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Transcript
Last Week -The land plant clade -The Tracheophytes -The fossil record - Overview of land plant phylogeny (more detail) - 4 central functions all land plants ‘do’ - Importance of physical laws in shaping plant form Function, and evolution - hydraulics Lecture 8 Feb. 6th, 2007 Overview Today (1) Cope’s Rule - why is bigger better? Plant size and plant hydraulics Basics of plant hydraulics that rely on vascular ‘tubes’ Hagen - Poiseuille equation describes how selection should govern the evolution of plant hydraulics (2) PaleoBotany Early split in vascular plant evolution ‘Leaf’ Evolution Convergent evolution Several important paleo plants Each major plant clade has increased in max. size over time Cope’s Rule (E.D. Cope 1896) Evolution of Angiosperms Evolution of Tracheophytes Fossil record - rapid evolutionary increases in the size and height of sporophytes Continued colonization on land required adaptations for Efficient internal transport Desiccation resistance or avoidance Reproduction without water Effective dispersal Competition (for multiple resources) What enabled plants to continue to increase in size? Two fundamental constraints influencing plant hydraulics and gas exchange (1) Increasing cost of resistance with increased height (2) Photosynthesis/transpiration compromise First ‘pipe’ The tracheid Second pipe - All tracheophytes The vessel - Mostly Angiosperms Tracheid diameter Throughout the evolution of the Tracheophytes . . . . . . with increases in transport distance (size) tracheid and vessel dimensions have changed Likely enabled plants to increase in size Engineering principles the most direct method is through ‘pipes’ Hagen - Poiseuille equation 8"li Zi = 4 ri Resistance of fluid flow = Zi li ri Length of pipe = li Radius of pipe = ri η = fluid viscosity What parameters can selection act on? What happens to resistance with changes in length vs. increases in radius? 8"li Zi = 4 ri Resistance decreases linearly with decreases in length But it decreases as r-4 Consider the advantage of the evolution of vessels in angiosperms… Tracheid diameter Throughout the evolution of the Tracheophytes . . . . . . with increases in transport distance (size) tracheid and vessel dimensions have changed Selection to lower resistance of transport . . . Likely enabled plants to increase in size Flux density of water through plants (Transpiration) On land . . Plants require 138.8 moles of water (lost) per mole of CO2 that has been photosynthetically fixed BUT AVAILABLE WATER MAY BE LIMITED Increase transpiration <-> increase photosynthesis Decrease transpiration <-> decrease photosynthesis Photosynthesis/Transpiration compromise Paleo Botany B Lycopods Horsetails Ferns Bryophytes Gymnosperms Angiosperms L -> A Tracheophytes B Zosterophyllophyta Trimerophya Rhyniophyta Antheridia and Archegonia All the ‘action’ happens within the Devonian! Tall and Diverse Floras by the Carboniferous Early Vascular Plants The Evolution of the Leaf •Leaves essentially govern terrestrial biogeochemical and Ecological processes. •Early land plants and vascular plants had photosynthetic stems with low stomate densities •‘Megaphyll leaves’ appeared ~ 360mya (end of Devonian) about 40my after the first land plants •These ‘Megaphyll leaves’ characterized by greater stomate densities •Greater densities of stomates = greater transpiration rates (water use) = greater carbon flux •Evolution of the leaf corresponds with dramatic drop in atmospheric CO2 ‘Leaves’ - are not all the same! Three analogous structures (1) ‘Leaves’ on gametophytes of nonvascular plants (2) Enations/microphylls of Zosterophyllophytes and Lycophyta Vascular system grows into Enation after development of ‘leaf-like’ extension (3) Megaphylls - true leaves that evolved from branch systems. Present in all seed plants Leaf develops with vascular system during development Two distinct lines of Land Plant Evolution Evolutionary Split in Vascular Plant Evolution (2) Megaphyll (1) Microphyll Remember that early split we talked about?? Extant Lycopods Sporangia Clubmosses