Residents of the coal-producing central Chinese city of Changsha al-most never see the sun, because it is hidden behind an atmospheredense with choking smog. Nine-tenths of the precipitation in Chang-sha is acid rain. China burns more coal than any other country in the

world, and the resulting untreated smoke leads to disastrous conditions such as thosein Changsha, the site of a major coal-fired power plant.

Coal is used for 75 percent of China’s energy needs—primarily to generate elec-tricity, but also directly for heating, smelting of metals, and other purposes. TheUnited States produces more than half of its electricity by burning coal, and indeedhas the largest coal reserves in the world. Extractable coal reserves in the U.S. exceedthe total amount of oil available for pumping in all other countries combined. Wheredid all this coal come from?

Coal comes from the remains of seedless plants that grew in great forests hundredsof millions of years ago. (The two other “fossilfuels”—petroleum and natural gas—comefrom the remains of plankton that lived in an-cient oceans.) Plant parts from those forestssank in swamps that were later covered by soil.Over millions of years, as the buried plant ma-terial was subjected to intense pressure and el-evated temperatures, coal formed.

At the time those ancient forests flourished,the plant world also included relatives of to-day’s mosses. These “mossy” ancestors werethe first plant life on dry land. Today, mossesare among the most abundant plants on Earth,yet they seem at first glance to lack adaptationsto life on land. Mosses have no advanced in-ternal “plumbing system” to move water andnutrients within their bodies, and their leafyphotosynthetic organs are only one cell thick.They require liquid water in order to repro-duce, and indeed, seem at first glance to behighly dependent on external moisture. Mossesand their relatives do have effective adapta-tions for life in terrestrial environments, how-ever, as is obvious from their wide distribution.Most live in moist habitats, but a few mosseseven live in deserts.

Plants without Seeds: From Sea to Land29

An Ingredient of Coal-Based SmogWhen coal burns, it produces the fly ashshown in this artificially colored image.When too much coal burns where thesmoke cannot blow away, the result is dis-astrous smog.

PLANTS WITHOUT SEEDS: FROM SEA TO LAND 571

The earliest terrestrial plants invadedthe land sometime during the Paleozoicera (see Table 22.1). These plants weretiny, but their metabolic activitieshelped convert parent rock into soil thatcould support the needs of their succes-sors. Larger and larger plants evolvedrapidly (in geological terms), and by theCarboniferous period (354–290 mya)great forests were widespread. How-ever, few of the trees in those forestswere like those we know today. During the tens of millionsof years since the Carboniferous, those early trees have beenreplaced by the modern trees whose adaptations and ap-pearance are familiar to us.

In this chapter, we will see how members of the plantkingdom invaded the land and evolved. Our descriptionshere will concentrate on those plants that lack seeds. The nextchapter completes our survey of the plant kingdom by con-sidering the seed plants, which dominate the terrestrial scenetoday.

The Plant Kingdom

The kingdom Plantae is monophyletic—all plants descendfrom a single common ancestor and form a branch of the evo-lutionary tree of life. The shared derived trait, or synapo-morphy, of the plant kingdom is development from embryosprotected by tissues of the parent plant. For this reason,plants are sometimes referred to as embryophytes. Plants re-tain the derived features that they share with green algae: theuse of chlorophylls a and b and the use of starch as a photo-synthetic storage product. Both plants and green algae havecellulose in their cell walls.

There are other ways to define “plant” and “plant king-dom” and still come out with a monophyletic group (clade).For example, combining plants as defined above with a groupof green algae called the charophytes results in a mono-phyletic plant kingdom with several shared derived traits, in-cluding the retention of the egg in the parent body. The addi-tion of the chlorophytes (the remainder of the green algae) tothe group just described gives another monophyletic group,with synapomorphies including the possession of chlorophyllb, that can be called a plant kingdom. There are no hard-and-fast criteria for defining a kingdom (or any other taxonomicrank), so these definitions of the plant kingdom are all valid.

In this book, we choose to use the first definition givenabove, in which the kingdom Plantae comprises only the em-bryophytes (Figure 29.1). Some botanists refer to a groupconsisting of the Plantae plus the green algae as the “greenplant kingdom,” to the red algae as the “red plant kingdom,”and to the stramenopiles as the “brown plant kingdom.”

There are ten surviving phyla of plantsThe surviving members of the kingdom Plantae fall naturallyinto ten phyla (Table 29.1). All members of seven of thosephyla possess well-developed vascular systems that trans-port materials throughout the plant body. We call these sevenphyla, collectively, the tracheophytes because they all pos-sess conducting cells called tracheids. The tracheophytes constitute a clade.

The remaining three phyla (liverworts, hornworts, andmosses), which lack tracheids, were once considered classesof a single larger phylum. In this book we use the term non-tracheophytes to refer collectively to these three phyla. Thenontracheophytes are sometimes collectively called bryo-phytes, but in this text we reserve that term for their most fa-miliar members, the mosses. Collectively, the nontracheo-phytes are not a monophyletic group. They are the threebasal clades of the plant kingdom.

Life cycles of plants feature alternation of generationsA universal feature of the life cycles of plants is the alterna-tion of generations. Recall from Chapter 28 that alternationof generations has two hallmarks:

� The life cycle includes both multicellular diploid indi-viduals and multicellular haploid individuals.

� Gametes are produced by mitosis, not by meiosis.Meiosis produces spores that develop into multicellularhaploid individuals.

If we begin looking at the plant life cycle at a single-cellstage—the diploid zygote—then the first phase of the cycle

Red algae

Stramenopiles

Chlorophytes

Charophytes

EmbryophytesPlantae

Ancestralorganism

“Green plants”

“Red plants”

“Brown plants”

29.1 What Is a Plant? There are three ways to define a plant king-dom, depending on which clade is chosen. In this book, we use themost restrictive definition: plants as embryophytes. Here, the twogreen algal clades are not considered plants.

features the formation, by mitosis and cytokinesis, of a mul-ticellular embryo and eventually the mature diploid plant(Figure 29.2). This multicellular, diploid plant is the sporo-phyte (“spore plant”).

Cells contained in sporangia (singular, sporangium,“spore vessel”) on the sporophyte undergo meiosis to pro-duce haploid, unicellular spores. By mitosis and cytokinesis,a spore forms a haploid plant. This multicellular, haploidplant is the gametophyte (“gamete plant”) that produceshaploid gametes. The fusion of two gametes (syngamy, or fer-

tilization) results in the formation of a diploid cell—the zy-gote—and the cycle repeats.

The sporophyte generation extends from the zygote throughthe adult, multicellular, diploid plant; the gametophyte gener-ation extends from the spore through the adult, multicellular,haploid plant to the gamete. The transitions between thegenerations are accomplished by fertilization and meiosis. Inall plants, the sporophyte and gametophyte differ genetically:The sporophyte has diploid cells, and the gametophyte hashaploid cells. In the three basal plant clades, the gametophytegeneration is larger and more self-sufficient, while the sporo-phyte generation is dominant in those groups that appearedlater in plant evolution.

Some protist life cycles also feature alternation of genera-tions, suggesting that the plants arose from one of these pro-tist groups. But which one?

