Reading Essentials for Biology Glencoe (794133), страница 66
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In both, the sporophyte is the dominant generation, andthe gametophyte generation develops within the sporophyte.However, in conifers the reproductive organs are in male andfemale cones. In the anthophytes, the reproductive organs are inthe flowers. Anthophytes are the only plants that produce flowersand fruits.What is the development of thefemale gametophyte?Inside each ovary is one or moreovules. In the ovule, a cell undergoesmeiosis and produces haploid megaspores. A megaspore is a femalespore that eventually can become thefemale gametophyte.
Four megasporesare produced, but three of them usuallydie. In most flowering plants, themegaspore’s nucleus undergoes mitosisthree times, producing eight haploidnuclei as illustrated in the figure tothe right. Together, these eight nucleimake up the female gametophyte.Six of the nuclei develop cell walls.One of the six becomes the egg cell.The two remaining nuclei, called polarREADING ESSENTIALSIdentify MainIdeas Highlight the mainpoint in each paragraph.
Studythe points and come up witha way to state each main pointin your own words.Embryo sac(femalegametophyte)Central cellOvuleEgg cellMicropyleChapter 24287NameDateClassSection24.3The Life Cycle of a Flowering Plant, continuednuclei, are enclosed in one cell. This cell, called the central cell, islocated at the center of the female gametophyte. The egg cell isnear the micropyle or opening of the ovule. Eventually the otherfive cells break apart and disappear.How does the male gametophyte develop?Pollen sacs in the anther produce haploid microspores.
Thenucleus of each microspore undergoes mitosis, forming two nuclei.A thick, protective wall surrounds these two nuclei. This structure isthe immature male gametophyte, or pollen grain. The nuclei withinthe pollen grain are the tube nucleus and the generative nucleus.When the pollen grains mature, the anther usually splits open.Pollen sacsMicrosporemother cell (2n)MEIOSISANTHERGenerativenucleusMicrospores(n)Pollen grain(male gametophyte)1. What are the three waysthat pollen is carried tothe stigma?________________________________________________________________________288Chapter 24What are some pollination adaptations?In flowering plants, pollination is the transfer of pollen grainsfrom the anther to the stigma.
Pollen grains can be carried to thestigma by wind, water, or animals. A high pollination rate means abetter rate of plant reproduction. This means the pistil of a flowerreceives enough pollen of its own species to fertilize the egg ineach ovule. Wind-pollinated plants produce large amounts ofpollen to help guarantee pollination.Most anthophytes have adaptations that allow pollen grains to bedeposited in the right place at the right time.
For example, windpollinated flowers lack structures that might block wind currents.Many anthopyhtes that are pollinated by animals produce nectar in their flowers. The animals that visit the flowers are lookingfor this concentrated liquid. Nectar is made up of proteins andREADING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.TubenucleusNameDateClassSection24.3The Life Cycle of a Flowering Plant, continuedsugars and collects in the cuplike area at the base of the petals.Insects and birds brush up against the anthers trying to get thenectar.
The pollen that attaches to them is carried to anotherflower as they continue to look for nectar, resulting in pollination.Some insects also gather pollen to use as food. By producing nectar and attracting animal pollinators, animal-pollinated plants donot need to produce as much pollen as wind-pollinated plants.Some nectar-feeding pollinators are attracted to a flower by itscolor or scent or both. Butterflies and bees are attracted to bright,colorful flowers. Some of these flowers have markings that humanscan’t see but are easily seen by insects. Flowers that are pollinatedby beetles and flies have a strong scent but are often dull in color.Some flowers have structural adaptations that give them a better chance of being pollinated by another plant of the same species.This is called cross-pollination.
It results in greater genetic variation than self-pollination. For example, the flowers of a certainspecies of orchid resemble female wasps. A male wasp visits theflower and attempts to mate with it. The male becomes coveredwith pollen, which is deposited on the next orchid he visits.2. Why do animal-pollinatedplants produce less pollenthan wind-pollinatedplants?________________________________________________________________________Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.What are the steps in fertilization?Animals or wind can bring a pollen grain to the stigma of thepistil, but that does not mean fertilization has occurred.
