Lab 8 - Primitive Plants -Bryophytes,Ferns and Fern Allies

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Introduction to Bryophytes - Mosses, Liverworts,and Hornworts

As we pass from mosses to ferns, we see a gradual transition fromprimitiveto modern traits. There are two major trends you should focus on intoday"slab. The first is a transition in life cycles, the second is a changeinbasic internal structure.

You are watching: Compared to the moss, the fern is better adapted to life on land because the fern

First, all plants undergo an alternation of generations,between a haploid gametophyte stage and a diploid sporophytestage. In the most primitive plants, like mosses, the gametophyte isdominant(i.e. it"s big and green). In higher plants like ferns and fern allies,the sporophyte stage is dominant. Gametophytes produce gametes (spermandeggs) in a special structure called a gametangium (-ia), whilesporophytesproduce spores in a special structure called a sporangium(-ia).

Second, all plants need to get water to their cells.Primitivebryophyteslike mosses and liverworts are so small that they can rely on diffusionto move water in and out of the plant. Mosses have a few strands ofwaterconducting tissue in their central stem, but nothing like the large andwell organized network of tubes in tracheophytes, or "tubeplants".The vascular tissues in the more advanced ferns and "fernallies"are made up of xylem and phloem, which conduct water,nutrients,and food throughout the plant body. We"ll look at these tissues in alaterlab.

Bryophytes also need a moist environment to reproduce. Theirflagellatedsperm must swim through water to reach the egg. So mosses andliverwortsare restricted to moist habitats. There are no mosses in the desert.Butmosses are surprisingly resistant to drying up, and can survive underveryharsh conditions. Mosses are the most abundant plants in both theArcticand the Antarctic. Asexual reproduction in bryophytes is accomplishedbyfragmentation or by tiny vegetative "sprouts" called gemmae,whichform in special little structures called gemmae cups.

Mosses and liverworts are lumped together as bryophytes,plantslacking true vascular tissues, and sharing a number of other primitivetraits. They also lack true stems, roots, or leaves, though they havecellsthat perform these general functions. The leafy green plant that we seewhen we look at a moss or a liverwort is really the gametophyte,which is the dominant stage in all bryophytes. The sporophytesofbryophytes do not have a free-living existence. They grow directly outof the fertilized egg in the archegonia, and remain dependent on theparentgametophyte for their nutrition.

Characteristics of Divisions

Division Bryophyta - (9,500 sp.) - mosses, Mnium, Sphagnum

Mosses come in two basic types, a cushiony type, with erect stalks,and a feathery type, which forms flattened mats of low-lying and highlybranched moss plants. In both cases, the leafy green gametophytes aredioeciousThey can be male plants, with antheridia at the top of the plant, orfemaleplants, with archegonia at the top. Remember that these gametophytesarealways haploid (1N) plants.

Sperm are produced within each antheridium, and an egg ineacharchegonium.Because the plant is already haploid, these gametes can be created bymitosis,simple cell division. The sperm swims to the archegonia through a thinfilm of water, drawn by a chemical attractant produced by the femaleplant,then swims down the neck of the archegonia to the egg. A good morningdewis more than sufficient water for the sperm to swim. Once the spermentersthe archegonia, it fuses with the egg. The 2N zygote develops into adiploidsporophyte plant, a small stalk that grows directly out of the top ofthearchegonium. This stalk is initially green, and photosynthetic, butlaterturns brown and becomes essentially a parasite on the femalegametophyte.

The sporophyte plant consists of a stalk, and a small capsuleon the top. Within the capsule, cells undergo meiosis to producetetradsof haploid spores. When the capsule is ripe, its hinged lid oroperculumopens up, and the spores are quickly dispersed by wind and water. Thesporesgerminate into a tiny green thread, which looks like a simple strand ofgreen algae. This similarity is one more clue that bryophytes aredescendedfrom green algae. This early threadlike stage is called the protonema(= first thread, plural = protonemata, like stigma/stigmata). The newadultgametophytes grow from a tiny bud that develops on the protonema.Eventuallythese gametophytes will grow to produce gametes, and the whole cyclewillstart over again. Mosses can also reproduce asexually by fragmentationor by growing little vegetative buds called gemma, which can break offand grow into a new tree .

