Plant Evolution, Diversity, and Adaptations PDF

Plant Diversity I: How Plants Colonized Land Chapter 29 Plant Evolution Over 325,000 plan t species iden tified Most live on land Plants supply oxygen, food sour ces, and habitat for many other terr estr ial organism s Ancestral traits in green algae Ma n y k e y t r ai ts of p l...

Plant Diversity I: How Plants Colonized Land Chapter 29 Plant Evolution Over 325,000 plan t species iden tified Most live on land Plants supply oxygen, food sour ces, and habitat for many other terr estr ial organism s Ancestral traits in green algae Ma n y k e y t r ai ts of p l an t s a ls o a p p ea r i n s o m e al g ae Fo r e x am p l e, pl a n ts an d s o m e a l g ae ar e m u l ti c el l u l ar , e u k ar y ot i c, p h o to s y n t h et i c a u to tr o p h s So m e al g ae , l i k e p l an t s , h a v e cel l u l o s e i n th e i r ce ll w a l l s an d c h lo r o p la s t s co n t ai n i n g ch l or o p h y l l a a n d b Ancestor was likely a charophyte Sa me arra ngement o f c ellulo se - synthesiz ing membrane pro teins Arranged in rings, rather than linear sets Similar structur e o f flagella ted sperm Sequence s imilarities in nuc lear, c hloropla st, and mitoc hondr ia l DNA Recent genetic a nalys is indic ates c harophytes in the c lade Zygnema tophyc eae ar e the clos est living relatives o f plants Adaptations for Terrestrial Life Benefits of life on land: Unfiltered sunlight, mo re plentiful CO 2 , and nutrient -ric h s oil Challenges to life on land: A scarcity of water Gravity Pla nts ac cumulated adaptations to terres tr ial c hallenges , r esulting in adaptive radiatio n Sporopollenin Ancestral adapt ation to terrest ri al life Pl ant spo res have a coating of sporopolleni n, a po lymer that prevents zygotes from drying out Ch ar op hyte s al so ha ve s p o ro p o ll eni n Ot her adaptations are derived co mpared to c haro phytes Divergence from algae Subject to controversy The placement of the boundary dividing plants from algae is the subject of ongoing debate The single defining trait of plant divergence is the embryo The traditional definition equates the kingdom Plantae with embryophytes (plants with embryos)  Four key traits appear in nearly all plants but are Derived absent in the charophytes Alternation of generations (where Traits in generations look different) Walled spores produced in sporangia Plants Apical meristems Alternation of Generations Life cycles of plants alternate between two generations of multicellular organisms: gametophytes and sporophytes The multicellular haploid gametophyte produces haploid gametes (sperm and eggs) by mitosis The multicellular diploid sporophyte produces haploid spores by meiosis Spores develop into gametophytes and fertilized eggs (zygotes) develop into sporophytes The Embryo Multicellular, Dependent Embryos This is parental care. The diploid embryo is retained and protected within the tissue of the female gametophyte Nutrients are transferred from parent to embryo through placental transfer cells Plants are called embryophytes because of this dependency of the embryo on the parent Walled Spores Produced in Sporangia The sporophyte produces spores in multicellular organs called sporangia Spore walls contain sporopollenin, which makes them resistant to harsh environments Apical Meristem Localized regions of cell division at the tips of roots and shoots are called apical meristems These cells divide continuously, enabling elongation of roots and shoots for better resource acquisition Additional derived traits The cuticle, a waxy covering of the epidermis that reduces water loss The stomata, pores that facilitate gas exchange between the outside air and internal plant tissues Origin and Diversification of Plants Microorganisms colonized land 3.2 billion years ago Plant spores first appear in the fossil record from about 470 million years ago Fossilized spores embedded in sporophyte tissue have been extracted from 450-million-year-old rocks Phylogeny Major Adaptive Radiation Events: Movement to land Vascular System Seeds Flowers First Plants Late Silurian Period Bryophytes: mosses, liverworts, hornworts No vascular system Vulnerable to gravity (small) Required water for reproduction Limited root system, able to grow over rocks, sand Evolution of a vascular system The evolution of lignin ~450 million years ago led to a vascular system, allowing plants to transport water and nutrients This made forests possible The first vascular plants still lacked seeds These were ferns, monilophytes, horsetail Seedless trees lost during Permian extinction Vascular seedless plants