The Plantae arose from a green algal cladeMuch evidence indicates that the closest living relatives ofthe plants are members of a clade of green algae called thecharophytes. The charophytes, along with some other greenalgae and the plants, form a clade that is sister to the chloro-phytes (see Figure 29.1), but we don’t yet know which charo-phyte clade is the true sister group to the plants. Stonewortsof the genus Chara are charophytes that resemble plants interms of their rRNA and DNA sequences, peroxisome con-

572 CHAPTER T WENT Y-NINE

Classification of Plantsa

PHYLUM COMMON NAME CHARACTERISTICS

NontracheophytesHepatophyta Liverworts No filamentous stage; gametophyte flat

Anthocerophyta Hornworts Embedded archegonia; sporophyte grows basally

Bryophyta Mosses Filamentous stage; sporophyte grows apically (from the tip)

TracheophytesNonseed tracheophytes

Lycophyta Club mosses Microphylls in spirals; sporangia in leaf axils

Pteridophyta Ferns and allies Differentiation between main axis and side branches

Seed plants

Gymnosperms

Cycadophyta Cycads Compound leaves; swimming sperm; seeds on modified leaves

Ginkgophyta Ginkgo Deciduous; fan-shaped leaves; swimming sperm

Gnetophyta Gnetophytes Vessels in vascular tissue; opposite, simple leaves

Pinophyta Conifers Seeds in cones; needlelike or scalelike leaves

Angiosperms

Angiospermae Flowering plants Endosperm; carpels; much reduced gametophytes; seeds in fruitaNo extinct groups are included in this classification.

29.1

Multicellularsporophyte

Multicellulargametophyte

DIPLOID (2n)

HAPLOID (n)Meiosis Fertilization

Spore Gametes

Zygote

29.2 Alternation of Generations A diploid sporophyte generationthat produces spores alternates with a haploid gametophyte genera-tion that produces gametes by mitosis.

tents, mechanics of mitosis and cytokinesis, and chloroplaststructure (Figure 29.3a). On the other hand, strong evidencefrom morphology-based cladistic analysis suggests that thesister group of the plants is a group of charophytes that in-cludes the genus Coleochaete (Figure 29.3b). Coleochaete-like al-gae have several features found in plants, such as plasmod-esmata and a tendency to protect the young sporophyte.

Whether they were more similar to stoneworts or toColeochaete, the ancestors of the plants lived at the margins ofponds or marshes, ringing them with a green mat. Fromthese marginal habitats, which were sometimes wet andsometimes dry, early plants made the transition onto land.

The Conquest of the Land

Plants, or their immediate ancestors in the green mat, first in-vaded the terrestrial environment between 400 and 500 mil-lion years ago. That environment differs dramatically fromthe aquatic environment. The most obvious difference is theavailability of the water that is essential for life: It is every-where in the aquatic environment, but hard to find and to re-tain in the terrestrial environment. Water provides aquaticorganisms with support against gravity; a plant on land,however, must either have some other support system orsprawl unsupported on the ground. A land plant must also

use different mechanisms for dispersing its gametes andprogeny than its aquatic relatives, which can simply releasethem into the water. How did organisms descended fromaquatic ancestors adapt to such a challenging environment?

Adaptations to life on land distinguish plants from green algaeMost of the characteristics that distinguish plants from greenalgae are evolutionary adaptations to life on land. Several ofthese features probably evolved in the common ancestor ofthe plants:

� The cuticle, a waxy covering that retards desiccation(drying)

� Gametangia, cases that enclose plant gametes and pre-vent them from drying out

� Embryos, which are young sporophytes contained withina protective structure

� Certain pigments that afford protection against the muta-genic ultraviolet radiation that bathes the terrestrialenvironment

� Thick spore walls containing a polymer that protects thespores from desiccation and resists decay

� A mutualistic* association with a fungus that promotesnutrient uptake from the soil

Further adaptations to the terrestrial environment appearedas plants continued to evolve. One of the most important ofthese later adaptations was the appearance of vascular tissues.

Most present-day plants have vascular tissuesThe first plants were nonvascular, lacking both water-con-ducting and food-conducting tissue. Although the term“nonvascular plants” is a time-honored name, it is mislead-ing when applied to the entire nontracheophyte group, be-cause some mosses (unlike liverworts and hornworts) dohave a limited amount of simple conducting tissue. Thus themore unwieldy name “nontracheophyte” is more descrip-tive. The first true tracheophytes—possessing specializedconducting cells called tracheids—arose later (Figure 29.4).

The nontracheophytes (the liverworts, hornworts, andmosses) have never been large plants. Except for some of themosses, they have no water-conducting tissue, yet some arefound in dry environments. Many grow in dense masses (see Figure 29.9a), through which water can move by capil-lary action. Nontracheophytes also have leaflike structuresthat readily catch and hold any water that splashes ontothem. These plants are small enough that minerals can be dis-tributed throughout their bodies by diffusion.

PLANTS WITHOUT SEEDS: FROM SEA TO LAND 573(a) Chara sp. (stonewort)

(b) Coleochaete sp.

29.3 The Closest Relatives of Land Plants The plant kingdomprobably evolved from a common ancestor shared with the charo-phytes, a green algal group. (a) Molecular evidence seems to favorstoneworts of the genus Chara as sister group to the plants. (b)Evidence from morphology indicates that the group including thiscoleochaete alga may be sister to the land plants.

*In a mutualistic association, both partners—here, the plant and the fun-gus—profit.

Familiar tracheophytes include the club mosses, ferns,conifers, and angiosperms (flowering plants). Tracheophytesdiffer from liverworts, hornworts, and mosses in crucialways, one of which is the possession of a well-developed vas-cular system consisting of specialized tissues for the trans-port of materials from one part of the plant to another. Onesuch tissue, the phloem, conducts the products of photosyn-thesis from sites where they are produced or released to siteswhere they are used or stored. The other vascular tissue, thexylem, conducts water and minerals from the soil to aerialparts of the plant; because some of its cell walls are stiffenedby a substance called lignin, xylem also provides support inthe terrestrial environment.

Nontracheophyte plants evolved tens of millions of yearsbefore the earliest tracheophytes, even though tracheophytesappear earlier in the fossil record. The oldest tracheophytefossils date back more than 410 million years, whereas theoldest nontracheophyte fossils are only about 350 millionyears old, dating from a time when tracheophytes were al-ready widely distributed. This finding simply shows that,given the differences in their structures and the chemicalmakeup of their cell walls, tracheophytes are more likely toform fossils than nontracheophytes are.

We will examine the adaptations of the tracheophytes laterin this chapter, concentrating first on the nontracheophytes.

The Nontracheophytes:Liverworts, Hornworts, and MossesMost liverworts, horn-worts, and mosses growin dense mats, usuallyin moist habitats. Thelargest of these plants are onlyabout 1 meter tall, and most areonly a few centimeters tall or long.Why have the nontracheophytes notevolved to be taller? The probable answeris that they lack an efficient system for con-ducting water and minerals from the soil to dis-tant parts of the plant body. To limit water loss, layers of ma-ternal tissue protect the embryos of all nontracheophytes. Allnontracheophyte clades also have a cuticle, although it is of-ten very thin (or even absent in some species) and thus nothighly effective in retarding water loss. Nontracheophyteslack the leaves, stems, and roots that characterize tracheo-phytes, although they have structures analogous to each.