While bryophytes in general are more interesting than important, inthe usual sense, a conspicuous exception are mosses of the genus Sphagnum.Sphagnummoss forms dense mats which become compressed into peat, which can beusedas fuel, although it’s very smoky. Peat also contains other plants suchas reeds, that grow amid the sphagnum. In dried form, peat moss isremarkablyabsorbent and, and has been used for diapers, for enriching poor gardensoils, and as a field dressing for wounds. Whereas cotton absorbs 4-6timesits dry weight, dried sphagnum can absorb 20 times its own weight influids!Peat bogs are very important and interesting ecosystems. Sphagnummosses greatly increase local acidity by releasing H+ ions, and the pHof peat bogs can drop to 4 or lower, perhaps the most acidic naturalenvironment.Peat bogs cover about 1% of the Earth’s land surface, an area abouthalfthe size of the United States.

Division Hepaticophyta - (9,000 sp.), liverworts, Marchantia,Conocephalum, Porella

Liverworts have the simplest bodies of all the green plants. Thegametophyte,the dominant stage, looks like a flat scaly leaf, with prominent lobes.It looks for all the world like a tiny flattened liver, hence thescientificname hepatico-phyta = liver plant. During the Middle Ages, thissimilaritycaused physicians to prescribe liverwort for diseases of the liver.Accordingto the Doctrine of Signatures the Creator had designed all of nature,includingplants, with our welfare in mind. People believed that plants had beenintentionally designed to resemble the organs of the body they weresupposedto heal! Hence liver-wort, wyrt being the Anglo-Saxon word for herb.Theshape of the liverwort was the signature of the Creator in nature. Canyou guess what walnuts were supposed to cure ? (diseases of thebrain)

Liverworts share the general properties of bryophytes, but are notveryclosely related to mosses or hornworts. Many botanists think they mayhaveevolved independently, from a different group of green algae. For onething,liverworts don’t store food as starch but as oils. If you get theaquaticliverwort Porella in lab, take a sniff of the jar, but not toodeep!It smells of rancid oils, oils that went a little funky while the plantwas being shipped. Another characteristic unique to liverworts is theirlack of stomata, which are found in all other plants, including mossesand hornworts.

In many species of liverworts, such as Marchantia, the oneyouwill most likely see in lab, the antheridia and archegonia are not ontopof the plant, but hanging down from the underside of odd littlestructuresthat look like tiny umbrellas. (These umbrella-shaped structures arecalledthe antheridiophore and archegoniophore). The bi-flagellated spermswimsto the egg, and fertilization takes place to form a diploid (2N)zygote.The tiny diploid sporophytes, which remain attrached to the parentplant,have a very simple structure. Meiosis within the sporophyte produces anumber of haploid spores. These spores are surrounded by curious longandtwisted moist cells called elaters. When the capsule dries and bursts,the elaters twist and jerk around in a way that scatters the spores inall directions. Liverworts can also reproduce asexually by means ofspecialstructures called gemmae cups. These little cups can be easilyseenon the surface of the plant. Each gemma cup contains a number of tinyplantletscalled gemmae, and a single drop of water will disperse them. Theselittlevegetative “clones” will then grow into a new gametophyte.

Division Anthocerophyta - (100 sp.), hornworts, Anthoceros

The green gametophytes of the hornwort look very much like aliverwort.But their small sporophytes more closely resemble those of mosses. Thesporophytes grow out of the gametophyte, and look like a little uprighthorn. Like mosses, hornworts have stomata, and so are probably morecloselyrelated to mosses and other plants than to the liverworts they matresemble.These plants are symbiotic with the cyanobacteria Nostoc. Thecyanobacteriafixes nitrogen for the hornwort.

