{"id":263,"date":"2017-08-16T10:49:26","date_gmt":"2017-08-16T10:49:26","guid":{"rendered":"http:\/\/americanboard.org\/Subjects\/biology\/?page_id=263"},"modified":"2020-12-29T08:26:56","modified_gmt":"2020-12-29T08:26:56","slug":"how-do-plants-grow","status":"publish","type":"page","link":"https:\/\/americanboard.org\/Subjects\/biology\/how-do-plants-grow\/","title":{"rendered":"How Do Plants Grow?"},"content":{"rendered":"<div class=\"twelve columns\" style=\"margin-top: 10%;\">\n<div class=\"advance\">\n<p><a class=\"button\" href=\"http:\/\/americanboard.org\/Subjects\/biology\/plant-biology-and-physiology\">Workshop Index<\/a>\u00a0<a class=\"button button-primary\" href=\"http:\/\/americanboard.org\/Subjects\/biology\/further-reading-7\">Further Reading \u27a1<\/a><\/p>\n<\/div>\n<p><!-- CONTENT BEGINS HERE --><\/p>\n<h1 id=\"title\">How Do Plants Grow?<\/h1>\n<h4>Objective<\/h4>\n<p>In this lesson, you will review plant growth, mechanisms, relationships, reproduction, and environmental responses.<\/p>\n<h4>Previously Covered:<\/h4>\n<ul>\n<li>Plants evolved from water to land and adapted to different environments.<\/li>\n<li>Plants are grouped as either nonvascular or vascular.<\/li>\n<li>Plants have an alternation of generations.<\/li>\n<\/ul>\n<section>\n<h3>Beginning of Growth<\/h3>\n<p><img decoding=\"async\" class=\"padding u-pull-left\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/germination.jpg\" alt=\"Germination of a seed\" \/><\/p>\n<p>A plant begins life after double fertilization occurs within the ovary of a flower. As cells divide by mitosis, an embryo and <abbr title=\"the triploid tissue of a seed\">endosperm<\/abbr> form. Then, the layers of tissue surrounding the embryo and endosperm toughen and become impermeable to water and oxygen. This causes the embryo to stop growing and a mature seed forms.<\/p>\n<p>At some point, the plant embryo will begin growing again. This is called <abbr title=\"the first stage of growth and development of a plant from a mature dormant seed\">germination<\/abbr>. Before germination can occur at all, water and oxygen must penetrate the seed coat. This results in the breaking of the seed coat. Many plants require heat or cold to germinate; still others require exposure to light. The first visible evidence of this process is the emergence of the embryonic root. Then the shoot will elongate and emerge from the soil. The leaves will then push through, unfurl, and begin to photosynthesize. Eventually, the food supply stored in the cotyledons is depleted, causing them to shrivel up and fall off.<\/p>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>The first sign of germination of a bean seed is the emergence of the<\/p>\n<ol>\n<li>shoot<\/li>\n<li>cotyledon(s)<\/li>\n<li>root<\/li>\n<li>endosperm<\/li>\n<\/ol>\n<p><a class=\"q-answer button button-primary\">Reveal Answer<\/a><\/p>\n<p class=\"q-reveal\">C is the correct choice. The shoot is the second structure to appear. The cotyledon is the third structure to appear. The endosperm is part of the seed formation process.<\/p>\n<\/section>\n<h3>Primary Growth<\/h3>\n<p>Plants grow by cell division that occurs in <abbr title=\"rapidly dividing cells at the growing points of a plant that differentiate into shoots, roots, and flowers\">meristems<\/abbr> located at the tips of the shoots and roots. This type of growth is called primary growth. It occurs in apical meristems, which are located at the tips of stems and roots, just behind the root cap. The area where the cells grow larger is the <em>zone of elongation<\/em>; farther back from this is the <em>zone of maturation<\/em>. In this zone, cells differentiate into the various cells making up the stem. The tissue produced is primary tissue.<\/p>\n<p>Primary growth also pushes roots through the soil. The root tip is covered by a <abbr title=\"a thimble-shaped protective cell structure over the apical meristem on a root tip\">root cap<\/abbr>. Growth in length is concentrated near the root tip. In the roots, meristems give rise to primary tissues:<\/p>\n<ul>\n<li><abbr title=\"the primary meristem that gives rise to epidermis\">Protoderm<\/abbr> gives rise to the epidermis.<\/li>\n<li><abbr title=\"the primary meristem of roots and shoots that gives rise to primary vascular tissue\">Procambium<\/abbr> gives rise to the stele, which is where the xylem and phloem develop.