Most nontracheophytes live on the soil or on other plants,but some grow on bare rock, dead and fallen tree trunks, andeven on buildings. Nontracheophytes are widely distributedover six continents and exist very locally on the coast of the

574 CHAPTER T WENT Y-NINE

Chlorophytes

Charophytes

Liverworts

Hornworts

Mosses

Club mosses

Ferns and allies

Gymnosperms

Floweringplants

Nonseedtracheophytes

Tracheophytes

KingdomPlantae

Nontracheophytes

Seedplants

Ancestralalga

First true vasculartissue

First seed plants

Protectedembryos

29.4 From Green Algae to Plants Three key characteristics thatemerged during plant evolution—protected embryos, vascular tis-sues, and seeds—are all adaptations to life in a terrestrial environ-ment. Plants with vascular tissue are called tracheophytes.

Liverworts

Hornworts

Mosses

Club mosses

Horsetails

Whisk ferns

Ferns

Gymnosperms

Flowering plants

seventh (Antarctica). They are successful plants, well adaptedto their environments. Most are terrestrial. Some live in wet-lands. Although a few nontracheophyte species live in freshwater, these aquatic forms are descended from terrestrialones. There are no marine nontracheophytes.

Nontracheophyte sporophytes are dependent on gametophytesIn nontracheophytes, the conspicuous green structure visibleto the naked eye is the gametophyte (Figure 29.5). In contrast,the familiar forms of tracheophytes, such as ferns and seedplants, are sporophytes. The gametophyte of nontracheo-phytes is photosynthetic and therefore nutritionally inde-pendent, whereas the sporophyte may or may not be photo-synthetic, but is always nutritionally dependent on thegametophyte and remains permanently attached to it.

A nontracheophyte sporophyte produces unicellular, hap-loid spores as products of meiosis within a sporangium, orcapsule. A spore germinates, giving rise to a multicellular,haploid gametophyte whose cells contain chloroplasts and are

thus photosynthetic. Eventually ga-metes form within specialized sexorgans, the gametangia. The arche-gonium is a multicellular, flask-shaped female sex organ with along neck and a swollen base, whichproduces a single egg (Figure 29.6a).The antheridium is a male sex or-gan in which sperm, each bearingtwo flagella, are produced in largenumbers (Figure 29.6b).

Once released, the sperm mustswim or be splashed by raindropsto a nearby archegonium on the

same or a neighboring plant. The sperm are aided in this taskby chemical attractants released by the egg or the archego-nium. Before sperm can enter the archegonium, certain cellsin the neck of the archegonium must break down, leaving awater-filled canal through which the sperm swim to com-plete their journey. Note that all of these events require liq-uid water.

On arrival at the egg, the nucleus of a sperm fuses withthe egg nucleus to form a zygote. Mitotic divisions of the zy-gote produce a multicellular, diploid sporophyte embryo.The base of the archegonium grows to protect the embryoduring its early development. Eventually, the developingsporophyte elongates sufficiently to break out of thearchegonium, but it remains connected to the gametophyteby a “foot” that is embedded in the parent tissue and absorbs water and nutrients from it. The sporophyte remainsattached to the gametophyte throughout its life. The sporo-phyte produces a capsule, within which meiotic divisionsproduce spores and thus the next gametophyte generation.

The structure and pattern of elongation of the sporophytediffer among the three nontracheophyte phyla—the liverworts(Hepatophyta), hornworts (Anthocerophyta), and mosses(Bryophyta). The probable evolutionary relationships of thesethree phyla and the tracheophytes can be seen in Figure 29.4.

PLANTS WITHOUT SEEDS: FROM SEA TO LAND 575

DIPLOID (2n)Sporophyte generation

HAPLOID (n)Gametophyte generation

Fertilization

Embryo (2n)

Ungerminatedspore

Gametophyte (n)

Germinatingspore

Sporophyte (2n)

Gametophytes (n)

Protonema with bud

Archegonium (n)

H2O

Antheridium (n)

Sperm (n)

Meiosis

Egg (n)

Fertilization in nontracheophytes requires water so that sperm can swim to eggs.

Egg (n)

The sporophyte is attached to and nutritionally dependent on the gametophyte.

Capsule

Photosyntheticfilament

Bud

Rhizoid

29.5 A Nontracheophyte LifeCycle The life cycle of nontra-cheophytes, illustrated here by amoss, is dependent on an exter-

nal source of liquid water. The visiblegreen structure of nontracheophytes isthe gametophyte.

Liverworts may be the most ancient surviving plant clade

The gametophytes of some liverworts (phylum Hepato-phyta) are green, leaflike layers that lie flat on the ground(Figure 29.7a). The simplest liverwort gametophytes, however,are flat plates of cells, a centimeter or so long, that produceantheridia or archegonia on their upper surfaces and anchor-ing and water-absorbing filaments called rhizoids on theirlower surfaces. Liverwort sporophytes are shorter than thoseof mosses and hornworts, rarely exceeding a few millimeters.

The liverwort sporophyte has a stalk that connects capsuleand foot. In most species, the stalk elongates and thus raisesthe capsule above ground level, favoring dispersal of sporeswhen they are released. The capsules of liverworts are simple:a globular capsule wall surrounding a mass of spores. In somespecies of liverworts, spores are not released by the sporophyteuntil the surrounding capsule wall rots. In other liverworts,however, the spores are thrown from the capsule by structures

that shorten and compress a “spring” as they dry out. Whenthe stress becomes sufficient, the compressed spring snapsback to its resting position, throwing spores in all directions.

Among the most familiar liverworts are species of thegenus Marchantia (Figure 29.7a). Marchantia is easily recog-nized by the characteristic structures on which its male andfemale gametophytes bear their antheridia and archegonia(Figure 29.7b). Like most liverworts, Marchantia also repro-duces asexually by simple fragmentation of the gametophyte.Marchantia and some other liverworts and mosses also repro-duce asexually by means of gemmae (singular, gemma), whichare lens-shaped clumps of cells. In a few liverworts, the gem-mae are loosely held in structures called gemmae cups, whichpromote dispersal of the gemmae by raindrops (Figure 29.7c).

Hornworts evolved stomata as an adaptation to terrestrial lifeThe phylum Anthocerophyta comprises the hornworts, sonamed because their sporophytes look like little horns (Fig-

(a) (b)Archegonia develop at the tip of a gametophyte. In the archegonium, the egg will be fertilized and begin development into a sporophyte.

Antheridia are also located at the tip of a gametophyte.

The large egg cell is in the center of the archegonium.

These antheridia contain a large number of sperm. When released, the sperm can be carried by water to an archegonium and then swim down its neck to the egg.

29.6 Sex Organs in Plants(a) Archegonia and (b) anthe-ridia of the moss Mnium(phylum Bryophyta). Thegametophytes of all plantshave archegonia andantheridia, but they are muchreduced in seed plants.

(a) Marchantia sp. (b) Marchantia sp. (c) Lunularia sp.

The disc-headed structures bear antheridia.

The umbrella-like structures bear archegonia.

These cups contain gemmae—small, lens-shaped outgrowths of the plant body, each capable of developing into a new plant.

29.7 Liverwort Structures Members of the phylum Hepatophytadisplay various characteristic structures. (a) Gametophytes. (b)Structures bearing antheridia and archegonia. (c) Gemmae cups.

576

ure 29.8). Hornworts appear at first glance to be liverwortswith very simple gametophytes. These gametophytes consistof flat plates of cells a few cells thick.

However, the hornworts, along with the mosses and tra-cheophytes, share an advance over the liverwort clade in theiradaptation to life on land. They have stomata—pores that,when open, allow the uptake of CO2 for photosynthesis andthe release of O2. Stomata may be a shared derived trait(synapomorphy) of hornworts and all other plants except liv-erworts, although hornwort stomata do not close and mayhave evolved independently.