All you see is Sporophyte! Within both lines many examples of convergent evolution Traits similar to seed like plants evolved independently with the Microphyll line! What does convergent evolution suggest? (1) Microphyll line of evolution A distinct line of evolution from seed plants Gave rise to the Lycophyta (Lycopods) Share common ancestor with Zosterophyllophyta Enations (leaf-like extensions) Increased photosynthetic surface areas Enations = Microphylls Not the same as leaves but functionally similar! Reached up to 78cm long (!) Microphyll Line of Evolution Not all leaves are the same! Enations (‘leaf-like’ extensions) (1) Independent evolution of leaf-like structures Microphyll Line of Evolution (2) Independent evolution of true roots (3) Independent evolution of vascular cambium and secondary growth But - vascular cambium could not undergo radial longnitudinal division Cambial cells became increasingly wider! No specimen larger than 10cm thick Microphyll Line of Evolution (4) Independent evolution of Heterospory (5?) Megaspore in some species retained within sporophyll Similar to ovules and seeds in seed plants -BUT Sperm still needed to swim (6) Tree-like habit Convergent evolution within the Lycophytes -‘Leaves’ - Roots -Secondary growth -Heterospory -‘Proto’ ovules/seeds -Tree-like habit (2) (1) Lycophytes dominated in the Devonian and Carboniferous -Comprised 2/3 of all plants within the Carboniferous -Common in swampy areas (restricted to wet environs!) But there are only 5 genera living today Lycopodium (club moss) Extinct Lycophytes Leaves = microphylls Lepidodendron Sporangia within cones (strobili) Strobili - reproductive structures which contain a high concentration of sporangia. Sigillaria Monopodial growth Sigillaria Lepidodendron fossils “Enations” www.twoguysfossils.com/ plants_usa.htm Early Carboniferous Forests (2) Megaphyll line of plant evolution The first plant with ‘true’ leaves (megaphylls) ~ 6m tall! Archaeopteris fissilis from the archive kept by Prof. E.M. Friis at the Stockholm Museum of Natural History True leaves first became widespread with the appearance of the genus Archaeopteris, the first forest tree, and typical of the Late Devonian period approximately 360-375 Myr ago. Archaeopteris fissilis had small, dissected leaves http://www.palaeobiology.org.uk/projects_01.htm (2) Megaphyll line of evolution Trimerophyta Mya ~370 - 380 Pteridophyta (Ferns) (2) --> spores (1) Two early evolutionary lines with the Megaphylls! Early seed plants(5-6 distinct groups) Progymnosperms (extinct) Mya ~363 Pteridosperma - ‘seed ferns’ (extinct) Cordaitles Similar to extant gymnosperms Seeds in ‘Cone’ like structures 300 mya Late Carboniferous Early seed plants Progymnosperms Apical dominance growth form ‘Seed Ferns’ Pteridosperma LEAVES bear ovules -> seeds Horrible Name! Don’t be confused - these are not Pteridophytes (ferns) Carboniferous plants 363-290mya Swamp, moist forests Dominated by 5 major plant groups Lycophytes Club mosses (extant - barely!!!) Spenophytes Horesetails (extant) Pteridophytes Ferns (extant) Progymnosperms (extinct) Early Seed Plants (many extinct) Carboniferous forest . . . . Wet - swamp forests First animals - insects, early reptiles “Today . . . . Fossil Fuel” (2) Megaphyll line of evolution continued . . . Descendents of progymnosperms (Cordaitles) gave rise to Gymnosperms with two more major evolutionary lines An evolutionary line which gave rise to Cycads Ginko Swimming sperm! Elements of a ‘pollen tube’ Another evolutionary line gave rise to Conifers Gnetophytes Sperm non-mobile Pollen tube Flowering Plants Carried to egg by growth of microgametophyte as a pollen tube (elaborate) Spermatophytes Evolution of the Seed Questions •What does the Hagen-Poiseuille equation tell us about plant evolution? •What is Cope’s Rule? •Name as many cases of convergent evolution in the microphyll line as you can •What does convergent evolution tell us about plant evolution and how plants work? •Compare the microphyll line of evolution with megaphyll line •Lepidodendron? •What is the photosynthesis/transpiration compromise?