Taxonomy

Kingdom Plantae

Bryophytes

Division Bryophyta - mosses (Mnium, Sphagnum; fr.Gr.bryon= moss)

Division Hepaticophyta - liverworts (Marchantia,Conocephalum,Porella; fr.Gr. hepato = liver)

Division Anthocerophyta - hornworts (= Anthocerotophyta; Anthoceros;fr.Granthos = flower, keras = horn)

Terms alternation of generations gametophyte gametangium (-ia) archegonium (-ia) antheridium (-ia) sporophyte sporangium (-ia) spore capsule protonema vascular system gemmae gemmae cupTo Do and View

Examine the living mosses on display. Notice the small capsuleson top of the tiny sporophytes. Mosses generally grow in one of twogrowthtypes: cushiony moss and feathery moss.

Examine slides of the antheridia and archegonia. The sausageshaped antheridia produce sperm, and the flask shaped archegoniaproduces eggs.

Examine slides of the moss capsule, and identify the spores.

Examine slides of the protonema. What type of algaedoesit remind you of? This resemblance is additional evidence that greenalgaegave rise to all higher plants.

Examine the terrestrial liverworts Marchantia and Conencephalum(oneor both should be on display). How does their growth habit differ fromthat of the mosses? Can you see any gemmae cups on the uppersurfaceof these plants?

Examine slides of the liverwort Marchantia.

Examine the aquatic liverworts like PorellaandRiccia(one or both should be on display). Notice how they differ from themoreterrestrial forms of liverwort.

Look at the preserved liverworts, and observe their distinctreproductive structures (they look like little green umbrellas). Howdoestheir life cycle differ from mosses?

Examine slides of the hornwort Anthocerophyta.

Things to Remember

Know the life cycle of the moss in detail, and be able torecognizethe various stages.

Hint: Be sure you understand the general life cycle ofplants,and can tell which stages are haploid gametophytes (1N) or diploidsporophytes(2N). We"ll learn several life cycles in lecture and in lab (moss,fern,pine, flowering plant), but all of them are variations on the samebasictheme.

Ecological, Evolutionary, and Economic Importance

Mosses are important in landscaping and gardening, especially peatmoss(Sphagnum).

Peat moss (Sphagnum) has been used historically as dressingsfor wounds.

Peat moss can be used as fuel.

Mosses are the most primitive living land plants.

Hornworts contain symbiotic colonies of the cyanobacteria Nostoc.

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shining club moss

Introduction to Tracheophytes - Ferns and Fern Allies

Tracheophytes (vascular plants) completed the conquest of theearth’s surface begun by the more primitive bryophytes. Just as theevolutionof spores was the key to the invasion of the land surface bybryophytes,the invention of complex vascular tissues let tracheophytes completetheconquest of dry land. There are about 250,000 species of vascularplants,grouped in nine divisions. Tracheophytes all have a well developedroot-shootsystem, with highly specialized roots, stems, and leaves, andspecializedvasculartissue (xylem and phloem) that function like miniature tubes toconductfood, water, and nutrients throughout the plant. Because ferns and fernallies posses true vascular tissues, they can grow to be much largerandthicker than the bryophytes.

The ferns and fern allies (non-seed tracheophytes) mark two majorevolutionarystrides. In these and in all more advanced plants, the leafy green diploidsporophyte now becomes the dominant stage. The tiny gametophytemay be either autotropophic (like the fern prothallus) orheterotrophic(like the gametophytes of some lycopsids), and is generally free livingand independent of the parental sporophyte. Unlike the vascularsporophytes,the gametophytes have no vascular tissue at all. These gametophytes aretherefore very small, and develop best in moist areas, where they canabsorbwater directly from their surroundings.

Like the bryophytes, ferns and fern allies are still restricted tomoisthabitats. Their flagellated sperm need a thin film of water to swimbetweenthe antheridium and the archegonium. And when the baby sporophyte growsup from the gametophyte, it is exposed to desiccation (drying up). Thisbasic strategy of a free-swimming sperm and a non-motile egg is sharedby plants, animals, and algae. It makes sense, because it means onlyoneset of gametes has to make the perilous journey outside of theorganism.

