Plant Biology - Seedless Vascular Plants - Quick Guides | Biology

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Study GuidePlant Biology–Seedless Vascular Plants1. Phylum Lycophyta: Club Mosses and More1.1 An Ancient Group of PlantsLycophytes are theoldest living seedless vascular plantson Earth. They belong to one of the twomain evolutionary lines of vascular plants. These two lines split very early—probably during theSilurian Period, and definitely by theDevonian Period.Because of this early split, lycophytes have evolvedindependentlyfrom other vascular plants forabout400 million years. Over time, they changed dramatically. They began as small, semi-aquaticplants and later grew intogiant tree-like formsthat dominated forests during theCoal Agefor nearly40 million years.As continents shifted and climates became drier, lycophytes gradually lost dominance. By the lateCarboniferous to early Permian, most of the large forms became extinct.Interestingly, lycophytes developed features similar to flowering plants, even though they are notclosely related. This is calledconvergent evolution. Traits likeleaves, wood, tree forms, and seed-like reproductive structuresevolved independently in both groups.1.2Lycophytes TodayToday, about1,200 speciesof lycophytes are still alive. They are grouped into three families:•Lycopodiaceae•Selaginellaceae(only one genus:Selaginella, about 700 species)•Isoetaceae(only one genus:Isoetes, about 100 species)Modern lycophytes are much smaller than their ancient relatives. None grow taller than aboutonemeter, even in tropical regions where they are most abundant.Many tropical species areepiphytes, meaning they grow high up in tree canopies. In temperateregions, lycophytes are usuallysmall, creeping, evergreen plants. In the past, people collectedthem to make simple evergreen coverings for graves.SomeSelaginellaspecies are famous as“resurrection plants.”These plants live in very dryenvironments. During droughts, they shut down their metabolism and curl into tight, brown balls that

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Study Guidelook dead. When water becomes available, they quickly uncurl, turn green, and begin photosynthesisagain—storing enough energy to survive the next dry period.Lycophytes are also found incold regions, including the Arctic, and often form aground coverinnorthern and mountain conifer forests.1.3Key Structural FeaturesLycophytes can be identified by two main features:1.Vascular tissue arrangement2.Microphyll leaves, which are small leaves withonly one vascular strandTheirsporangia(structures that produce spores) arekidney-shaped, just like those of their ancientancestors. These sporangia are carried onsporophylls, which are grouped together into cone-likestructures calledstrobili.InSelaginellaandIsoetes, a small scale-like structure called aliguleis present near the base of theleaf. This feature helps distinguish these groups from other lycophytes.1.4Life Cycle of Lycopods (Lycopodiaceae)Members of the Lycopodiaceae arehomosporous, meaning they produce only one type of spore.Each spore grows into abisexual gametophytethat produces both eggs and sperm.Some gametophytes growundergroundand depend on amycorrhizal fungusfor nutrients. Othersgrow above ground. This gametophyte stage can last a very long time—up to 15 yearsin somespecies—before reproduction occurs.After fertilization, theembryo develops slowlyinto a sporophyte. The young sporophyte may stayattached to the gametophyte and rely on it for nutrition for an extended period.1.5Life Cycle ofSelaginellaSelaginellaspecies areheterosporous, meaning they produce two different types of spores:•Megaspores, which develop intofemale gametophytes•Microspores, which develop intomale gametophytes

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Study GuideThe female gametophyte developsinside the megaspore. When the spore wall breaks, thearchegonia(structures containing eggs) are exposed.The male gametophyte producesbiflagellate sperm, which are also released when the spore wallbreaks. These sperm swim to the archegonia to fertilize the egg.After fertilization, the embryo developswithin the archegonium, which stays inside themegagametophyte. This setup is similar to flowering plants, but there is an important difference:No integuments form, and thereforeno seed coat develops. This structure is considered anearlystep toward seed evolution, but it is not a true seed.1.6TheIsoetesGroupThe third lycophyte group,Isoetes, is alsoheterosporous. Its sporangia are found onquill-likesporophyllsthat grow in a tight cluster around a short, swollen stem called acorm.A unique feature ofIsoetesis the presence of acambiumwithin the corm. This cambium producessecondary tissues, a trait that is uncommon among seedless vascular plants.SummaryLycophytes are a fascinating group of plants with a long evolutionary history. Although most of theirancient tree-sized forms are extinct, modern lycophytes show important evolutionary traits—especiallythose related to vascular tissue, leaves, and reproduction. Studying them helps us understand howcomplex land plants, including seed plants, evolved over time.

