Reading Essentials for Biology Glencoe (794133), страница 63
Текст из файла (страница 63)
Indoleacetic (in doh luh SEE tihk)acid (IAA), an auxin, is produced in apicalShoot tipShoot tip removedmeristems of plant stems. IAA weakens theBuds growBudsinto sidecell wall, allowing a cell to stretch and growinhibitedbrancheslonger. Stem growth is the result of new cellsfrom the apical meristem and their increasingcell lengths.Auxins have other effects on plant growthand development. Auxin produced in the apicalmeristem keeps side branches from growing.Removing the stem tip reduces the amount ofauxin and allows branches to develop.Auxin also delays fruit formation and keepsfruit from dropping off the plant. When theB Once the tip is removed, theamount of auxin in the plant decreases, ripeAuxin produced in the tip ofthe main shoot inhibits theside branches start to grow.fruit drops to the ground and trees begin togrowth of the side branches.shed their leaves.272Chapter 23READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Plant HormonesNameDateClassSection23.3Plant Responses, continuedHow do gibberellins affect plants?Another group of plant growth hormones called gibberellins(jih buh REH lunz) help cells elongate.
Often dwarf plants areshort because the plant does not produce gibberellins. If gibberellins are applied to the tips of these dwarf plants, they will growtaller. Gibberellins increase the rate at which seeds begin to growand buds develop. Farmers use gibberellins to increase the formation of fruit. Florists use gibberellins to cause flower buds to open.1. Infer Why might a floristneed to use gibberellins?________________________________________________________________________What other hormones affect plant growth?Copyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.The hormones called cytokinins (si tuh KI nihnz) stimulatethe production of proteins needed for mitosis. Without cytokinins,plant cells would grow larger but never divide.
The effects ofcytokinins are often enhanced by the presence of other hormones.The plant hormone ethylene (EH thuh leen) is a gas made ofcarbon and hydrogen. Ethylene is produced and released by fruitwhile it ripens. It causes cell walls to weaken and soften. Ethylenespeeds the ripening of fruits and causes the breakdown of complexcarbohydrates to simple sugars. This gives ripe fruit its sweettaste.
Many farmers often pick unripe fruit and then use ethyleneto ripen the fruit later.2. Infer How might placing a piece of ripe fruitin a brown paper bagwith unripe fruit causethe unripe fruit to ripen?________________________________________________________________________Plant ResponsesTropism is a change in a plant’s growthbecause of an external stimulus. The tropism iscalled positive if the plant grows toward thestimulus. The tropism is called negative if theplant grows away from the stimulus.The growth of a plant toward light is calledphototropism. It is caused by the hormonegroup auxins.
There is more auxin on the sideof the stem that is away from the light. Thiscauses the cells on that side to lengthen. Thedifference in size between the lengthened cellsand the cells on the other side causes the stemto bend toward the light.Another tropism causes roots to grow downward and stems to grow upward. Gravitropismis plant growth in response to gravity. Positivegravitropism causes roots to grow into the soiland anchor the plant. Stems exhibit negativegravitropism as they grow away from the ground.READING ESSENTIALSLightsourceElongated cellsChapter 23273NameDateClassSection23.3Plant Responses, continued3. Why are tropisms notreversible?________________________________________________________________________Growth response to touch is called thigmotropism.
Duringearly growth, if the tendrils of sweet peas and passion vines toucha fence or trellis, they coil around it.Tropisms involve growth and are not reversible. The positionof a stem that has grown in a particular direction cannot bechanged. However, if the direction of the stimulus is changed, thestem will begin growing in another direction.A nastic movement is a responsive movement of a plant that isnot dependent on the direction of the stimulus.
An example of anastic movement is the response of Mimosa pudica leaflets whenthey are touched. Touching causes a drop in water pressure in thecells at the base of each leaflet. The cells become limp and theleaflets change direction. A nastic movement is reversible becauseit does not involve plant growth.Another example of nastic movement is the sudden closing ofthe hinged leaf of a Venus’s-flytrap. If an insect triggers sensitivehairs on the inside of the leaf, the leaf snaps shut. In both examples, the leaves return to their original positions once the stimulusis removed.Mini Glossaryauxins (AWK sunz): group of plant hormonesthat promote cell elongation; move througha plant by active transporthormone: chemical produced in one part of anorganism and transported to another part,where it causes a physiological changecytokinins (si tuh KI nihnz): group of hormonesthat stimulate mitosis and cell divisionnastic movement: responsive movement of aplant not dependent on the direction ofthe stimulusethylene (EH thuh leen): plant hormone thatpromotes the ripening of fruitsgibberellins (jih buh REH lunz): group of planthormones that cause plants to grow tallerby stimulating cell elongation; transportedin vascular tissuetropism: change in a plant’s growth because ofan external stimulus1.
Read the key terms and definitions in the Mini Glossary above. On the lines below explainhow the key terms auxins and tropism relate to each other.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________274Chapter 23READING ESSENTIALSCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.◗ After You ReadNameDateClassSection23.3Plant Responses, continued2.
In Column 1 are some new concepts from this section. Column 2 gives an example of eachconcept. Write the letter of the example on the line next to the concept that matches it.Column 1Column 2_____1. plant hormonesa. growth responses to external stimuli_____2. tropismsb.
sometimes caused by changes in cell pressure_____3. nastic movement_____4. phototropismc. affect plant growth and functionsd. growth response to touche. growth of plant toward light_____5. thigmotropismCopyright © by Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.Visit the Glencoe Science Web site at science.glencoe.com to findyour biology book and learn more about plant responses.READING ESSENTIALSChapter 23275NameDateClassSection24.1 Life Cycles of Mosses, Ferns,and ConifersSC.F.2.4.1 The student understands the mechanisms of asexual and sexual reproduction and knows the differentgenetic advantages and disadvantages of asexual and sexual reproduction.
Also covers SC.F.1.4.2◗ Before You ReadPrevious sections have briefly covered alternation of generations. Write any questions you haveabout alternation of generations on the lines below. If, after you read this section you still have questions, be sure to ask your teacher.______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________◗ Read to LearnCreate a Quiz After youhave read this section, createa quiz based on what youhave learned.
After you havecompleted writing the quizquestions, be sure to answerthem.1. Is the gametophyte or thesporophyte the dominantgeneration in most plantspecies?________________________________________________________________________276Chapter 24Most plant cycles include an alternation of generations. Asyou learned earlier, alternation of generations consists of a sporophyte (spore-producing) generation, and a gametophyte (gameteproducing) generation.All cells of a sporophyte are diploid, containing two of eachkind of chromosome. To create haploid spores, containing one ofeach kind of chromosome, certain cells of a sporophyte undergomeiosis. The haploid spores undergo cell divisions and form amulticellular, haploid gametophyte.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