<\/li>\n<li>Ground meristems give rise to ground tissue system; ground tissue stores food and fills the region between the <abbr title=\"the xylem and phloem combined; vascular tissue\">stele<\/abbr> and the epidermis.<\/li>\n<\/ul>\n<h3>Secondary Growth<\/h3>\n<p>Most plants undergo secondary growth, in which the plant increases in width as well as length. The tissues that result from this growth are secondary tissues. The effects of secondary growth are most dramatic in woody plants. Two lateral meristems function in this process: <abbr title=\"cambium that gives rise to secondary vascular tissues (phloem and xylem) located between the xylem and bark\">vascular cambium<\/abbr> and <abbr title=\"a lateral meristem producing cork and part of the periderm\">cork cambium<\/abbr>. Cell division in the lateral meristems adds layers of new cells around the outside of the plant&#8217;s body.<\/p>\n<p><img decoding=\"async\" class=\"padding u-pull-left\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/cambium.jpg\" alt=\"Cambium structure\" \/><\/p>\n<p>The vascular cambium produces xylem and phloem. This layer is just beneath the bark. Xylem is produced in the interior of the vascular cambium and phloem is produced in the exterior. The new ring of xylem tissue formed every growing season and the older xylem become the wood of trees. Older xylem layers are dead, yet can still conduct water for several years. The phloem layer is never as thick as the xylem layer. Phloem tissue forms the inner half of tree bark.<\/p>\n<p>The cork cambium lies within the bark of a woody stem. It produces the cork cells of the outer bark. Cork tissue functions as a barrier that helps protect the stem from physical damage and pathogens. The layers of cork tissue and the cork cambium make up the <abbr title=\"plant tissue derived from cork cambium and replaces the epidermis, the cells being dead; also called bark\">periderm<\/abbr>, which is the protective coat of the secondary plant body that replaces the epidermis of the primary plant body.<\/p>\n<p>The lateral meristems also develop and produce secondary growth in the roots. The vascular cambium forms within the stele and produces xylem and phloem. As the stele grows, the epidermis is split and shed. A cork cambium forms from the <abbr title=\"a cylinder of cells lying inside the endodermis that conducts water and nutrients to vascular tissue\">pericycle<\/abbr> of the stele and produces periderm, which will become the secondary dermal tissue.<\/p>\n<h3>Summary<\/h3>\n<ul>\n<li>Evolution is the great organizing principle of biology. Similarities between organisms at every level from molecules to organ systems demonstrate this theory.<\/li>\n<li>Each organ system evidences development from the simplest multicellular animals, such as cnidarians, through complex vertebrates, such as mammals.<\/li>\n<\/ul>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>Cell division that results in the increased width of a tree occurs in the<\/p>\n<ol>\n<li>lateral meristems<\/li>\n<li>apical meristems<\/li>\n<li>primary tissues<\/li>\n<li>sieve tube vessels<\/li>\n<\/ol>\n<p><a class=\"q-answer button button-primary\">Reveal Answer<\/a><\/p>\n<p class=\"q-reveal\">The correct choice is A; cell division in the lateral meristems occurs in secondary growth which increases the length and width of a tree. Apical meristems are responsible for cell growth at the tips of roots and shoots. Primary tissues form leaves, roots, fruit pulp, and stem pith. Sieve tube vessels are part of the food-conducting phloem.<\/p>\n<\/section>\n<h3>Classification<\/h3>\n<p>Plants are classified into three main groups depending on how long it takes them to produce flowers and how long they live.<\/p>\n<ul>\n<li><em>Annuals<\/em> are plants that grow from seed to maturity, flower, produce seeds and die in the course of one growing season. Examples are marigolds, corn, and peas.<\/li>\n<li><em>Biennials<\/em> are plants that live for two years. In the first growing season, they grow roots, stems, and leaves. In the second season, new leaves and stems grow from the roots. The plant also produces flowers. Once the flowers produce seeds, the plant dies. Examples are foxglove, sugar beets, carrots, and turnips.<\/li>\n<li><em>Perennials<\/em> are plants that live for more than two years. The majority of vascular plants are perennials. Examples are trees, shrubs, and some grasses.