diversified into small herbaceous plants and giant trees by the Carboniferous period and formed forests Seedless trees survived for millions of years in moist swamps, but were lost to climate drying during the Permian period Lycophytes and horsetails declined after Permian About 1,200 species of small lycophytes remain now Only one genus of horsetails remains, 15 species total Lignin led to coal Decay of lignin was slow in Carboniferous swamps; undecayed organic material slowly turned into peat Evolution of fungi that could decompose lignin slowed coal formation Oceans formed, piling up marine sediments, and exerting heat and pressure on the peat Over millions of years, peat converted to coal, which people have burned since the Industrial Revolution Evolution of Seeds Characteristics found in living seed plants date back to the late Devonian period (380 million years ago) For example, Archaeopteris was a heterosporous tree with a woody stem, but it did not bear seeds A 360-million-year-old fossil from the genus Elkinsia provides the earliest evidence of seed plants Oldest gymnosperm fossils are about 305 mullion years old Gymnosperms replaced seedless vascular plants in the drying climate of the late Carboniferous period Had adaptations for drought tolerance and produced seeds and pollen Gymnosperms dominant in Mesozoic Gymnosperms dominated terrestrial ecosystems during the Mesozoic era, 252 to 66 million years ago They served as food for herbivorous dinosaurs Recent fossil discoveries show that gymnosperms were pollinated by insects over 100 million years ago Angiosperms began to replace gymnosperms near the end of the Mesozoic era Evolution of Flowers (Angiosperms) Appear suddenly in fossil record 100 million years ago We currently do not understand how angiosperms arose from earlier seed plants Angiosperms originated in the early Cretaceous period, about 140 million years ago The earliest pollen fossils with angiosperm characteristics are 130 million years old They dominated some terrestrial ecosystems by the mid-Cretaceous, about 100 million years ago They diversified through the mass extinctions at the end of the Cretaceous, 66 million years ago Now widespread Selective advantage Heterosporous plants can specialize in adaptations for fertilization and seed dispersal Plant Diversity All plants exhibit alternation of generations and have a sporophyte phase and a gametophyte phase Diploid sporophyte produces spores through meiosis Haploid gametophyte produces gametes through mitosis Bryophytes Herbaceous (non-woody plants) Liverworts, phylum Hepatophyta Mosses, phylum Bryophyta Hornworts, phylum Anthocerophyta Basal group in plant lineage Gametophyte is dominant form Gametophyte is dominant form Larger and longer-living than sporophyte Sporophytes present for only part of life cycle Spores grow protonema 1 cell thick Can absorb water and nutrients and photosynthesize Forms buds that develop into gametophyte Both are on same plant Bryophytes Most bryophytes are constrained in height by Lack of rigid support tissues Lack of vascular tissue for long- distance transport Some moss species have conducting tissues that enable growth up to 60 cm (2 feet) tall Rhizoids Root-like structures that anchor gametophytes to the substrate Do not absorb nutrients Lack specialized conducting cells and do not participate in water or mineral absorption Gametophytes Gametophytes can produce multiple gametangia, structures that produce gametes Archegonia, female gametangia, produce a single nonmotile egg Antheridia, male gametangia, produce many motile sperm Fertilization requires water Flagellated sperm swim to the egg through a film of water in response to chemical attractants The fertilized egg and resulting embryo are retained within the archegonium Many bryophytes also reproduce asexually For example, some mosses produce brood bodies, small plantlets that detach and form new plants Bryophyte Sporophytes Attached to and dependent on the gametophyte The gametophyte supplies sugars, amino acids, minerals, and water to the sporophyte Smallest sporophytes of all extant plant groups A typical bryophyte sporophyte consists of three major parts The foot absorbs nutrients from the gametophyte The seta (stalk) conducts nutrients to the sporangium The sporangium, also called a capsule, produces spores by meiosis A peristome at the top of the capsule disperses spores when conditions are dry Liverworts have Smaller than mosses and hornwort simple sporophyte Lack stomata Liverwort Liverworts, Phylum Hepatophyta Liver-shaped gametophytes Some gametangia on stalks, others stemlike with