Reading Essentials for Biology Glencoe (794133), страница 64
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The gametophyte produceshaploid gametes, sex cells. The female gamete is an egg and themale gamete is a sperm. When a sperm fertilizes an egg, a diploidzygote forms. This is sexual reproduction. The zygote canundergo cell divisions and form an embryo sporophyte. If theembryo develops to maturity, the cycle can begin again.This basic life cycle pattern is the same for most plants. However, the pattern may vary. For example, recall that in mosses, thegametophyte is the familiar form.
In flowering plants, the sporophyte is the familiar form. Most people have never seen the femalegametophyte of a flowering plant. Botanists refer to the bigger,visible plant as the dominant generation. The dominant generationlives longer and can stay alive without the other generation. Inmost plant species, the sporophyte is the dominant generation.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Alternation of GenerationsNameDateClassSection24.1Life Cycles of Mosses, Ferns, and Conifers, continuedHow are new plants reproduced asexually?In a process called vegetative reproduction, new plants areproduced from existing plant organs or parts of organs.
The newplants have the same genetic makeup as the original plant. Forexample, some thallose liverwort gametophytes can producecuplike structures. Inside, tiny pieces of tissue called gemmae(JEH mee) develop. If gemmae fall from the cup to the ground,they can grow into other liverwort gametophytes. They aregenetically identical to the liverwort that produced the gemmae.Life Cycle of MossesThe gametophyte generation is the dominant generation inmosses. The diagram below shows the life cycle of mosses.DevelopingsporophyteMitosisSporophyte (2n)Zygote (2n)Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Gametophyte (n)SpermSPOROPHYTEGENERATION2nEggFertilizationAntheridiumCapsuleMeiosisArchegoniumGAMETOPHYTEGENERATIONnOpen capsuleGerminatingsporesMalegametophyte(n)READING ESSENTIALSFemalegametophyte(n)RhizoidsProtonemaChapter 24277NameDateClassSectionLife Cycles of Mosses, Ferns, and Conifers, continued2.
How do moss gametophytes reproduce by vegetative reproduction?________________________________________________________________________A haploid spore can germinate and grow to form a protonema.A protonema (proh tuh NEE muh) is a small green filament, orstrand, of cells that can develop into the gametophyte. In somemosses, male and female reproductive structures form on separategametophytes. In others, male and female reproductive structuresare on the same gametophyte.
The archegonium is the eggproducing female reproductive structure. The antheridium is thesperm-producing male reproductive structure.Sperm from an antheridium swim in a continuous film of waterto an egg in an archegonium. If fertilization occurs, a diploidzygote forms. The zygote undergoes cell divisions to become thesporophyte. The sporophyte is a stalk with a capsule on top. Thesporophyte is attached to and depends on the gametophyte.
Cellsin the sporophyte capsule undergo meiosis, producing haploidspores. When the capsule is mature, it bursts open and releasesspores. If the spores land in a favorable environment, they can germinate and develop into a new plant. The cycle starts again.Some moss gametophytes also reproduce by vegetative reproduction. If the gametophyte becomes dry and brittle, it can breakinto pieces. When moisture returns, each piece can grow and forma protonema, a green filament of cells, and then a gametophyte.Life Cycle of FernsIn ferns, the dominant generation is the sporophyte.
The fernsporophyte includes fronds and roots that grow from a rhizome,which is an underground stem. The underside of some fern frondsare covered in rusty brown spots. These are the sori, which areclusters of sporangia. Remember that sporangia are sacs thatcontain spores. Meiosis occurs within the sporangia, producinghaploid spores. When environmental conditions are right, thesporangia open, releasing the haploid spores.A spore develops to form a tiny heart-shaped gametophytecalled a prothallus. The prothallus produces both archegoniaand antheridia on its surface.