<\/li>\n<\/ul>\n<h3>Growth Hormones<\/h3>\n<p>Hormones control the division, growth, maturation, and differentiation of plant cells. They are manufactured primarily in apical meristems, in young leaves, and in growing seeds and developing fruits. Plant hormones control a plant&#8217;s branching pattern, the rate at which its stems elongate, and its responses to environmental conditions.<\/p>\n<p><abbr title=\"the hormone that regulates plant growth in the apical regions\">Auxin<\/abbr> in the stems causes the stem to grow toward light and away from the pull of gravity. High concentrations of auxin stimulate young stem cells to elongate. Auxin in the roots causes the roots to grow away from light and toward the pull of gravity. High concentrations of auxin inhibit root cell elongation. Auxin also can cause cell division in meristematic regions to stop or start. Finally, the high concentration of auxin produced by the apical meristem inhibits the growth of lateral buds near the tip of the shoot.<\/p>\n<p><abbr title=\"a plant growth hormone that induces cell division and differentiation\">Cytokinins<\/abbr> are manufactured by the growing roots. They stimulate cell division and cause dormant seeds to sprout. Several of the effects produced by cytokinins are the opposite of effects produced by auxins.<\/p>\n<p><abbr title=\"a plant hormone that regulates plant growth characteristics, and stimulates flowering and seed germination\">Gibberellin<\/abbr> is manufactured by all higher plants. It stimulates growth in leaves and stems, but not in the roots. In stems, gibberellin stimulates cell elongation and cell division. It can also signal seeds to break dormancy and begin germination.<\/p>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>What is the hormone that controls the growth of plant roots?<\/p>\n<ol>\n<li>Gibberellin<\/li>\n<li>Cytokinin<\/li>\n<li>Auxin<\/li>\n<li>Glucose<\/li>\n<\/ol>\n<p><a class=\"q-answer button button-primary\">Reveal Answer<\/a><\/p>\n<p class=\"q-reveal\">The correct choice is B. Gibberellin is responsible for leaf and stem growth. Auxin is responsible for stem elongation. Glucose is not a hormone; it is a product of photosynthesis and fuels plant processes.<\/p>\n<\/section>\n<h3>Photosynthesis<\/h3>\n<p>Plants convert the energy of sunlight into the energy in the chemical bonds of carbohydrates (sugars and starches) in a process called photosynthesis. The overall process can be summarized by the following chemical equation:<\/p>\n<p class=\"center\"><strong>CO<sub>2<\/sub> + H<sub>2<\/sub>O + light \u2192 C<sub>6<\/sub>H<sub>12<\/sub>O<sub>6<\/sub> + O<sub>2<\/sub><\/strong><\/p>\n<p>Photosynthesis occurs primarily within leaves. Cells in the leaves contain chloroplasts, which contain chlorophyll. Chlorophyll is the light-absorbing substance needed for photosynthesis.<\/p>\n<p>Photosynthesis requires:<\/p>\n<ul>\n<li>Sunlight<\/li>\n<li>Pigments<\/li>\n<li>Carbon dioxide<\/li>\n<li>Water<\/li>\n<li>Energy-storing compounds<\/li>\n<\/ul>\n<p>The stages of photosynthesis are:<\/p>\n<ul>\n<li>Energy is captured from sunlight.<\/li>\n<li>Light energy is converted to chemical energy.<\/li>\n<li>Chemical energy powers the synthesis of organic molecules, using carbon from carbon dioxide.<\/li>\n<\/ul>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>Photosynthesis produces<\/p>\n<ol>\n<li>carbon dioxide<\/li>\n<li>water<\/li>\n<li>alcohol<\/li>\n<li>glucose<\/li>\n<\/ol>\n<p><a class=\"q-answer button button-primary\">Reveal Answer<\/a><\/p>\n<p class=\"q-reveal\">Hope you chose D, because the products of photosynthesis are glucose and oxygen. Photosynthesis does not produce carbon dioxide, water, or alcohol.