chapter 29

Fig. 29-1

Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Concept 29.1: Land plants evolved from green algaeGreen algae called charophytes are the closest relatives of land plants


Morphological and Molecular EvidenceMany characteristics of land plants also appear in a variety of algal clades, mainly algaeHowever, land plants share four key traits only with charophytes:Rose-shaped complexes for cellulose synthesisPeroxisome enzymesStructure of flagellated spermFormation of a phragmoplast

Fig. 29-230 nm


Comparisons of both nuclear and chloroplast genes point to charophytes as the closest living relatives of land plantsNote that land plants are not descended from modern charophytes, but share a common ancestor with modern charophytes

Fig. 29-340 m5 mmChara species, a pond organism Coleochaete orbicularis, adisk-shaped charophyte thatalso lives in ponds (LM)


Adaptations Enabling the Move to LandIn charophytes a layer of a durable polymer called sporopollenin prevents exposed zygotes from drying outThe movement onto land by charophyte ancestors provided unfiltered sun, more plentiful CO2, nutrient-rich soil, and few herbivores or pathogensLand presented challenges: a scarcity of water and lack of structural support


The accumulation of traits that facilitated survival on land may have opened the way to its colonization by plantsSystematists are currently debating the boundaries of the plant kingdomSome biologists think the plant kingdom should be expanded to include some or all green algaeUntil this debate is resolved, we will retain the embryophyte definition of kingdom Plantae

Fig. 29-4ANCESTRALALGARed algaeChlorophytesCharophytesEmbryophytesViridiplantaeStreptophytaPlantae


Derived Traits of PlantsFour key traits appear in nearly all land plants but are absent in the charophytes:Alternation of generations (with multicellular, dependent embryos)Walled spores produced in sporangiaMulticellular gametangiaApical meristems


Additional derived traits such as a cuticle and secondary compounds evolved in many plant speciesSymbiotic associations between fungi and the first land plants may have helped plants without true roots to obtain nutrients


Alternation of Generations and Multicellular, Dependent EmbryosPlants alternate between two multicellular stages, a reproductive cycle called alternation of generationsThe gametophyte is haploid and produces haploid gametes by mitosisFusion of the gametes gives rise to the diploid sporophyte, which produces haploid spores by meiosis


The diploid embryo is retained within the tissue of the female gametophyteNutrients are transferred from parent to embryo through placental transfer cellsLand plants are called embryophytes because of the dependency of the embryo on the parent

Fig. 29-5aGametophyte(n)Gamete fromanother plantnnMitosisGameteFERTILIZATIONMEIOSISMitosisSporenn2nZygoteMitosisSporophyte(2n)Alternation of generations

Fig. 29-5bEmbryoMaternal tissueWall ingrowthsPlacental transfer cell(outlined in blue)Embryo (LM) and placental transfer cell (TEM)of Marchantia (a liverwort)2 m10 m


Walled Spores Produced in SporangiaThe sporophyte produces spores in organs called sporangiaDiploid cells called sporocytes undergo meiosis to generate haploid sporesSpore walls contain sporopollenin, which makes them resistant to harsh environments

Fig. 29-5cSporesSporangiumSporophyteLongitudinal section ofSphagnum sporangium (LM)GametophyteSporophytes and sporangia of Sphagnum (a moss)


Multicellular GametangiaGametes are produced within organs called gametangiaFemale gametangia, called archegonia, produce eggs and are the site of fertilizationMale gametangia, called antheridia, are the site of sperm production and release

Fig. 29-5dFemale gametophyteMalegametophyteAntheridiumwith spermArchegoniumwith eggArchegonia and antheridia of Marchantia (a liverwort)


Apical MeristemsPlants sustain continual growth in their apical meristemsCells from the apical meristems differentiate into various tissues

Fig. 29-5eApicalmeristemof shootDevelopingleavesApical meristemsApical meristemof rootRoot100 m100 mShoot


The Origin and Diversification of PlantsFossil evidence indicates that plants were on land at least 475 million years agoFossilized spores and tissues have been extracted from 475-million-year-old rocks

Fig. 29-6(a) Fossilized spores(b) Fossilized sporophyte tissue


Those ancestral species gave rise to a vast diversity of modern plantsLand plants can be informally grouped based on the presence or absence of vascular tissueMost plants have vascular tissue; these constitute the vascular plantsNonvascular plants are commonly called bryophytes


Seedless vascular plants can be divided into cladesLycophytes (club mosses and their relatives)Pterophytes (ferns and their relatives)Seedless vascular plants are paraphyletic, and are of the same level of biological organization, or grade