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ground pine (Lycopodium) showing strobili

The ferns and fern allies germinate from spores. These plants aremostlyhomosporous- their spores are identical and you can"t differentiate which willgrow into male or female plants. They are also monoecious -boththe archegonia and antheridia (male and female reproductive structures)are borne on the same plant. Contrast these primitive vascular plantswiththe more advanced seed plants, the gymnosperms and angiosperms, whichgerminatefrom seeds rather than from spores. Seed plants are all heterosporous.It is easy to differentiate the larger female megaspore fromthesmaller male microspore. The sperm of seed plants have noflagella.They lack antheridia, and only a few still have an archegonia. Unlikethemore primitive ferns and fern allies, seed plants are mostly dioecious,having separate male and female plants.

In many of these primitive plants, certain leaves are specializedforreproduction. These modified leaves, or sporophylls, bear thesporangiaat their bases. These sporophylls usually branch out from a shortenedstem,forming a club shaped structure called a strobilus. The pineconeand the flower are elaborate variations on these primitive strobili.

There are four divisions of non-seed tracheophytes, vascular plantsthat reproduce by means of spores, the Psilophyta, Lycophyta,Sphenophyta,and Pterophyta. Before these non-seed tracheophytes evolved, thebryophyteswere the dominant form of plant life. The evolutionary edge of having amore efficient conducting system, and a well-developed root-shootsystemenabled them to outcompete bryophytes. If you’re lucky enough to seegroundpine or other club mosses growing in the shade of a large pine tree,thinkfor a moment about how these tiny plants were once the masters of theplanet,forming vast forests with trunks from 20 to 100 feet tall!

Taxonomy

Kingdom Plantae

Tracheophytes (vascular tissue, no seeds)

Division Lycophyta - club moss, quillworts (Lycopodium,Selaginella)

Division Sphenophyta - horsetails (Equisetum; fr.L.equus= horse)

Division Psilophyta - whisk fern (Psilotum)

Division Pterophyta - true ferns (Pteris; fr.Gr.pteridion= little wing)

Terms homosporous heterosporous megaspores microspores rhizomes sporophylls strobilus (-i) sori indusium (-ia) prothallus fiddleheadsCharacteristics of Divisions

Division Psilophyta - (fr Gr. psilo = smooth), whiskferns,Psilotum

There are only two living genera of whisk ferns, sole survivors of alarge and widespread group of early land plants. In addition to thelivingDivision Psilophyta, the psilopsids, there are two extinct divisions ofprimeval vascular plants. The primitive whisk ferns resemble theseextinctpioneers in many ways. They are the only living vascular plants thatlacka root-shoot system, a characteristic they share with both extinctDivisionsof ancestral vascular plants. Some recent molecular evidence suggeststhatone, or even both, of the living genera of psilopsids may actually bemoreclosely related to ferns, like a fern that has reverted to moreprimitivetraits. If this is true, then Psilophyta will join the ranks of thenumerousextinct Divisions of plants.

Psilopsids are found in tropical and subtropical areas, and occursthroughoutthe southern US. I once found one growing on my back porch under theleavesof a spider plant. Whisk ferns are a common weed in greenhouses alloverthe world. They are simple green upright stems, with dichotomousbranching.They have no leaves, and no true roots. The outer tissues of the stemdoall the photosynthesizing. A portion of the stem called a rhizome runsalong the ground, or just below it. A rhizome is a horizontalstemthat spreads the plant around. Roots grow out the bottom of therhizome,and a new plant can arise at the same point from the top.

The green stem-like plant is the diploid sporophyte, the dominantstagein the life cycle. In the small sporangia (bright yellow) that formalongthe upper stems, the spore mother cell forms haploid spores by meiosis.Their gametophytes are tiny little thread-like underground plants thatlack chlorophyll, and live as heterotrophs in the soil, looking andactingmuch like a tiny fungi. It actually contains a symbiotic fungi, thesamemycorrhizae that live in the rhizomes of the adult sporophyte.