<\/p>\n<\/section>\n<p><object class=\"old_animation\" id=\"flashPlayer\" width=\"750\" height=\"450\" classid=\"clsid:d27cdb6e-ae6d-11cf-96b8-444553540000\" codebase=\"http:\/\/fpdownload.macromedia.com\/pub\/shockwave\/cabs\/flash\/swflash.cab#version=6,0,0,0\" align=\"middle\"><param name=\"allowScriptAccess\" value=\"sameDomain\" \/><param name=\"movie\" value=\"flashPlayer.swf?sURL=photosynthesis.swf\" \/><param name=\"quality\" value=\"high\" \/><param name=\"bgcolor\" value=\"#ffffff\" \/><embed src=\"http:\/\/americanboard.org\/Subjects\/Images\/biology\/flash\/photosynthesis\/flashplayer.swf?sURL=http:\/\/americanboard.org\/Subjects\/Images\/biology\/flash\/photosynthesis\/photosynthesis.swf\" quality=\"high\" bgcolor=\"#ffffff\" width=\"750\" height=\"450\" name=\"flashPlayer\" align=\"middle\" allowscriptaccess=\"sameDomain\" type=\"application\/x-shockwave-flash\" pluginspage=\"http:\/\/www.macromedia.com\/go\/getflashplayer\" \/><\/object><\/p>\n<div class=\"slider-container new_animation\">\n<iframe loading=\"lazy\" src=\"https:\/\/americanboard.org\/Subjects\/animation_sliders\/plant_grow\/plants_HTML5 Canvas.html\" width=\"750\" height=\"450\" frameborder=\"0\" scrolling=\"no\"><br \/>\n<\/iframe><\/div>\n<h3>Light Reactions<\/h3>\n<p>The process of photosynthesis is divided into two parts: light reactions and dark reactions. The light reactions require light. In these reactions, the energy of sunlight is captured and used to make energy-storing compounds. The light reactions occur in the photosynthetic membranes of chloroplasts. The light reactions can be divided into four basic processes: light absorption, electron transport, oxygen production, and ATP formation. These processes are closely linked and dependent on each other.<\/p>\n<ul style=\"list-style: circle;\">\n<li>\n<h4>Light Absorption<\/h4>\n<p>The photosynthetic membrane contains clusters of pigment, or photosystems. There are two photosystems in green plants. Each one contains hundreds of chlorophyll molecules, as well as other pigments. The other pigments absorb light in the regions of the spectrum that chlorophyll does not. This causes electrons in pigment molecules to be raised to an excited state. Only a special pair of chlorophyll molecules can process the light energy.<\/li>\n<li>\n<h4>Electron Transport<\/h4>\n<p>The high-energy electrons produced in the previous process travel from a special pair of chlorophyll molecules to an adjacent molecule called an electron carrier. This begins electron transport. The high-energy electrons are transferred along a series of electron-carrier molecules in the photosynthetic membrane. At the end of the chain, an enzyme passes the high-energy electrons to an electron carrier called NADP +, which is converted into NADPH. Adding a pair of electrons to this electron carrier requires energy. The energy and electrons stored in the bonds of NADPH will be used later on in the overall process.<\/li>\n<li>\n<h4>Oxygen Production<\/h4>\n<p>As the light continues to shine, the photosynthetic membrane contains a system that provides new electrons to chlorophyll to replace the ones that converted to NADPH. These electrons are taken from water. Four electrons are removed from two water molecules, leaving four hydrogen ions and two oxygen atoms. The two oxygen atoms combine to form oxygen gas that is eventually released into the air.<\/li>\n<li>\n<h4>ATP Formation<\/h4>\n<p>The hydrogen ions left behind when water is &#8220;split&#8221; are released into the photosynthetic membrane. In addition, more hydrogen ions are pumped across the membrane as electrons are passed from chlorophyll to NADP +. Eventually, an enzyme attaches a phosphate molecule to ADP, forming ATP. This is the second way in which the energy of sunlight is trapped in chemical form. These energy-storing molecules will be converted to a more convenient form during the dark reactions.<\/li>\n<\/ul>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>The oxygen that results from photosynthesis comes directly from the<\/p>\n<ol>\n<li>splitting of carbon dioxide molecules by the Calvin cycle<\/li>\n<li>splitting of water molecules to provide electrons for photosystem II<\/li>\n<li>action of proton pumps in the electron transport chain<\/li>\n<li>absorption of photons by carotenoids in the photosystems<\/li>\n<\/ol>\n<p><a class=\"q-answer button button-primary\">Reveal Answer<\/a><\/p>\n<p class=\"q-reveal\">B is the correct choice, because after the water molecules are split, the resulting two oxygen atoms combine to form oxygen gas. Choices A, C, and D do not produce oxygen.<\/p>\n<\/section>\n<h3>Dark Reactions<\/h3>\n<p><img decoding=\"async\" class=\"padding u-pull-right\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/calvincyclesummary.jpg\" alt=\"Calvin cycle\" \/><\/p>\n<p>The dark reactions generally take place in sunlight; however, light does not play a role in the dark reactions. In this part of photosynthesis, carbon dioxide is used to make a complex organic molecule. The dark reactions form a cycle, or circular series of reactions. The dark reactions are also known as the <em>Calvin cycle<\/em>.