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Study Guide2. Phylum Sphenophyta: Horsetails2.1A Once-Giant Group, Now Much SmallerLong ago, during theCarboniferous Period, horsetails were part of a large group ofwoody, tree-sized plantsthat filled ancient forests. Today, only oneherbaceous (non-woody) genus,Equisetum, remains. This genus includes about15 living species.Even though modern horsetails are much smaller, they closely resemble their ancient ancestors.Because of this,Equisetummay bethe oldest living genus of vascular plantsand one that haschanged very little over time.2.2How to Recognize a HorsetailHorsetails are among theeasiest plants to identify. They have several distinctive features:•Jointed, ribbed, hollow stems•Stems contain large amounts ofsilica, which makes them rough

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Study GuideoWhen two stems are rubbed together, they make arasping sound•This rough texture led to the old common name“scouring rush,”because early settlersused them for cleaning potsAt eachnode(joint) on the stem, there is a ring ofsmall leaves fused into a sheath. In manyspecies, each node also produces awhorl of branches, giving the plant its familiar“horsetail”appearance.The aboveground shoots grow from a widespreadrhizome systembelow the soil.2.3Habitat and GrowthHorsetails needmoist environments, at least during part of the growing season. This is becausetheirsperm must swim through a thin film of waterto reach the eggs—just like in ferns and otherfern allies.However, horsetails do not rely only on sexual reproduction. They often spreadvegetatively throughtheir rhizomes, allowing them to colonize areas quickly without forming spores.2.4Distribution and Ecological RoleEquisetumgrowsalmost worldwide, except inAustralia, New Zealand, and Antarctica. Mostspecies are found in theNorthern Hemisphere, especially between40° and 60° north latitude.Despite their long history, modern horsetails havelittle commercial valueand play only aminor rolein most ecosystemstoday.2.5Life Cycle: Similar to FernsThe life cycle ofEquisetumisvery similar to that of ferns, with differences mainly in structure ratherthan process.Spore Production•Sporangiaare grouped together in cone-like structures calledstrobili•In some species, these strobili grow at the tips ofnormal green shoots•In other species, the plant produces ashort-lived, non-green, unbranched shootthatexists only to make sporesInside the strobili, sporangia hang in clusters offive to tenfrom umbrella-shaped structures calledsporangiophores.

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Study Guide2.6Spores and ElatersEach spore is surrounded byfour thickened bands called elaters. These elaters arehygroscopic,meaning they respond to changes in moisture.•When the air is dry, the elaterstwist and move•This motion helps the sporesfloat and dispersemore easily(These elaters are different from the elaters found in mosses.)2.7Gametophyte Stage and FertilizationOnce a spore germinates, it forms asmall, green, flat (thalloid) gametophyte. This gametophyte:•Anchors itself to moist, nutrient-rich surfaces usingrhizoids•Producesmultiflagellate sperm, which require water to swimThe sperm swim to thearchegonium, whereone sperm fertilizes the single egg. The resultingzygotedevelops into an embryo inside the archegonium.At first, theyoung sporophyte stays attached to the gametophyte, receiving nourishment. Once itsroots, stems, and shoots are developed enough, the sporophyte becomesindependent, completingthe life cycle.Key TakeawayHorsetails are living fossils—simple, tough plants with an ancient design. While they no longerdominate forests, their structure and life cycle give us valuable insight into theearly evolution ofvascular plantsand how modern plant groups developed over time.3. Phylum Psilotophyta: Whisk FernsLet’s take a closer look atPsilotophyta, a small and unusual group of plants commonly calledwhiskferns. Even though they look ancient, there’s more to their story than meets the eye.

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