO PLANTS until about 500 million years ago.Until that time all life had been essentially aquatic…Terrestrial Earth – barrenIt’s hard to Imagine…
3 Evolution of Plants WATER TO LAND!!!!!!! The evolutionary history of plants is a story of ADAPTATION TO TERRESTRIAL CONDITIONS from water.WATER TO LAND!!!!!!!Water…To Land
4 Since their beginnings, Plants have been very successful… Plants have diversified into about 290,000 living species.
5 The Ancestors of Land Plants… Were most likely some type of Green Algae
6 Relatives to ancient plants – Green Alage CharaColeochaete“Kingdom” ProtistsSpirogyraVolvox
7 The closest algal relatives of land plants today… Charophyceans (Charophytes)It is likely that plants and charophytes share a common ancestor.In other words, charophytes did not give rise to plants, but share a common ancestor with them.This makes charophytes the nearest living algal relative to plants.
8 Plant and Algae shared characteristics These characteristics are shared with many algae.MulticellularEukaryoticPhotosynthetic autotrophsCellulose cell wallsChlorophylls a and b presentStore surplus carbs in form of STARCHWhat then is it that links plants specifically to charophytes?Evidence for thegreen algae/plantconnection
9 What links the plants to the charophytes? 4 additional characteristicsShape of the complexes that make cellulose (rose – shaped)Enzymes contained in the peroxisomesSperm structure is similarCertain details of cell division processesAlso…the DNA!
10 Ancient Algae and the Water to Land Transition Some ancient algae (like modern charophytes) inhabited shallow waters at edges of ponds/lakesSurvived occasional dryingHad coatings on spores to protect from dryingNatural selection would favor individual algae that can survive periods OUT of water.Led to ADAPTIVE RADIATION of plant ancestors onto LAND
11 Why move to Land? Land has some big assets! Bright sunlight unfiltered by waterAbundance of CO2 in atmosphereSoil rich in mineral nutrientsFew herbivores and pathogens (at least at first!)
12 Problems in taking advantage of Land’s assets… However…to take advantage of these assets plants had to figure out how to do without water!! This means:Prevent Bodies from drying out…Prevent Gametes from drying outFertilization without waterPrevent Embryos from drying outPLANTS have solutions to these problems. ALGAE do NOT.
13 So, to distinguish plants from Algae we use… …the solutions to the problems of land living that plants developed but that water-living algae did NOT develop.Adaptations to land
14 Problem 1 – accessing resources on land Location of Resources in TWO Very Different PlacesAbove ground – light,Below ground – water and mineralsHow to access these two different environments simultaneously is a huge problemRemember – algae do NOT have this problemContinually bathed in uniform water environmentNo need to protect tissues from drying outAll tissues have equal access to light, dissolved nutrients, etc.
15 Problem 1 – accessing resources on land: SOLUTION Apical MeristemsSmall regions of growth in tips of roots and shootsincrease access to resources in different directions: soil/light.Cells produced in these meristems differentiate into tissues suited for these two different environments.Example:Protected epidermis for leaves exposed to airCuticle; stomataRoots unprotected and designed for absorptionApical Meristems
16 Problem 1 – accessing resources on land: SOLUTION Access to light and soil, generally require a plant to gain length in both directionsBut how to get substances transported across long distances within the plant’s body?VASCULAR TISSUETissue that can move LARGE QUANTITIES of material through a plantWithout it, osmosis is the only alternativeSLOW; limits size a land organism can attain.
18 Problem 2: Evaporation of water from tissues on Land - Algae is constantly bathed in waterEvaporation from tissues is NOT a concernAlso, algae tissue freely accepts nutrients from waterthus no need for access to nutrients in soilNo need for support – water supports the algae and floats it near the surface to receive lightAlgae on land would rapidly lose water from tissues with no way to replenish; also, access to light would be limited without support structures
19 Problem 2: Evaporation of water from tissues on Land Solution Protective tissuesEpidermis and secretion of waxy cuticlePrevents water lossStomataAllows gas exchange even though cuticle is covering the plantCross-section through a leaf
20 Problem 3 – Drying out of sex cells and embryos Aquatic organisms do NOT have to “worry” aboutGametes drying out during developmentGametes drying out when releasedEmbryos drying out during developmentEmbryos drying out when releasedLAND organisms DO have to “worry” about these things.Water > Land transition required solving these problems
21 Problem 3 – Drying out of sex cells and embryos: Solution Protection of Gametes duringProductionGamete production is protected inside structures called gametangiaPrevents drying out of sperm/eggDispersalIn more advanced plants, gametes are protected during dispersal in structures (pollen) that can travel through air.NOT dependent on swimming sperm.