A seed is an embryo and nutrients surrounded by a protective coatSeed plants form a clade and can be divided into further clades:Gymnosperms, the naked seed plants, including the conifersAngiosperms, the flowering plants

Table 29-1

Fig. 29-7Origin of land plants (about 475 mya)123123Origin of vascular plants (about 420 mya)Origin of extant seed plants (about 305 mya)ANCES-TRALGREENALGALiverwortsHornwortsMossesLycophytes (club mosses,spike mosses, quillworts)Pterophytes (ferns,horsetails, whisk ferns)GymnospermsAngiospermsSeed plantsSeedlessvascularplantsNonvascularplants(bryophytes)Land plantsVascular plantsMillions of years ago (mya)500450400350300500


Concept 29.2: Mosses and other nonvascular plants have life cycles dominated by gametophytesBryophytes are represented today by three phyla of small herbaceous (nonwoody) plants:Liverworts, phylum HepatophytaHornworts, phylum AnthocerophytaMosses, phylum BryophytaMosses are most closely related to vascular plants

Fig. 29-UN1Nonvascular plants (bryophytes)Seedless vascular plantsGymnospermsAngiosperms


Bryophyte GametophytesIn all three bryophyte phyla, gametophytes are larger and longer-living than sporophytesSporophytes are typically present only part of the time

Fig. 29-8-1KeyHaploid (n)Diploid (2n)Protonemata(n)BudBudMalegametophyte(n)Femalegametophyte (n)

GametophoreRhizoidSporesSporedispersalPeristomeSporangiumMEIOSISSetaCapsule(sporangium)FootMaturesporophytesCapsule withperistome (SEM)Femalegametophytes2 mm


GametophoreRhizoidSporesSporedispersalPeristomeSporangiumMEIOSISSetaCapsule(sporangium)FootMaturesporophytesCapsule withperistome (SEM)Femalegametophytes2 mmRaindropSpermAntheridiaEggArchegoniaFERTILIZATION(within archegonium)


GametophoreRhizoidSporesSporedispersalPeristomeSporangiumMEIOSISSetaCapsule(sporangium)FootMaturesporophytesCapsule withperistome (SEM)Femalegametophytes2 mmRaindropSpermAntheridiaEggArchegoniaFERTILIZATION(within archegonium)Zygote(2n)EmbryoArchegoniumYoungsporophyte(2n)

Fig. 29-8aCapsule withperistome (SEM)2 mm


A spore germinates into a gametophyte composed of a protonema and gamete-producing gametophoreRhizoids anchor gametophytes to substrateThe height of gametophytes is constrained by lack of vascular tissuesMature gametophytes produce flagellated sperm in antheridia and an egg in each archegoniumSperm swim through a film of water to reach and fertilize the egg


Animation: Moss Life Cycle


Bryophyte SporophytesBryophyte sporophytes grow out of archegonia, and are the smallest and simplest sporophytes of all extant plant groupsA sporophyte consists of a foot, a seta (stalk), and a sporangium, also called a capsule, which discharges spores through a peristomeHornwort and moss sporophytes have stomata for gas exchange

Fig. 29-9aThallusGametophore offemale gametophyteMarchantia polymorpha,a thalloid liverwortMarchantia sporophyte (LM)SporophyteFootSetaCapsule(sporangium)500 m

Fig. 29-9bPlagiochiladeltoidea,a leafyliverwort

Fig. 29-9cAn Anthoceroshornwort speciesSporophyteGametophyte

Fig. 29-9dGametophyteSetaCapsuleSporophyte(a sturdyplant thattakes monthsto grow)Polytrichum commune,hairy-cap moss


The Ecological and Economic Importance of MossesMoses are capable of inhabiting diverse and sometimes extreme environments, but are especially common in moist forests and wetlandsSome mosses might help retain nitrogen in the soil

Fig. 29-10RESULTSWith mossWithout mossAnnual nitrogen loss(kg/ha)0123456


Sphagnum, or peat moss, forms extensive deposits of partially decayed organic material known as peatSphagnum is an important global reservoir of organic carbon

Fig. 29-11(a) Peat being harvested(b) Tollund Man, a bog mummy

Fig. 29-11a(a) Peat being harvested

Fig. 29-11b(b) Tollund Man, a bog mummy


Concept 29.3: Ferns and other seedless vascular plants were the first plants to grow tallBryophytes and bryophyte-like plants were the prevalent vegetation during the first 100 million years of plant evolution Vascular plants began to diversify during the Devonian and Carboniferous periodsVascular tissue allowed these plants to grow tallSeedless vascular plants have flagellated sperm and are usually restricted to moist environments