Hornworts have two characteristics that distinguish themfrom both liverworts and mosses. First, the cells of hornwortseach contain a single large, platelike chloroplast, whereas thecells of other nontracheophytes contain numerous small, lens-shaped chloroplasts. Second, of all the nontracheophytesporophytes, those of the hornworts come closest to being ca-pable of indeterminate growth (growth without a set limit).Liverwort and moss sporophytes have a stalk that stopsgrowing as the capsule matures, so elongation of the sporo-phyte is strictly limited. The hornwort sporophyte, however,has no stalk. Instead, a basal region of the capsule remains ca-pable of indefinite cell division, continuously producing newspore-bearing tissue above. The sporophytes of some horn-worts growing in mild and continuously moist conditions canbecome as tall as 20 centimeters. Eventually the sporophyte’sgrowth is limited by the lack of a transport system.

To support their metabolism, the hornworts need accessto nitrogen. Hornworts have internal cavities filled with mu-cilage; these cavities are often populated by cyanobacteriathat convert atmospheric nitrogen gas into a form usable bythe host plant.

We have presented the hornworts as sister to the clade con-sisting of mosses and tracheophytes, but this is only one pos-

sible interpretation of the current data. The exact evolutionarystatus of the hornworts is still unclear, and in some phyloge-netic analyses they are placed as the most ancient plant clade.

Water and sugar transport mechanisms emerged in the mossesThe most familiar nontracheophytes are the mosses (phylumBryophyta). There are more species of mosses than of liver-worts and hornworts combined, and these hardy little plantsare found in almost every terrestrial environment. They areoften found on damp, cool ground, where they form thickmats (Figure 29.9a). The mosses are probably sister to the tra-cheophytes (see Figure 29.4).

Many mosses contain a type of cell called a hydroid, whichdies and leaves a tiny channel through which water can

PLANTS WITHOUT SEEDS: FROM SEA TO LAND 577

Anthoceros sp.

The sporophytes of hornworts can reach 20 cm in height.

Gametophytes are flat plates a few cells thick.

29.8 A Hornwort The sporophytes of hornworts can resemble little horns.

(a)

(b) The teeth of this moss capsule help expel the thousands of spores.

29.9 The Mosses (a) Dense moss forms hummocks in a valley onNew Zealand’s South Island. (b) The moss capsule, from which sporesare dispersed, grows at the tip of the plant.

travel. The hydroid may be the progenitor of the tracheid, thecharacteristic water-conducting cell of the tracheophytes, butit lacks lignin (a waterproofing substance that also lendsstructural support) and the cell wall structure found in tra-cheids. The possession of hydroids and of a limited systemfor transport of sucrose by some mosses (via cells called lep-toids) shows that the old term “nonvascular plant” is some-what misleading when applied to mosses.

In contrast to liverworts and hornworts, the sporophytesof mosses and tracheophytes grow by apical cell division, inwhich a region at the growing tip provides an organized pat-tern of cell division, elongation, and differentiation. Thisgrowth pattern allows extensive and sturdy vertical growthof sporophytes. Apical cell division is a shared derived traitof mosses and tracheophytes.

The moss gametophyte that develops following spore ger-mination is a branched, filamentous structure called a pro-tonema (see Figure 29.5). Although the protonema looks a bitlike a filamentous green alga, it is unique to the mosses. Someof the filaments contain chloroplasts and are photosynthetic;others, called rhizoids, are nonphotosynthetic and anchor theprotonema to the substratum. After a period of linear growth,cells close to the tips of the photosynthetic filaments dividerapidly in three dimensions to form buds. The buds eventu-ally differentiate a distinct tip, or apex, and produce the fa-miliar leafy moss shoot with leaflike structures arranged spi-rally. These leafy shoots produce antheridia or archegonia (seeFigure 29.6). The antheridia release sperm that travel throughliquid water to the archegonia, where they fertilize the eggs.

Sporophyte development in most mosses follows a pre-cise pattern, resulting ultimately in the formation of an ab-sorptive foot anchored to the gametophyte, a stalk, and, atthe tip, a swollen capsule, the sporangium. In contrast tohornworts, whose sporophytes grow from the base, the mosssporophyte stalk grows at its apical end, as tracheophytes do.Cells at the tip of the stalk divide, supporting elongation ofthe structure and giving rise to the capsule. For a while, thearchegonial tissue grows rapidly as the stalk elongates, buteventually the archegonium is outgrown and is torn apart bythe expanding sporophyte.

The lid of the capsule is shed after the completion of meio-sis and spore development. In most mosses, groups of cellsjust below the lid form a series of toothlike structures sur-rounding the opening. Highly responsive to humidity, thesestructures dig into the mass of spores when the atmosphere isdry; then, when the atmosphere becomes moist, they flingout, scooping out the spores as they go (Figure 29.9b). Thespores are thus dispersed when the surrounding air ismoist—that is, when conditions favor their subsequent ger-mination.

Mosses of the genus Sphagnum often grow in swampyplaces, where the plants begin to decompose in the water af-

ter they die. Rapidly growing upper layers compress thedeeper-lying, decomposing layers. Partially decomposedplant matter is called peat. In some parts of the world, peoplederive the majority of their fuel from peat bogs. Sphagnum-dominated peatlands cover an area approximately half aslarge as the United States—more than 1 percent of Earth’ssurface. Long ago, continued compression of peat composedprimarily of other nonseed plants gave rise to coal.

With their simple system of internal transport, the mossesare, in a sense, vascular plants. However, they are not tra-cheophytes because they lack true xylem and phloem.

Introducing the Tracheophytes

Although they are an extraordinarily large and diversegroup, the tracheophytes can be said to have been launchedby a single evolutionary event. Sometime during the Paleo-zoic era, probably well before the Silurian period (440 mya),the sporophyte generation of a now long-extinct plant pro-duced a new cell type, the tracheid (Figure 29.10). The tra-cheid is the principal water-conducting element of the xylemin all tracheophytes except the angiosperms, and even in theangiosperms, tracheids persist alongside a more specializedand efficient system of vessels and fibers derived from them.

The evolution of a tissue composed of tracheids had twoimportant consequences. First, it provided a pathway forlong-distance transport of water and mineral nutrients froma source of supply to regions of need. Second, its stiff cellwalls provided something almost completely lacking—andunnecessary—in the largely aquatic green algae: rigid struc-tural support. Support is important in a terrestrial environ-ment because plants tend to grow upward as they competefor sunlight to power photosynthesis. Thus the tracheid setthe stage for the complete and permanent invasion of landby plants.

The tracheophytes feature another evolutionary novelty:a branching, independent sporophyte. A branching sporo-phyte can produce more spores than an unbranched body,and it can develop in complex ways. The sporophyte of a tra-cheophyte is nutritionally independent of the gametophyteat maturity. Among the tracheophytes, the sporophyte is thelarge and obvious plant that one normally notices in nature.This pattern is in contrast to the sporophyte of nontracheo-phytes such as mosses, which is attached to, dependent on,and usually much smaller than the gametophyte.

The present-day evolutionary descendants of the early tra-cheophytes belong to seven distinct phyla (see Figure 29.10).The tracheophytes have two types of life cycles, one that in-volves seeds and another that does not. The nonseed tra-cheophytes (the two basal phyla) include the club mossesand the ferns and their relatives: horsetails and whisk ferns.We will describe these phyla in detail after taking a closer

578 CHAPTER T WENT Y-NINE

look at tracheophyte evolution. The five phyla of seed plantswill be described in the following chapter.