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Division Lycophyta - (1,000 sp., fr. Gr. lycos=wolf), clubmosses,quillworts, Lycopodium (podus=foot)

Their are only five living genera of lycopsids, but at one time fromthe distant Devonian, about 400 mya, well into the Carboniferous, theywere the dominant form of vegetation on the face of the Earth. Now theyare reduced to a shadow of their glorious past, inconspicuous littleplantsin the forest understory. The tropical species are small epiphytes(plantsthat grow on other plants).

Their roots grow from special underground stems called rhizomes, asdo most of these primitive tracheophytes. In some species thesporophyllsare mixed in with the scale-like leaves. In many species, thesporophyllsare organized into strobili, hence the common name of “club moss”. Thesperm swim down the strobilus to the archegonia, and the zygote thatformsis retained in the cone, which ripens and falls to the ground. Thegametophytesare independent and free-living, They are curious creatures that lookandact nothing like their sporophyte parents. They can be eitherheterotrophicor autotrophic, and usually have a symbiotic fungi associated withthem.Many of the lycopsids are heterosporous. Selaginella is a goodexampleof a heterosporous plant.

Division Sphenophyta - (15 sp., one genus, fr. Gr.sphen=wedge),horsetails, Equisitum

In waste places, disturbed areas like trails and railroad beds, andin odd corners of fields and forests you might find another small plantquietly dreaming of its former splendor, the horsetail. Horsetailsappearedin the late Devonian, and were among the dominant forest trees forhundredsof millions of years. Only one genus of Sphenophyta still exists, thegenusEquisetum,and it may be the oldest living genus of plants on earth. Horsetailstoweredamong the Carboniferous forests, reaching heights of 30-60 feet. Muchofthe coal deposits we exploit for fuel today were formed from horsetailsand other trees during the Carboniferous, toward the end of thePaleozoic.

Horsetails have true roots, stems, and leaves, though the leaves arelittle more than flattened stems. Their hollow, ribbed stems arejointed,kind of like a stalk of bamboo, and a whorl of leaves arises at eachjoint.The plants are spread vegetatively by rhizomes. The stems feel veryrough,because the epidermal tissues are impregnated with tiny grains ofsilica(sand). This probably helps protect the plant against herbivores. Theserough stems made this plant ideal for pioneer women to use forscrubbingpots and pans, hence its other common name, “scouring rush”.

The green plant we see is the diploid sporophyte generation. Thestalkscan be highly branched vegetative stalks, which actually look likehorsetails, or straight unbranched reproductive stalks, which are tippedwitha large strobilus containing the sporangia. The homosporous sporesdevelopinto a teeny-tiny green gametophyte, just a few mm long, that lookslikethe gametophyte of a fern. The gametophyte is haploid, free-living, andautotrophic.

Division Pterophyta - (12,000 sp., fr. Gr. pteridion=littlewing),ferns

Ferns probably evolved from the psilopsids, sometime in theDevonian,relatively early on in land plant evolution. They are very abundant anddiverse, ranging in size from a single centimeter to trees 24 meterstallwith 5 meter fronds. Ferns have been better competitors with seedplantsthan other seedless vascular plants, and are a conspicuous part of thelandscape throughout the world, but especially in the tropics, where75%of their 12,000 species occur.

Ferns are relatively advanced plants, with true roots, stems andleaves.The blade of the fern is called a frond, and the little individualleafletsare called pinnae. Ferns have true leaves, what botanists callmacrophylls.While the leaves of more primitive plants, which are calledmicrophylls,are simply extensions of the epidermis of the stem, the leaves of fernsand higher plants were formed as a web of tissue stretched betweensmallterminal branches. The leaves of higher plants, as well as the modifiedleaves that make up the pine cone and the flower.

The life cycle of the fern is typical of other non-seed vascularplants.The leafy green plant is the sporophyte. Fertile fronds developsclustersof small sporangia on the underside of the frond. These clusters ofsporangiaare called sori (sing. sorus). Sori are often protected by atinyumbrella-like cap called an indusium (-ia). Ferns are mostlyhomosporous,though some are heterosporous. The heterosporous state is a moreadvancedcondition, that seems to have evolved independently in several groupsofplants.