Fig. 29-UN2Nonvascular plants (bryophytes)Seedless vascular plantsGymnospermsAngiosperms


Origins and Traits of Vascular PlantsFossils of the forerunners of vascular plants date back about 420 million yearsThese early tiny plants had independent, branching sporophytesLiving vascular plants are characterized by: Life cycles with dominant sporophytesVascular tissues called xylem and phloemWell-developed roots and leaves

Fig. 29-12Sporophytes of Aglaophyton major


Life Cycles with Dominant SporophytesIn contrast with bryophytes, sporophytes of seedless vascular plants are the larger generation, as in the familiar leafy fernThe gametophytes are tiny plants that grow on or below the soil surfaceAnimation: Fern Life Cycle

Fig. 29-13-1KeyHaploid (n)Diploid (2n)MEIOSISSporedispersalSporangiumSporangiumMaturesporophyte(2n)SorusFiddlehead

Fig. 29-13-2KeyHaploid (n)Diploid (2n)MEIOSISSporedispersalSporangiumSporangiumMaturesporophyte(2n)SorusFiddleheadSpore(n)YounggametophyteMaturegametophyte(n)ArchegoniumEggAntheridiumSpermFERTILIZATION

Fig. 29-13-3KeyHaploid (n)Diploid (2n)MEIOSISSporedispersalSporangiumSporangiumMaturesporophyte(2n)SorusFiddleheadSpore(n)YounggametophyteMaturegametophyte(n)ArchegoniumEggAntheridiumSpermFERTILIZATIONNewsporophyteGametophyteZygote(2n)


Transport in Xylem and Phloem Vascular plants have two types of vascular tissue: xylem and phloemXylem conducts most of the water and minerals and includes dead cells called tracheidsPhloem consists of living cells and distributes sugars, amino acids, and other organic productsWater-conducting cells are strengthened by lignin and provide structural supportIncreased height was an evolutionary advantage


Evolution of RootsRoots are organs that anchor vascular plantsThey enable vascular plants to absorb water and nutrients from the soilRoots may have evolved from subterranean stems


Evolution of LeavesLeaves are organs that increase the surface area of vascular plants, thereby capturing more solar energy that is used for photosynthesis


Leaves are categorized by two types:Microphylls, leaves with a single veinMegaphylls, leaves with a highly branched vascular systemAccording to one model of evolution, microphylls evolved first, as outgrowths of stems

Fig. 29-14Vascular tissueSporangiaMicrophyll(a) Microphylls(b) MegaphyllsOvertoppinggrowthMegaphyllOther stemsbecome re-duced and flattened.Webbingdevelops.


Sporophylls and Spore VariationsSporophylls are modified leaves with sporangiaSori are clusters of sporangia on the undersides of sporophyllsStrobili are cone-like structures formed from groups of sporophylls


Most seedless vascular plants are homosporous, producing one type of spore that develops into a bisexual gametophyteAll seed plants and some seedless vascular plants are heterosporousHeterosporous species produce megaspores that give rise to female gametophytes, and microspores that give rise to male gametophytes

Fig. 29-UN3Homosporous spore productionSporangiumon sporophyllSingletype of sporeTypically abisexualgametophyteEggsSpermEggsSpermHeterosporous spore productionMegasporangiumon megasporophyllMegasporeFemalegametophyteMalegametophyteMicrosporeMicrosporangiumon microsporophyll


Classification of Seedless Vascular PlantsThere are two phyla of seedless vascular plants:Phylum Lycophyta includes club mosses, spike mosses, and quillwortsPhylum Pterophyta includes ferns, horsetails, and whisk ferns and their relatives

Fig. 29-15aLycophytes (Phylum Lycophyta)Selaginella apoda,a spike mossIsoetesgunnii,a quillwortStrobili(clusters ofsporophylls)2.5 cmDiphasiastrum tristachyum, a club moss1 cm

Fig. 29-15bSelaginella apoda,a spike moss1 cm

Fig. 29-15cIsoetesgunnii, a quillwort

Fig. 29-15dStrobili(clusters ofsporophylls)Diphasiastrum tristachyum, a club moss2.5 cm

Fig. 29-15ePterophytes (Phylum Pterophyta)Athyriumfilix-femina,lady fernVegetative stemStrobilus onfertile stem1.5 cm25 cm2.5 cmPsilotumnudum,a whiskfernEquisetumarvense,fieldhorsetail

Fig. 29-15fAthyriumfilix-femina,lady fern25 cm

Fig. 29-15gEquisetumarvense,fieldhorsetailVegetative stemStrobilus onfertile stem1.5 cm