leaflike appendages No stomata on sporophyte Hornworts, Phylum Anthocerophyta Symbiosis with nitrogen fixing bacteria Sporophytes are horned-shaped Up to 5cm tall Splits open to release spores Gametophytes 1-2 cm Multiple sporophytes attached Mosses, Phylum Bryophyta Visible sporophytes Green and photosynthetic when immature Turn brown before releasing spores Ecological Importance Mosses are common in moist forests and wetlands They also inhabit extremely cold, hot, and dry environments, rehydrating after complete desiccation Some colonize and help retain nitrogen in bare, sandy soils Peat Sphagnum, or “peat moss,” forms extensive deposits of partially decayed organic material known as peat Peat can be used as a source of fuel The low temperature, pH, and oxygen level of peatlands inhibit decay of moss and other organisms Some peatlands have preserved corpses for thousands of years Peatland Peatlands cover 3% of Earth’s land surface but contain one- third of the world’s soil carbon Overharvesting of Sphagnum contributes to global warming by releasing stored CO2 Decomposition will start releasing even more CO2 if water levels drop with warming temperatures Seedless vascular plants Seedless vascular plants have an extensive vascular transport system, but do not produce seeds Still require wet environment for sperm to travel to egg They are divided into two clades: Lycophytes are club mosses and their relatives Monilophytes are ferns and their relatives New adaptations Sporophyte and gametophyte sometimes independent Sporophyte becomes dominant Has branching structure Larger and more complex Vascular tissue Xylem and phloem Roots and leaves Sporophylls: Spore bearing leaves Larger size through vascular transport Two vessel types: Zylem Conducts most of the water and minerals Xylem cells are dead at functional maturity and are lignified: strengthened by the polymer lignin Phloem Organic molecules like sugar Remain living Xylem Phloem Benefits Long distance transport of nutrients supports plant tissue away from water source Lignin in xylem supports taller growth Height led to competition for sunlight and greater spore dispersal Eventually led to forests  Both anchor plants and absorb soil nutrients Evolution of Roots Closely resemble stem tissue of early vascular plants Likely evolved from stems Evolution of Leaves Leaves increase surface area for light capture and conduct most of the photosynthesis in plants They are categorized by two types: Microphylls, small, often spine- shaped leaves with a single vein, are found only in lycophytes Megaphylls, larger leaves with a highly branched vascular system, are found in all other plant groups  Sporophylls and Spore Variations Sporophylls are modified leaves with sporangia Sori are clusters of sporangia on the undersides of fern sporophylls (sorus is one sori) Most seedless vascular plants are homosporous They have one type of sporophyll and sporangium, which produces one type of spore The spores usually produce bisexual gametophytes Sporophylls and Many lycophytes (club mosses) and most gymnosperms have strobili, clumps of sporophylls in cone-like structures Spore Variations Plant Sex Terms Alternation of generations makes terms confusing Fern gametophyte can be called bisexual, hermaphroditic or monoecious The gametophyte has antheridia and archegonia Sporophyte does not have a sex: homosporous Even more confusing in angiosperms (flowering plants) heterosporous  Lycophytes (Phylum Lycophyta) Lycophytes grow in diverse habitats Some gametophytes are photosynthetic; others form below ground symbioses with fungi Sporophytes have both leaf-forming upright stems, and ground-hugging root-forming stems Spikemosses and quillworts are all heterosporous; clubmosses are homosporous Clubmosses and spikemosses not true moss (have vascular structure) Monilophytes Ferns, horsetails, whisk ferns and relatives (Phylum More closely related to seed plants than lycophytes Monilophyta) Both have megaphylls and branching roots Ferns Most widespread seedless vascular plant, 12,000+ species Most diverse in tropical forests Fern sporophytes have large megasporophylls (fronds) that are divided into leaflets A frond is coiled at the tip (the fiddlehead), which unfurls as the leaf grows Most species are homosporous, and have springlike devices for spore dispersal Horsetail - Equisetum Horsetail sporophytes have jointed stems with rings of small leaves or branches Stems high in silicon, have historically been used for scouring pots and pans as well as herbal remedies Some have separate fertile and vegetative stems Gametophytes are bisexual