<\/p>\n<p>The Calvin cycle employs a complex battery of enzymes that are found in the <abbr title=\"the site of photosynthesis' dark reaction, containing water soluble enzymes\">stroma<\/abbr> of a chloroplast . Photosynthesis itself usually occurs in the <abbr title=\"a phospholipid bilayer membrane in chloroplasts, which is folded into grana\">thylakoid membranes<\/abbr> of a chloroplast. A total of six carbon dioxide molecules must enter the Calvin cycle to produce one six-carbon sugar molecule. The Calvin cycle takes place in five steps.<\/p>\n<p><img decoding=\"async\" class=\"padding u-pull-left\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/photosynthesis.jpg\" alt=\"Photosynthesis\" \/><\/p>\n<ul>\n<li>The cycle begins when a carbon atom from a CO2 molecule is added to a five-carbon molecule (the starting material).<\/li>\n<li>The resulting six-carbon molecule is unstable and immediately splits, forming two three-carbon molecules. These first two steps tend to be relatively slow. In order to compensate, the chloroplast has an enzyme that catalyzes the reaction.<\/li>\n<li>Then, energy in the form of a phosphate group from ATP is added to the three-carbon molecules, and they are reduced by the addition of hydrogen from NADPH.<\/li>\n<li>The reduced three-carbon molecules may combine to make glucose, a six-carbon sugar, or may be used to make other organic molecules.<\/li>\n<li>Most of the reduced three-carbon molecules are used to regenerate the five-carbon starting material, thus completing the cycle.<\/li>\n<\/ul>\n<p>Finally, photosynthesis and another process called <abbr title=\"the process within a cell that breaks down sugar and other organic compounds in order to release energy; either anaerobic or aerobic\">cellular respiration<\/abbr> form a continuous cycle, because the products of one process are the starting materials for the other.<\/p>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>Which of these is produced by the dark reactions?<\/p>\n<ol>\n<li>Oxygen<\/li>\n<li>Water<\/li>\n<li>ATP<\/li>\n<li>Glucose<\/li>\n<\/ol>\n<p><a class=\"q-answer button button-primary\">Reveal Answer<\/a><\/p>\n<p class=\"q-reveal\">D is the correct choice, as glucose is produced in the Calvin cycle dark reaction. Oxygen and ATP are produced in the Calvin cycle light reaction. Water is required for, not produced by, photosynthesis.<\/p>\n<\/section>\n<h3>Symbiotic Relationships<\/h3>\n<p>There are several examples of symbiotic relationships among plants and other organisms, including:<\/p>\n<ul>\n<li>lichens<\/li>\n<li>mycorrhizae<\/li>\n<li>acacia trees<\/li>\n<li>pollination<\/li>\n<li>nitrogen fixation<\/li>\n<li>mistletoe<\/li>\n<\/ul>\n<table>\n<tbody>\n<tr>\n<td><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/lichen.jpg\" alt=\"Lichens\" \/><\/td>\n<td>Lichens are symbiotic relationships between a fungus and a photosynthetic organism, usually algae. In this partnership, the alga carries out photosynthesis, providing the fungus with a source of organic nutrients. The fungus, in turn, provides the alga with water and minerals that it has collected from the surfaces on which it grows.<\/td>\n<\/tr>\n<tr>\n<td><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/mycorrhizae.jpg\" alt=\"Mycorrhizae\" \/><\/td>\n<td>Mycorrhizae are symbiotic relationships between plant roots and fungi. The fungal filaments aid in the direct transfer of phosphorus and other minerals from the soil into the roots of the plant, while the plant supplies carbohydrates to the fungus. Over 95% of all vascular plants have mycorrhizae. This type of symbiotic relationship is an example of <abbr title=\"a relationship of two species living together in which both benefit\">mutualism<\/abbr>.<\/td>\n<\/tr>\n<tr>\n<td><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/acacia.jpg\" alt=\"Acacia\" \/><\/td>\n<td>Certain species of Central and South American acacia trees have hollow thorns that house stinging ants. The ants feed on sugar produced by nectaries on the tree. The acacia benefits from the housing and feeding of the ants, because the ants attack everything that touches the tree. The ants sting other insects, remove fungal spores and other debris, and clip surrounding vegetation that happens to grow close to the foliage of the acacia.