Fig. 29-15h2.5 cmPsilotumnudum,a whiskfern


Phylum Lycophyta: Club Mosses, Spike Mosses, and QuillwortsGiant lycophytes thrived for millions of years in moist swampsSurviving species are small herbaceous plantsClub mosses and spike mosses have vascular tissues and are not true mosses


Phylum Pterophyta: Ferns, Horsetails, and Whisk Ferns and RelativesFerns are the most diverse seedless vascular plants, with more than 12,000 speciesThey are most diverse in the tropics but also thrive in temperate forestsHorsetails were diverse during the Carboniferous period, but are now restricted to the genus EquisetumWhisk ferns resemble ancestral vascular plants but are closely related to modern ferns


The Significance of Seedless Vascular PlantsThe ancestors of modern lycophytes, horsetails, and ferns grew to great heights during the Devonian and Carboniferous, forming the first forestsIncreased photosynthesis may have helped produce the global cooling at the end of the Carboniferous periodThe decaying plants of these Carboniferous forests eventually became coal

Fig. 29-16

Fig. 29-UN4GametophyteMitosisMitosisSporeGameteMitosisnnnn2nMEIOSISFERTILIZATIONZygoteSporophyteHaploidDiploid1234Alternation of generationsApical meristemsMulticellular gametangiaWalled spores in sporangiaArchegoniumwith eggAntheridiumwith spermSporangiumSporesApical meristemof shootDevelopingleaves

Fig. 29-UN4aGametophyteMitosisMitosisMitosisSporeGameteZygotennnn2nMEIOSISFERTILIZATIONHaploidDiploidSporophyteAlternation of generations

Fig. 29-UN4bApical meristemof shootApical meristems

Developingleaves

Fig. 29-UN4cArchegoniumwith eggAntheridiumwith spermMulticellular gametangia

Fig. 29-UN4dWalled spores in sporangiaSporangiumSpores

Fig. 29-UN5

Fig. 29-UN6

Fig. 29-UN7


You should now be able to:Describe four shared characteristics and four distinct characteristics between charophytes and land plantsDistinguish between the phylum Bryophyta and bryophytesDiagram and label the life cycle of a bryophyteExplain why most bryophytes grow close to the ground and are restricted to periodically moist environments


Describe three traits that characterize modern vascular plants and explain how these traits have contributed to success on landExplain how vascular plants differ from bryophytesDistinguish between the following pairs of terms: microphyll and megaphyll; homosporous and heterosporousDiagram and label the life cycle of a seedless vascular plant

Figure 29.1 How did plants change the world?Figure 29.2 Rosette cellulose-synthesizing complexes

Figure 29.3 Examples of charophytes, the closest algal relatives of land plantsFigure 29.4 Three possible plant kingdomsFigure 29.5 Derived traits of land plants

Figure 29.5 Derived traits of land plantsFigure 29.5 Derived traits of land plantsFigure 29.5 Derived traits of land plantsFigure 29.5 Derived traits of land plantsFigure 29.6 Ancient plant spores and tissue (colorized SEMs)

Table 29.1Figure 29.7 Highlights of plant evolutionFigure 29.8 The life cycle of a mossFigure 29.8 The life cycle of a mossFigure 29.8 The life cycle of a mossFigure 29.8 The life cycle of a mossFigure 29.9 Bryophyte diversity

Figure 29.9 Bryophyte diversity

Figure 29.9 Bryophyte diversity

Figure 29.9 Bryophyte diversityFigure 29.10 Can bryophytes reduce the rate at which key nutrients are lost from soils?Figure 29.11 Sphagnum, or peat moss: a bryophyte with economic, ecological, and archaeological significanceFigure 29.11 Sphagnum, or peat moss: a bryophyte with economic, ecological, and archaeological significanceFigure 29.11 Sphagnum, or peat moss: a bryophyte with economic, ecological, and archaeological significanceFigure 29.12 Sporophytes of Aglaophyton major, an ancient relative of present-day vascular plantsFigure 29.13 The life cycle of a fernFigure 29.13 The life cycle of a fernFigure 29.13 The life cycle of a fernFigure 29.14 Hypotheses for the evolution of leaves

Figure 29.15 Seedless vascular plant diversity

Figure 29.15 Seedless vascular plant diversityFigure 29.15 Seedless vascular plant diversity

Figure 29.15 Seedless vascular plant diversityFigure 29.15 Seedless vascular plant diversityFigure 29.15 Seedless vascular plant diversity



Figure 29.16 Artists conception of a Carboniferous forest based on fossil evidence

chapter 29

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