Sperm released by antheridia swimthrough a continuous film of water to eggs in archegonia. Iffertilization occurs, the resulting diploid zygote develops intoa sporophyte.At first, the developing sporophyte depends on the gametophytefor its nutrition. However, as soon as the sporophyte producesgreen fronds, it can carry on photosynthesis and live on its own.The prothallus dies and decomposes as the sporophyte matures.The mature fern consists of a rhizome (underground stem) from278Chapter 24READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.24.1NameDateClassSection24.1Life Cycles of Mosses, Ferns, and Conifers, continuedProthallusGrowingprothallusArchegoniumSpores (n)AntheridiumEggRhizoidsSporangiumMeiosisGAMETOPHYTEGENERATIONnSpermCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.FertilizationSPOROPHYTEGENERATION2nZygoteSorusFrondsRhizomeCell divisionsRootswhich roots and fronds grow.
If pieces of the rhizome break off,new fern plants can develop from the pieces by vegetative reproduction. The cycle continues when sporangia develop on thefronds and spores are released. The figure above shows the lifecycle of the fern.READING ESSENTIALSChapter 24279NameDateClassSection24.1Life Cycles of Mosses, Ferns, and Conifers, continuedOvuleFemale coneMegasporesMale coneMicrosporemother cellsMicrosporesFemalegametophyteYoungseedlingMeiosisAdultsporophyteSPOROPHYTEGENERATION2nMalegametophyte(pollen grain)Two archegoniawith egg cellsGAMETOPHYTEGENERATIONnSeedSeed coatEggCotyledonsPollen grainEmbryoMicropyleStoredfoodSperm nucleusOne eggis fertilizedMaturing pollen grainThe Life Cycle of ConifersThe dominant generation in conifers is the sporophyte. Unlikeferns, conifers produce seeds. The stages of a conifer’s life cycleare shown in the figure above. Adult conifer plants produce maleand female cones on separate branches of the same plant.
Conescontain sporangia on their scales.280Chapter 24READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.FertilizationNameDateClassSection24.1Life Cycles of Mosses, Ferns, and Conifers, continuedFemale cones are larger than male cones. On the upper surfaceof each scale of a female cone, two ovules develop. Each ovulecontains a sporangium with a diploid cell. The diploid cell undergoes meiosis and produces four megaspores.
A megaspore is afemale spore that eventually becomes the female gametophyte.One of the four megaspores lives and grows by cell division intothe female gametophyte.The female gametophyte depends on the sporophyte for protection and nutrition. Inside the female gametophyte are two ormore archegonia, each containing an egg.Male cones have sporangia that undergo meiosis to producemale spores called microspores. Each microspore can developinto a male gametophyte, or pollen grain.
Each pollen grain hasa hard, water-resistant outer covering.Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.How does pollination take place?In conifers, pollination is the transfer of pollen grains from themale cone to the female cone. Wind blows the pollen grains fromthe male cones. If a pollen grain falls near the opening of an ovulein the female cone, pollination can occur.
The opening of theovule is called the micropyle (MI kruh pile). The pollen graingets trapped in a sticky drop of fluid that covers the micropyle.As the fluid evaporates, the pollen grain is pulled closer to themicropyle. Even though pollination has occurred, fertilizationdoes not take place for at least a year. The pollen grain and thefemale gametophyte will mature during this time.3. Analyze What are thedifferences between maleand female reproductivecones?________________________________________________________________________When are mature seeds released?As the pollen grain matures, it produces a pollen tube thatgrows through the micropyle into the ovule.
A sperm nucleusfrom the pollen grain moves through the pollen tube to the egg.If fertilization occurs, a zygote forms. The female gametophytenourishes the zygote until it develops into an embryo with severalcotyledons. The cotyledons will nourish the sporophyte afterdevelopment begins. A seed coat forms around the ovule as themature seed is produced. Mature seeds are released when thefemale cone opens.READING ESSENTIALSChapter 24281NameDateClassSection24.1Life Cycles of Mosses, Ferns, and Conifers, continued◗ After You ReadMini Glossarymegaspore: female spore formed by some plantsthat can develop into a female gametophytemicropyle (MI kruh pile): the opening in theovule through which the pollen tube entersmicrospore: male spore formed by some plantsthat can develop into a male gametophyte,or pollen grainprotonema (proh tuh NEE muh): in mosses, asmall green filament of haploid cells thatdevelops from a spore; can develop intothe gametophytevegetative reproduction: type of asexual reproduction in plants where a new plant is produced from existing plant organs or partsof organs1.
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