22 Problem 3 – Drying out of sex cells and embryos: Solution Protect delicate Embryos from drying out, too!Egg fertilization occurs inside gametangiumZygote develops into embryo inside gametangiumEmbryo is retained and nourished inside gametangiumEmbryos dispersed in structures that also prevent drying out:SporesSeeds
23 However, solutions to all these problems were NOT achieved at once… These solutions were generally achieved in STEPSWe can use these steps in evolution of land adaptations as a way to determine a “plant family tree” or PHYLOGENY
24 A new period of adaptive radiation followed Green AlgaeMossFernsConifersFlowering PlantsEach of these circled branch points indicates some new adaptation that led to even more success on land…A new period of adaptive radiation followedAlgae/PlantCommonAncestor
25 Plant Classification Domain Eukarya Kingdom Plantae (currently excludes the algae, but there is DISAGREEMENT about this!)Some think green algae should be included within the plant kingdom!10 major phyla (actually called divisions in plants)3 of these are nonvascular (bryophytes)7 are vascular and include the vast majority of plants (93%)
26 Generalizations Regarding Plant Groups Plants can be loosely and informally placed into two major groupsNonvascular (the bryophytes/mosses)More ancient / primitiveRely on cell to cell diffusion for transportSmall (like mosses)No true roots, stems or shootsVascular (everything else)More recent / advancedUse sophisticated “plumbing” (vascular tissue) to move food/waterCan become quite largeTrue roots, stems and shoots (by definition, roots/stems/shoots are all structures that contain vascular tissue)
27 Generalizations Regarding Plant Groups Vascular Plants can be informally grouped intoSeedless Vascular PlantsFirst to arise from nonvascular ancestorsPossess (obviously) vascular tissue, but that’s about all that sets them apart from nonvascular plantsGymnospermsNaked seed plants / Conifers– 1st seeds, but seeds unprotectedAngiospermsFlowering plants – seed development protected inside ovaries
28 FIRST Period of Plant Evolution: Origin of Plants from Aquatic Ancestors Living groups from this first period of plant evolution:Division Bryophyta – the mosses10,000 speciesDivision Hepatophyta – Liverworts6,500 speciesDivision Anthocerophyta – Hornworts100 species
29 Typical Bryophyte (moss) life cycle Plant Life Cycles called ALTERNATION OF GENERATIONSLife cycle alternates between two different multicellular bodies, with each form producing the other.One body form is haploidThe other body form is diploidDo NOT confuse with haploid/diploid stages of animals, etc. The haploid form in plants is a multicellular body, not just a gamete.
30 Alternation of Generations – Why do we care? ALL plants undergo this pattern called alternation of generationsWhy do we care?Differences in plant life cycles give us important clues about evolutionary advances of each major plant group
31 Haploid spores land in suitable environment Sperm must SWIM to eggDIPLOID Sporophyte grows out of haploid female gametophyteEgg/ ovumMITOSISFertilization In female gametophyte creates DIPLOID ZYGOTEMEIOSIS occurs in spore capsuleMale GametophyteHaploid generationFemale GametophyteHaploid generationSpore capsule opens and releases HAPLOID sporesHaploid spores germinate and grow into haploid gametophytesHaploid spores land in suitable environment
32 FIRST Period of Plant Evolution: Origin of Plants from Aquatic Ancestors Adaptations to Terrestrial LifeCuticleJacketed gametangia protect gametes and embryosBUT, still tied to water because…Still have swimming spermMUST have water for fertilization to occurLACK vascular tissueMeans no real means of efficiently conducting food and waterSize is limited – very smallNo seeds – spores used for embryo dispersalGametophyte is dominant generationDiploid generation is present ONLY as ZYGOTE!!