Tracheophytes have been evolving for almost half a billion yearsThe evolution of an effective cuticle and of protective layersfor the gametangia (archegonia and antheridia) helped makethe first tracheophytes successful, as did the initial absenceof herbivores (plant-eating animals) on land. By the late Sil-urian period, tracheophytes were being preserved as fossilsthat we can study today. Two groups of nonseed tracheo-phytes that still exist made their first appearances during theDevonian period (409–354 mya): the lycopods (club mosses)and the pteridophytes (including horsetails and ferns). Theirproliferation made the terrestrial environment more hos-pitable to animals. Amphibians and insects arrived soon af-ter the plants became established.

Trees of various kinds appeared in the Devonian periodand dominated the landscape of the Carboniferous. Mightyforests of lycopods up to 40 meters tall, horsetails, and tree

ferns flourished in the tropical swamps of what would be-come North America and Europe (Figure 29.11). The remnantsof those forests are with us today as huge deposits of coal.

In the subsequent Permian period, the continents came to-gether to form a single gigantic land mass, called Pangaea.The continental interior became warmer and drier, but latein the period glaciation was extensive. The 200-million-yearreign of the lycopod–fern forests came to an end as they werereplaced by forests of seed plants (gymnosperms), whichdominated until other seed plants (angiosperms) becamedominant less than 80 million years ago.

The earliest tracheophytes lacked roots and leavesThe earliest known tracheophytes belonged to the now-extinct phylum Rhyniophyta. The rhyniophytes were amongthe only tracheophytes in the Silurian period. The landscapeat that time probably consisted of bare ground, with standsof rhyniophytes in low-lying moist areas. Early versions ofthe structural features of all the other tracheophyte phyla ap-peared in the rhyniophytes of that time. These shared fea-tures strengthen the case for the origin of all tracheophytesfrom a common nontracheophyte ancestor.

PLANTS WITHOUT SEEDS: FROM SEA TO LAND 579

Commonancestor

Nontracheophytes

Club mosses

Horsetails

Tracheophytes

Seed plants

Gymno-sperms

Nonseedtracheophytes

PteridophytesWhisk ferns

Ferns

Cycads

Conifers

Ginkgos

Gnetophytes

Angiosperms

Tracheids;branching,independentsporophyte

Multiflagellate sperm, complex leaves

Seeds

Flowers, carpels, triploid endosperm

29.10 The Evolution of Today’s Plants The nine phyla of extanttracheophytes are divided between those that produce seeds andthose that do not.

In 1917, the British paleobotanists Robert Kidston andWilliam H. Lang reported their finding of well-preserved fos-sils of tracheophytes embedded in Devonian rocks near Rhynie,Scotland. The preservation of these plants was remarkable, con-sidering that the rocks were more than 395 million years old.These fossil plants had a simple vascular system of phloem andxylem. Some of the plants had flattened scales on the stems,which lacked vascular tissue and thus were not comparable tothe true leaves of any other tracheophytes.

These plants also lacked roots. They were apparently an-chored in the soil by horizontal portions of stem, called rhi-zomes, that bore water-absorbing rhizoids. These rhizomes alsobore aerial branches, and sporangia—homologous with thenontracheophyte capsule—were found at the tips of thesebranches. Their branching pattern was dichotomous; that is,the shoot apex divided to produce two equivalent newbranches, each pair diverging at approximately the same anglefrom the original stem (Figure 29.12). Scattered fragments ofsuch plants had been found earlier, but never in such profusionor so well preserved as those discovered by Kidston and Lang.

29.11 An Ancient ForestThis reconstruction is of a Carboniferous forest that once thrived inwhat is now Michigan. The dominant “trees” are lycopods of thegenus Lepidodendron; ferns are also abundant.

Rhizome

Dichotomous branching

Sporangia

Rhizoids

29.12 An Ancient TracheophyteRelative This extinct plant,Aglaophyton major (phylumRhyniophyta), lacked roots andleaves. It had a central column ofxylem running through its stems,but true tracheids were lacking.The rhizome is a horizontalunderground stem, not a root.The aerial stems were less than50 cm tall, and some weretopped by sporangia. Other verysimilar rhyniophytes such asRhynia did have tracheids.

The presence of xylem indicated that these plants were tra-cheophytes. But were they sporophytes or gametophytes?Close inspection of thin sections of fossil sporangia revealedthat the spores were in groups of four. In almost all livingnonseed tracheophytes (with no evidence to the contraryfrom fossil forms), the four products of meiosis and cytoki-nesis remain attached to one another during their develop-ment into spores. The spores separate only when they aremature, and even after separation their walls reveal the ex-act geometry of how they were attached. Therefore, a groupof four closely packed spores is found only immediately af-ter meiosis, and a plant that produces such a group must bea diploid sporophyte—and so the Rhynie fossils must havebeen sporophytes. Gametophytes of the Rhyniophyta werealso found; they, too, were branched, and depressions at theapices of the branches contained archegonia and antheridia.

Although they were apparently ancestral to the other tra-cheophyte phyla, the rhyniophytes themselves are longgone. None of their fossils appear anywhere after the De-vonian period.

Early tracheophytes added new featuresA new phylum of tracheophytes—the Lycophyta (clubmosses)—also appeared in the Silurian period. Another—thePteridophyta (ferns and fern allies)—appeared during theDevonian period. These two groups arose from rhyniophyte-like ancestors. These new groups featured specializations notfound in the rhyniophytes, including one or more of the fol-lowing: true roots, true leaves, and a differentiation betweentwo types of spores.

THE ORIGIN OF ROOTS. The rhyniophytes had only rhizoidsarising from a rhizome with which to gather water andminerals. How, then, did subsequent groups of tracheo-phytes come to have the complex roots we see today?

It is probable that roots had their evolutionary origins asa branch, either of a rhizome or of the aboveground portionof a stem. That branch presumably penetrated the soil and

branched further. The underground portion could anchor theplant firmly, and even in this primitive condition it could ab-sorb water and minerals. The discovery of fossil plants fromthe Devonian period, all having horizontal stems (rhizomes)with both underground and aerial branches, supported thishypothesis.

Underground and aboveground branches, growing insharply different environments, were subjected to very dif-ferent selection pressures during the succeeding millions ofyears. Thus the two parts of the plant axis—the abovegroundshoot system and the underground root system—divergedin structure and evolved distinct internal and externalanatomies. In spite of these differences, scientists believe thatthe root and shoot systems of tracheophytes are homolo-gous—that they were once part of the same organ.

THE ORIGIN OF TRUE LEAVES. Thus far we have used the term“leaf” rather loosely. We spoke of “leafy” mosses and com-mented on the absence of “true leaves” in rhyniophytes. Inthe strictest sense, a leaf is a flattened photosynthetic struc-ture emerging laterally from a main axis or stem and pos-sessing true vascular tissue. Using this precise definition aswe take a closer look at true leaves in the tracheophytes, wesee that there are two different types of leaves, very likely ofdifferent evolutionary origins.