The haploid spores are formed by meiosis inside the sporangium. Theyare ejected in a miniature explosion caused by the unequal drying ofthealternate thick and thin-walled cells that line the outer surface. Thetop pulls slowly back until it reaches a critical point and then snapsforward at an incredible speed. At that size scale, the expulsion offernspores is one of the most explosive events in nature. The sporesgerminateinto tiny gametophytes. The little heart shaped gametophyte is called aprothallus,literally “first-body” (pl prothalli). the prothallus has no vasculartissue.Its small size lets it rely entirely on diffusion. Its tiny rhizoidsareassociated with mycorrhizal fungi. The little prothallus is green, andphotosynthetic, and bears either antheridia and archegonia, orsometimesboth together, on its upper surface (lab slides have both on sameprothallus).The archegonia are always found at the arch of the heart, and theantheridiaare tucked away among the tiny rhizoids at the other end. The spermswimsto the egg to fuse into a diploid zygote. The new sporophyte growsdirectlyout of the top of the gametophyte. When it first begins to uncurl, thefrond looks like the scrolled neck of a violin or fiddle, and thisstageof development is called a fiddlehead.

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To Do and View

Examine the living lycopsids on display. Why are they calledclub mosses? Notice that quillworts and Selaginella are verydifferentin appearance from the club mosses.

Examine slides of Selaginella"s strobilus. Identify megasporesand microspores.

Examine the living horsetails on display. Notice theprominentstrobiliofthe reproductive stalks, and the bushy growth form of the vegetativestalks(if available).

Examine the living whisk ferns on display. Psilopsidshave a simple dichotomous branching pattern. You may see tiny yellowsporangiaon the branches. Whisk ferns lack strobili. These primitive plants areclosely related to ferns.

Examine the living ferns on display. Can you see any fiddleheads?Look for the rhizomes. Rhizomes are modified horizontal stemsbearingroots, that run along or just underneath the ground, and spread fernsandfern allies around.

Examine the living fern prothallus on display under adissectingmicroscope. Note its characteristic heart shape. Some prothalli mayhavea tiny new fern emerging from the notch of the "arch" heart, where thearchegoniaarelocated.

Examine slides of the prothallus: Find the archegonia(can you see an egg?).

Examine the fern leaflet on display under the dissectingmicroscope.Notice the prominent indusia, and the small sporangiapeekingout from beneath. You"ll see a few groups of sporangia that have losttheirindusium. If you"re lucky, as the heat of the microscope dries out theleaflet, you might see the sporangia eject its spores. But don"tblink- it"s one of the fastest and most explosive acts in nature. Use highpowerto observe the spores.

Examine slides of the sori and indusia; use high powerto observe the spores.

Things to Remember

Know the life cycle of the fern. Recognize specimens andslidesof the various stages.

Economic, Ecological, and Evolutionary Importance

Ferns and fern allies are primarily responsible for our moderndepositsof coal.

The fiddleheads of some species of ferns are edible.

See more: What Does Nadia Mean In Spanish ? What Does The Name Nadia Mean In Spanish

Ferns are important for the florist, gardening and landscapeindustries.

Consider This

Why are all these plants restricted to wet habitats?

Which group of protists gave rise to these plants? (How do we know?)

Why is the epidermis of the horsetail so rough? What does it needprotectionfrom?

All of the fern allies in this lab once towered 50-100 feet or more.What happened?

Links to Explore

Bryophyte taxonomy, images, and more at:

http://bryophytes.plant.siu.edu/

Get info about bryophytes from the Missouri Botanical Garden at:

http://www.mobot.org/mobot/tropicos/most/

Lie down among the ferns at the American Fern Society, try theirlengthylist of links:

http://amerfernsoc.org

Basic info on ferns and fern allies, complete with images, courtesyof Texas A&M:

http://www.csdl.tamu.edu/FLORA/fsb/fsbfern1.htm

Look for ferns on the "tree of life" at

http://phylogeny.arizona.edu/tree/eukaryotes/green_plants/embryophytes/filicopsida/filicopsida.html