Severalevents have to take place before fertilization occurs. Rememberthat inside each pollen grain are two haploid nuclei. One is thetube nucleus, the other is the generative nucleus. The tubenucleus directs the growth of the pollen tube down through thepistil to the ovary. The generative nucleus divides by mitosis,producing two sperm nuclei.
The sperm nuclei move down thepollen tube to the micropyle.Inside the ovule is the female gametophyte. One of the spermunites with the egg cell forming a diploid zygote, which beginsthe new sporophyte generation. The other sperm nucleus fuseswith the central cell, which contains the polar nuclei. This createsa cell with a triploid (3n) nucleus. This process, in which onesperm fertilizes the egg and the other sperm joins with the centralcell, is called double fertilization. Double fertilization is uniqueto anthophytes.The triploid nucleus will divide many times, eventually formingthe endosperm of the seed. The endosperm is food storage tissuethat supports development of the embryo in anthophyte seeds.READING ESSENTIALS3.
What is doublefertilization?________________________________________________________________________Chapter 24289NameDateClassSection24.3The Life Cycle of a Flowering Plant, continuedDouble FertilizationPollen grainStigmaStyleTwo spermnucleiPollen tubeOvaryTube nucleusOne spermfertilizes thecentral cell(3n)Central cellOvuleEgg cellMany flower ovaries contain more than one ovule. Each ovulecan become a seed as long as at least one pollen grain lands on thestigma for each ovule in the ovary.
Think of the hundreds of seedsin a watermelon. Each seed is the result of a pollen grain pollinating a flower, followed by the process of double fertilization.Seeds and FruitsHow do seeds form?4. What are the functions ofthe seed coat?Once fertilization takes place, seed development begins. Insidethe ovule, the zygote divides and develops into the embryo plant.While this is happening, the triploid central cell develops intothe seed’s endosperm. The wall of the ovule becomes the seedcoat. The seed coat helps with seed dispersal and helps protectthe embryo until it begins growing. The embryo inside a seedis the next sporophyte generation.
The formation of seedsand the fruits that enclose them help ensure the survival of thenext generation.________________________________________________________________________290Chapter 24How does fruit form?As seeds develop, the surrounding ovary grows larger andbecomes the fruit. Sometimes other flower organs become partof the fruit. A fruit is a structure that contains the seeds ofan anthophyte.READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.One spermfertilizes theegg cell (2n)NameDateClassSection24.3The Life Cycle of a Flowering Plant, continuedFruitA When the eggs in the ovules of ablueberry flower have been fertilized,the petals, stamens, and stigmaeventually wither and fall away.OvaryC The remains of the sepalsand some dried stamensusually can be seen at thetop of a blueberry fruit.OvulesSepalsStamenB The wall of the blueberryCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.flower’s ovary becomes fleshyas the seeds develop.FusedpetalsEach flowering plant species has different flowers and fruit thatcan be used for identification.
Some plants develop fleshy fruitssuch as apples, grapes, melons, tomatoes, and cucumbers. Otherplants develop dry fruits such as peanuts, sunflower “seeds,” andwalnuts. In dry fruits, the ovary around the seeds hardens as thefruit matures. Some plants that we call vegetables or grains arefruits.
Did you know that green peppers are actually fruits?What are some methods of seed dispersal?A fruit protects the seeds within it. A fruit can help scatterseeds away from parent plants and into new habitats. This scattering, or dispersal, is important because it reduces competition forsunlight, soil, and water between the parent plant and offspring.Animals carry fruit away from the parent plant. While eating thefruit, they may spit the seeds out. Seeds that are eaten usually passunharmed through the digestive system, then are deposited in theREADING ESSENTIALSChapter 24291NameDateClassSectionThe Life Cycle of a Flowering Plant, continued5.
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