34 2nd Period of Plant Evolution – Diversification of Vascular Plants Group known as SEEDLESS VASCULAR PLANTSAdaptationsFrom Previous Group:Cuticle and gametangia of previous plantsNew to SEEDLESS VASCULAR PLANTS:Sporophyte is dominant GenerationVASCULAR TISSUE added!!Can efficiently move food and water throughout bodyCan attain larger sizeStill share these primitive characteristics:Still depend on swimming spermStill no seeds – use spores for embryo dispersal
35 2nd Period of Plant Evolution – Diversification of Vascular Plants Living groups from this period of plant evolutionDivision Psilophyta – whisk ferns10-13 speciesDivision Lycophyta – club mosses1,000 speciesDivision Sphenophyta – horsetails15 speciesDivision Pterophyta – ferns12,000 species
39 3rd Period of Plant Evolution – Diversification of Vascular Plants Group known as GYMNOSPERMS or “Naked Seed Plants”AdaptationsFrom previous groups:Cuticle, gametangia, dominant sporophyte and vascular tissueNEW to GYMNOSPERMS:SEEDS – to further protect embryos from dessication (and other hazards) and aid in dispersal.Pollen – air transported sperm – no longer need water to transport swimming spermStill primitive in that…NO FlowersNO Fruit
40 3rd Period of Plant Evolution – Diversification of Vascular Plants “naked seeds” refers to seeds NOT being enclosed in any specialized chambers (fruit) Seeds are exposed to environment on highly modified leaf surfaces (ex. Cones)A seed consists ofEmbryoStored food (starch)Protective covering (seed coat)Pollen – airborne sperm (sort of)Highly reduced male gametophyteNo longer dependent on water for fertilization
41 3rd Period of Plant Evolution – Diversification of Vascular Plants Living groups from this period of plant evolutionDivision Coniferophyta (conifers)Largest groupNeedle-like leaves550 speciesDivision Cycadophyta100 speciesDivision Gingkophyta1 speciesDivision Gnetophyta70 species
42 4th Period of Plant Evolution – Diversification of Vascular Plants Emergence of Flowering Plants - AngiospermsAdaptationsFrom previous groups:Cuticle, gametangia, dominant sporophyte, vascular tissue, seeds and pollen of previous groupsNEW to ANGIOSPERMS:FLOWERSFRUITSDouble fertilizationFert of egg to make embryoFert of 2 polar nuclei to make endospermTRIPLOID tissue (endosperm – stored food2 polar nuclei + 1 sperm nuc. = 3n
44 4th Period of Plant Evolution – Diversification of Vascular Plants Advantages of flowers and fruitsFlowers draw pollinators which are very efficient at transferring pollen from flower to flowerFruits aid in seed dispersal either by water, wind or animalsOther plants depend on water or wind for gamete and seed dispersal which is not near as precise.Angiosperms may use wind also, but fruits allow for elaborate structures to carry seeds FURTHER by wind.
45 4th Period of Plant Evolution – Diversification of Vascular Plants Advantage of triploid tissue in seedsHas greater ability to pull food resources from “mother” than embryo does. Thus nutrients can be stored in the seed for later use.Also, fact that endosperm must be fertilized ensures that endosperm will not be made if fertilization has not occurred. Avoids waste.
46 4th Period of Plant Evolution – Diversification of Vascular Plants Living groups from this period of evolutionONE Divison – Anthophyta235,000 speciesTwo Important ClassesMonocotDicot
49 Alternation of Generations Haploid gametophytes and diploid sporophytes take turns producing one another.GametophytePlant that makes GAMETESHAPLOID PLANTMAKES HAPLOID CELLSSporophyteDiploid plantCells inside undergo MEIOSISPlant that makes SPORES – HAPLOID CELLSWhy make spores?Efficient at dispersal of “babies”Wind dispersed
50 Evolutionary Trends in Alternation of Generations Initially in the evolution of land plants, gametophytes were the dominant generation.Larger than sporophyteSporophyte dependent on gametophyte for everything – sporophyte NOT photosynthetic
51 Evolutionary Trends in Alternation of Generations Trend in plant evolution was for SPOROPHYTE to become dominant and GAMETOPHYTE to become dependentWhy?Diploid tissue is less subject to the negative effects of mutation than haploid tissueIn diploid tissue there are two copies of genes. If one gets damaged, the other may be able to function normally.
52 Why Study Plant Life Cycles? Features of the life cycles are key evolutionary adaptations to terrestrial environmentsExample: replacement of flagellated sperm by pollen