The first leaf type, the microphyll, is usually small andonly rarely has more than a single vascular strand, at least inplants alive today. Plants in the phylum Lycophyta (clubmosses), of which only a few genera survive, have such sim-ple leaves. The evolutionary origin of microphylls is thoughtby some biologists to be sterile sporangia (Figure 29.13a). Theprincipal characteristic of this type of leaf is that its vascular

PLANTS WITHOUT SEEDS: FROM SEA TO LAND 581

Megaphyll

Overtopping

Microphyll

(a) (b)

Vasculartissue

Sporangia

Sporangium

Time

Time

A sporangium evolvedinto a simple leaf.

Flat plates of photosynthetic tissue developed between branches.

2

The end branches evolved into the veins of leaves.

3

A branching stem system became progressively reduced and flattened.

1

29.13 The Evolution of Leaves (a) Microphylls are thought tohave evolved from sterile sporangia. (b) The megaphylls of pterido-phytes and seed plants may have arisen as photosynthetic tissuedeveloped between branch pairs that were “left behind” as dominantbranches overtopped them.

strand departs from the vascular system of thestem in such a way that the structure of the stem’svascular system is scarcely disturbed. This wastrue even in the fossil lycopod trees of the Car-boniferous period, many of which had leavesmany centimeters long.

The other leaf type is found in ferns and seedplants. This larger, more complex leaf is called amegaphyll. The megaphyll is thought to havearisen from the flattening of a dichotomouslybranching stem system and the development ofovertopping (a pattern in which one branch differ-entiates from and grows beyond the others). Thischange was followed by the development of pho-tosynthetic tissue between the members of over-topped groups of branches (Figure 29.13b). Mega-phylls may have evolved more than once, indifferent phyla of tracheophytes showing over-topping of branches.

HOMOSPORY AND HETEROSPORY. In the mostancient of the present-day tracheophytes, boththe gametophyte and the sporophyte are inde-pendent and usually photosynthetic. Spores pro-duced by the sporophytes are of a single type,and they develop into a single type of gameto-phyte that bears both female and male reproduc-tive organs. The female organ is a multicellulararchegonium, typically containing a single egg.The male organ is an antheridium, containingmany sperm. Such plants, which bear a singletype of spore, are said to be homosporous(Figure 29.14a).

A different system, with two distinct types ofspores, evolved somewhat later. Plants of this typeare said to be heterosporous (Figure 29.14b). Onetype of spore, the megaspore, develops into alarger, specifically female gametophyte (amegagametophyte) that produces only eggs. Theother type, the microspore, develops into asmaller, male gametophyte (a microgametophyte)that produces only sperm. The sporophyte pro-duces megaspores in small numbers in megaspo-rangia on the sporophyte, and microspores inlarge numbers in microsporangia.

The most ancient tracheophytes were all homo-sporous, but heterospory evidently evolved inde-pendently several times in the early descendantsof the rhyniophytes. The fact that heterosporyevolved repeatedly suggests that it affords selective advan-tages. Subsequent evolution in the plant kingdom featuredever greater specialization of the heterosporous condition.

Some tracheophyte clades arose and became extinct in thecourse of evolution. The earliest clades to arise and surviveto this day belong to the nonseed tracheophytes.

582 CHAPTER T WENT Y-NINE

Gametophyte(n)

DIPLOID (2n)

HAPLOID (n)Meiosis Fertilization

Archegonium (å)(n)

Antheridium (ç)(n)

Eggs (n)

Eggs (n)

Sperm (n)

Zygote (2n)

Embryo (2n)Sporangium (2n)

Spore mother cell (2n)

Spore (n)

(a) Homospory

(b) Heterospory

Sporophyte(2n)

DIPLOID (2n)

HAPLOID (n)Meiosis

Sperm (n)

Zygote (2n)

Embryo (2n)

Sporophyte(2n)

Megaspore (n)

Microsporangium (2n)

Spore mothercell (2n)

Spore mothercell (2n)

Microspore (n)

Microgametophyte (ç)(n)

Megagametophyte (å)(n)

Homosporous plants produce a single type of spore.

The spores of heterosporous plants produce male and female gametophytes.

The spores of homosporous plants produce a single type of gametophyte with both male and female reproductive organs.

Fertilization

Heterosporous plants produce two types of spores: a larger megaspore and a smaller microspore.

Megasporangium (2n)

29.14 Homospory and Heterospory (a) Homosporous plants bear a sin-gle type of spore. Each gametophyte has two types of sex organs, antheridia(male) and archegonia (female). (b) Heterosporous plants, which bear twotypes of spores that develop into distinctly male and female gametophytes,evolved later.

The Surviving Nonseed TracheophytesThe nonseed tracheophytes have a large, independent sporo-phyte and a small gametophyte that is independent of thesporophyte. The gametophytes of the surviving nonseed tra-cheophytes are rarely more than 1 or 2 centimeters long andare short-lived, whereas their sporophytes are often highlyvisible; the sporophyte of a tree fern, for example, may be 15or 20 meters tall and may live for many years.

The most prominent resting stage in the life cycle of a non-seed tracheophyte is the single-celled spore. This featuremakes their life cycle similar to those of the fungi, the greenalgae, and the nontracheophytes, but not, as we will see inthe next chapter, to that of the seed plants. Nonseed tracheo-phytes must have an aqueous environment for at least onestage of their life cycle because fertilization is accomplishedby a motile, flagellated sperm.

The ferns are the most abundant and diverse group ofnonseed tracheophytes today, but the club mosses and horse-tails were once dominant elements of Earth’s vegetation. Afourth group, the whisk ferns, contains only two genera. Inthis section we’ll look at the characteristics of these fourgroups and at some of the evolutionary advances that ap-peared in them.

The club mosses are sister to the other tracheophytesThe club mosses andtheir relatives (togethercalled lycopods, phy-lum Lycophyta) divergedearlier than all other living tra-cheophytes—that is, the remainingtracheophytes share an ancestor thatwas not ancestral to the Lycophyta.There are relatively few surviving species ofclub mosses.

The lycopods have roots that branch dichoto-mously. The arrangement of vascular tissue in their stems issimpler than in the other tracheophytes. They bear only mi-crophylls, and these simple leaves are arranged spirally onthe stem. Growth in club mosses comes entirely from apicalcell division, and branching is dichotomous, by a division ofthe apical cluster of dividing cells.

The sporangia in many club mosses are contained withinconelike structures called strobili (singular, strobilus; Figure29.15). A strobilus is a cluster of spore-bearing leaves insertedon an axis tucked into the upper angle between a specializedleaf and the stem. (Such an angle is called an axil.) Other clubmosses lack strobili and bear their sporangia in the axil be-tween a photosynthetic leaf and the stem. This placementcontrasts with the apical sporangia of the rhyniophytes.There are both homosporous species and heterosporous

species of club mosses. Although only a minor element ofpresent-day vegetation, the Lycophyta are one of two phylathat appear to have been the dominant vegetation during theCarboniferous period. One type of coal (cannel coal) isformed almost entirely from fossilized spores of the tree ly-copod Lepidodendron—which gives us an idea of the abun-dance of this genus in the forests of that time (see Figure29.11). The other major elements of Carboniferous vegetationwere horsetails and ferns.

Horsetails, whisk ferns, and ferns constitute a cladeOnce treated as distinctphyla, the horsetails,whisk ferns, and fernsform a clade, the phylumPteridophyta (pteridophytes,or “ferns and fern allies”). Withinthat clade, the whisk ferns and thehorsetails are both monophyletic; theferns are not. However, about 97 percent ofall fern species, including those with whichyou are most likely to be familiar, do belong to asingle clade, the leptosporangiate ferns. In the pteridophytes—and in all seed plants—there is differentiation (overtopping)between the main axis and side branches.

HORSETAILS GROW AT THE BASES OF STEM SEGMENTS. Like theclub mosses, the horsetails are represented by only a few

PLANTS WITHOUT SEEDS: FROM SEA TO LAND 583

Liverworts

Hornworts

Mosses

Club mosses

Horsetails

Whisk ferns

Ferns

Gymnosperms

Flowering plants Liverworts

Hornworts

Mosses

Club mosses

Horsetails

Whisk ferns

Ferns

Gymnosperms

Floweringplants

(a) Lycopodium obscurum (b)

Strobilus

Microsporangium

29.15 Club Mosses (a) Strobili are visible at the tips of this clubmoss. Club mosses have microphylls arranged spirally on their stems.(b) A thin section through a strobilus of a club moss, showingmicrosporangia.

present-day species. All are in a single genus,Equisetum. These plants are sometimes called“scouring rushes” because silica deposits found intheir cell walls made them useful for cleaning.They have true roots that branch irregularly. Theirsporangia curve back toward the stem on the endsof short stalks called sporangiophores (Figure29.16a). Horsetails have a large sporophyte and asmall gametophyte, both independent.

The small leaves of horsetails are reducedmegaphylls and form in distinct whorls (circles)around the stem (Figure 29.16b). Growth in horse-tails originates to a large extent from discs of di-viding cells just above each whorl of leaves, soeach segment of the stem grows from its base.Such basal growth is uncommon in plants, al-though it is found in the grasses, a major group offlowering plants.

PRESENT-DAY WHISK FERNS RESEMBLE THE MOST ANCIENT

TRACHEOPHYTES. There once was some disagree-ment about whether rhyniophytes are entirelyextinct. The confusion arose because of the exis-tence today of two genera of rootless, spore-bearing plants,Psilotum and Tmesipteris, collectively called the whisk ferns.Psilotum nudum (Figure 29.17) has only minute scalesinstead of true leaves, but plants of the genus Tmesipterishave flattened photosynthetic organs—reduced mega-phylls—with well-developed vascular tissue. Are these twogenera the living relics of the rhyniophytes, or do they havemore recent origins?

Psilotum and Tmesipteris once were thought to be evolu-tionarily ancient descendants of anatomically simple ances-tors. That hypothesis was weakened by an enormous holein the geological record between the rhyniophytes, whichapparently became extinct more than 300 million years ago,and Psilotum and Tmesipteris, which are modern plants.DNA sequence data finally settled the question in favor of amore modern origin of the whisk ferns from fernlike ances-tors. These two genera are a clade of highly specializedplants that evolved fairly recently from anatomically morecomplex ancestors by loss of complex leaves and true roots.Whisk fern gametophytes live below the surface of theground and lack chlorophyll. They depend upon fungalpartners for their nutrition.

Ferns evolved large, complex leavesThe sporophytes of the ferns, like those of the seed plants,have true roots, stems, and leaves. Their leaves are typicallylarge and have branching vascular strands. Some specieshave small leaves as a result of evolutionary reduction, but

even these small leaves have more than one vascular strand,and are thus megaphylls.

The ferns constitute a group that first appeared during theDevonian period and today consists of about 12,000 species.The ferns are not a monophyletic group, although, as alreadymentioned, 97 percent of the species—the leptosporangiateferns—do constitute a monophyletic group. The leptospo-rangiate ferns differ from the other ferns in having sporan-gia with walls only one cell thick, borne on a stalk.

584 CHAPTER T WENT Y-NINE

(a) Equisetum arvense (b) Equisetum palustre

SporangiumLeaves

Fertile shootSporangiophore

29.16 Horsetails (a) Sporangia and sporangiophores of a horsetail. (b) Vege-tative and fertile shoots of the marsh horsetail. Reduced megaphylls can be seenin whorls on the stem of the vegetative shoot on the right; the fertile shoot on theleft is ready to disperse its spores.

Psilotum nudum

29.17 A Whisk Fern Psilotum nudum was once considered bysome to be a surviving rhyniophyte and by others to be a fern. It isnow included in the phylum Pteridophyta, and it is widespread in theTropics and Subtropics.

Ferns are characterized by fronds (large leaves with com-plex vasculature; Figure 29.18a). During its development, thefern frond unfurls from a tightly coiled “fiddlehead” (Figure29.18b). Some fern leaves become climbing organs and maygrow to be as much as 30 meters long.

Because they require water for the transport of the malegametes to the female gametes, most ferns inhabit shaded,moist woodlands and swamps. Tree ferns can reach heightsof 20 meters. Tree ferns are not as rigid as woody plants, andthey have poorly developed root systems. Thus they do notgrow in sites exposed directly to strong winds, but rather in

ravines or beneath trees in forests. The sporangia of ferns arefound on the undersurfaces of the fronds, sometimes cover-ing the whole undersurface and sometimes only at the edges.In most species the sporangia are found in clusters called sori(singular, sorus) (Figure 29.19).

The sporophyte generation dominates the fern life cycleInside the sporangia, fern spore mother cells undergo meio-sis to form haploid spores. Once shed, the spores travel greatdistances and eventually germinate to form independent ga-metophytes. Old World climbing fern, Lygodium microphyllum,is currently spreading disastrously through the Florida Ever-glades, choking off the growth of other plants. This rapidspread is testimony to the effectiveness of windborne spores.

Fern gametophytes have the potential to produce both an-theridia and archegonia, although not necessarily at the sametime or on the same gametophyte. Sperm swim through wa-ter to archegonia—often to those on other gametophytes—where they unite with an egg. The resulting zygote developsinto a new sporophyte embryo. The young sporophytesprouts a root and can thus grow independently of the ga-metophyte. In the alternating generations of a fern, the ga-metophyte is small, delicate, and short-lived, but the sporo-phyte can be very large and can sometimes survive forhundreds of years (Figure 29.20).

Most ferns are homosporous. However, two groups ofaquatic ferns, the Marsileaceae and Salviniaceae, are derivedfrom a common ancestor that evolved heterospory. Themegaspores and microspores of these plants (which germi-nate to produce female and male gametophytes, respectively)are produced in different sporangia (megasporangia and mi-crosporangia), and the microspores are always much smallerand greater in number than the megaspores.

(a) Adiantum pedatum

(b)

(c) Marsilea mutica

29.18 Fern Fronds Take Many Forms (a) The fronds of Northernmaidenhair fern form a pattern in this photograph. (b) The “fiddle-head” (developing frond) of a common forest fern; this structure willunfurl and expand to give rise to a complex adult frond such as thosein (a). (c) The tiny fronds of a water fern.

Dryopteris intermedia

29.19 Fern Sori Are Clusters of Sporangia Sori, each containingmany spore-producing sporangia, have formed on the underside ofthis frond of the Midwestern fancy fern.

PLANTS WITHOUT SEEDS: FROM SEA TO LAND 585

A few genera of ferns pro-duce a tuberous, fleshy gameto-phyte instead of the charac-teristic flattened, photosyntheticstructure produced by mostferns. Like the gametophytes ofwhisk ferns, these tuberousgametophytes depend on amutualistic fungus for nutri-tion; in some genera, even thesporophyte embryo must be-come associated with the fun-gus before extensive develop-ment can proceed. In Chapter31 we will see that there aremany other important plant–fungus mutualisms.

All the tracheophytes wehave discussed thus far dis-perse themselves by spores. In the next chap-ter we will discuss the plants that dominatemost of Earth’s vegetation today, the seedplants, whose seeds afford new sporophytesprotection unavailable to those of the nonseedtracheophytes.

Chapter Summary

The Plant Kingdom� Plants are photosynthetic eukaryotes that develop fromembryos protected by parental tissue. Like the green algae, theyuse chlorophylls a and b and store carbohydrates as starch.Review Figure 29.1� Plant life cycles feature alternation of gametophyte (haploid)and sporophyte (diploid) generations. Both generations includemulticellular organisms. Review Figure 29.2� There are ten surviving phyla of plants. The three basal phylaare nontracheophytes, and the remaining seven phyla are tra-cheophytes. Review Table 29.1� Plants arose from a common green algal ancestor in thecharophyte clade, either a stonewort or a member of the groupthat includes Coleochaete. Descendants of this ancestral charo-phyte colonized the land.

The Conquest of the Land� The acquisition of a cuticle, gametangia, a protected embryo,protective pigments, thick spore walls with a protective poly-mer, and a mutualistic association with a fungus are all definingcharacters of plants, and all are associated with the adaptationof plants to life on land.

� Tracheophytes are characterized by possession of a vascularsystem, consisting of water- and mineral-conducting xylem andnutrient-conducting phloem. Nontracheophytes lack a vascularsystem. Review Figure 29.4

The Nontracheophytes: Liverworts, Hornworts, andMosses� Nontracheophytes either lack vascular tissues completely or,in the case of certain mosses, have only a rudimentary system ofwater- and food-conducting cells. � The nontracheophyte sporophyte generation is smaller thanthe gametophyte generation and depends on the gametophytefor water and nutrition. Review Figures 29.5, 29.6. See Web/CDTutorial 29.1� The nontracheophytes include the liverworts (phylumHepatophyta), hornworts (phylum Anthocerophyta), and moss-es (phylum Bryophyta). � Hornwort sporophytes grow at their basal end. � Hornworts, mosses, and tracheophytes have surface pores(stomata) that allow gas exchange and minimize water loss.� In mosses and tracheophytes, the sporophytes grow by apicalcell division.� The hydroids of mosses, through which water may travel,may be ancestral to tracheids, the water-conducting cells of thetracheophytes.

586 CHAPTER T WENT Y-NINE

Embryo

Archegonialwall

Horizontal stem

Sori (clustersof sporangia)

Roots

Root

Sporangium

Sporophyte

Mature sporophyte(typically 0.3–1 m tall)

Mature gametophyte (about 0.5 cm wide)

Rhizoids

AntheridiumSperm

Spore tetrad

Germinatingspore

Egg

Archegonium

FertilizationDIPLOID (2n)

HAPLOID (n)Meiosis

29.20 The Life Cycle of a Fern Themost conspicuous stage in the fern lifecycle is the mature, diploid sporophyte.

Introducing the Tracheophytes� The tracheophytes have vascular tissue with tracheids andother specialized cells designed to conduct water, minerals, andproducts of photosynthesis.� Present-day tracheophytes are grouped into seven phyla. Thetwo basal phyla are nonseed tracheophytes, and the rest areseed plants. Review Figure 29.10� In tracheophytes, the sporophyte is larger than the gameto-phyte and independent of the gametophyte generation.� The earliest tracheophytes, known to us only in fossil form,lacked roots and leaves. Review Figure 29.12� Roots may have evolved from rhizomes or from branchesthat penetrated the ground. Microphylls are thought to haveevolved from sporangia, and megaphylls may have resultedfrom the flattening and reduction of an overtopping, branchingstem system. Review Figure 29.13� Heterospory, the production of distinct female megasporesand male microspores, evolved on several occasions fromhomosporous ancestors. Review Figure 29.14. See Web/CDActivities 29.1 and 29.2

The Surviving Nonseed Tracheophytes� Club mosses (phylum Lycophyta) have microphylls arrangedspirally.� Among the pteridophytes (phylum Pteridophyta), horsetailshave reduced megaphylls in whorls. Whisk ferns lack roots; onegenus has minute scales rather than leaves, and the other hasreduced megaphylls with vascular tissue. Leaves with morecomplex vasculature are characteristic of all other phyla of tra-cheophytes. � The ferns are not a clade, although 97 percent of fern speciesdo constitute a clade. Ferns have megaphylls with branching vas-cular strands. Review Figure 29.20. See Web/CD Activity 29.3

Self-Quiz1. Plants differ from photosynthetic protists in that only plants

a. are photosynthetic.b. are multicellular.c. possess chloroplasts.d. have multicellular embryos protected by the parent.e. are eukaryotic.

2. Which statement about alternation of generations in plantsis not true?a. It is heteromorphic.b. Meiosis occurs in sporangia.c. Gametes are always produced by meiosis.d. The zygote is the first cell of the sporophyte generation.e. The gametophyte and sporophyte differ genetically.

3. Which statement is not evidence for the origin of plantsfrom the green algae?a. Some green algae have multicellular sporophytes and

multicellular gametophytes.b. Both plants and green algae have cellulose in their cell walls.c. The two groups have the same photosynthetic and

accessory pigments.d. Both plants and green algae produce starch as their

principal storage carbohydrate.e. All green algae produce large, stationary eggs.

4. The nontracheophytesa. lack a sporophyte generation.b. grow in dense masses, allowing capillary movement of

water.

c. possess xylem and phloem.d. possess true leaves.e. possess true roots.

5. Which statement is not true of the mosses?a. The sporophyte is dependent on the gametophyte.b. Sperm are produced in archegonia.c. There are more species of mosses than of liverworts and

hornworts combined.d. The sporophyte grows by apical cell division.e. Mosses are probably sister to the tracheophytes.

6. Megaphyllsa. probably evolved only once.b. are found in all the tracheophyte phyla.c. probably arose from sterile sporangia.d. are the characteristic leaves of club mosses.e. are the characteristic leaves of horsetails and ferns.

7. The rhyniophytesa. possessed vessel elements.b. possessed true roots.c. possessed sporangia at the tips of stems.d. possessed leaves.e. lacked branching stems.

8. Club mosses and horsetailsa. have larger gametophytes than sporophytes.b. possess small leaves.c. are represented today primarily by trees.d. have never been a dominant part of the vegetation.e. produce fruits.

9. Which statement about ferns is not true?a. The sporophyte is larger than the gametophyte.b. Most are heterosporous.c. The young sporophyte can grow independently of the

gametophyte.d. The frond is a megaphyll.e. The gametophytes produce archegonia and antheridia.

10. The leptosporangiate fernsa. are not a monophyletic group.b. have sporangia with walls more than one cell thick.c. constitute a minority of all ferns.d. are pteridophytes.e. produce seeds.

For Discussion1. Mosses and ferns share a common trait that makes water

droplets a necessity for sexual reproduction. What is that trait?

2. Are the mosses well adapted to terrestrial life? Justify youranswer.

3. Ferns display a dominant sporophyte generation (with largefronds). Describe the major advance in anatomy that enablesmost ferns to grow much larger than mosses.

4. What features distinguish club mosses from horsetails? Whatfeatures distinguish these groups from rhyniophytes? Fromferns?

5. Why did some botanists once believe that the whisk fernsshould be classified together with the rhyniophytes?

6. Contrast microphylls with megaphylls in terms of structure,evolutionary origin, and occurrence among plants.

PLANTS WITHOUT SEEDS: FROM SEA TO LAND 587