<\/td>\n<\/tr>\n<tr>\n<td><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/pollination.jpg\" alt=\"Pollination\" \/><\/td>\n<td>Some angiosperms have adaptations that attract animals that function in pollination or seed dispersal. For example, pollen is spread when the pollinator consumes nectar, and seeds are dispersed by animals that eat the fruit that encloses them.<\/td>\n<\/tr>\n<tr>\n<td><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/nitrogenfixation.jpg\" alt=\"Nitrogen fixation\" \/><\/td>\n<td>Many plant families include species that form symbiotic relationships with nitrogen-fixing bacteria that give roots a built-in source of fixed nitrogen for assimilation into organic compounds. This is especially common in the legume plant family, which includes peas, beans, soybeans, peanuts, alfalfa, and clover. Nodules on the roots contain the bacteria. Meanwhile, the plant provides the bacteria with carbohydrates and other organic compounds.<\/td>\n<\/tr>\n<tr>\n<td><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/mistletoe2.jpg\" alt=\"Mistletoe\" \/><\/td>\n<td>Lichens are symbiotic relationships between a fungus and a photosynthetic oMistletoe lives as a parasite on oaks and other trees. Mistletoe is photosynthetic, but it supplements its nutrition using projections to siphon xylem sap from the vascular tissue of host trees.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>Mycorrhizae are symbiotic relationships of fungi and<\/p>\n<ol>\n<li>algae<\/li>\n<li>lichens<\/li>\n<li>roots<\/li>\n<li>chloroplasts<\/li>\n<\/ol>\n<p><a class=\"q-answer button button-primary\">Reveal Answer<\/a><\/p>\n<p class=\"q-reveal\">The correct choice is C. Mycorrhizae are mutualistic relationships between fungi and plant roots. Mycorrhizae do not occur in algae, lichens, or chloroplasts.<\/p>\n<\/section>\n<h3>Plant Reproduction<\/h3>\n<p>The life cycles of most <em>algae<\/em> include both a diploid and a haploid generation. Diploid cells have the normal number of chromosomes for a species, whereas haploid cells only have half the number of chromosomes. Most species of algae reproduce by <abbr title=\"the regular alternation of forms or reproduction modes in an organism's life cycle; this includes alternation between haploid and diploid phases or between asexual and sexual reproductive cycles\">alternation of generations<\/abbr>.<\/p>\n<p>A species called <em>Chlamydomonas<\/em> usually reproduces asexually by mitosis. However, if conditions become unfavorable, it can switch to a stage that reproduces sexually.<\/p>\n<p><em>Mosses<\/em> generally reproduce by alternation of generations. They can reproduce sexually only when standing water is present. Some species have both male and female reproductive organs on one gametophyte; other species have male and female reproductive structures on separate gametophytes.<\/p>\n<p><em>Ferns<\/em> reproduce through alternation of generations, as well. Sexual reproduction in ferns depends upon the presence of standing water for sperm to swim to eggs. The same individual produces both the eggs and sperm.<\/p>\n<p>Familiar <em>gymnosperms<\/em>, such as pine trees, are diploid sporophytes. The male cones carry structures called microsporangia, which produce pollen grains. The female cones carry structures called megasporangia, which produce female gametophytes. The female gametophytes produce ovules, in which egg cells form.<\/p>\n<p>Pollen grains are carried by wind to female cones, where they land near an ovule. The grain splits open and grows a pollen tube, which grows into the ovule. Eventually, the sperm inside the tube break out. One fertilizes the egg and the other disintegrates. The resulting zygote will eventually become an embryo encased in what later develops into a seed.<\/p>\n<p>A pine tree will produce both male and female cones on the same individual.<\/p>\n<p>Like all other types of plants, <em>angiosperms<\/em> reproduce by alternation of generations. However, angiosperms have evolved a specialized plant structure to help them with sexual reproduction: the flower.<\/p>\n<p>Located in each plant ovary are ovules. The ovules produce female gametophytes, which are embryo sacs containing eight nuclei. These gametophytes are smaller than that of mosses and ferns. The nuclei move around and eventually the nucleus closest to the ovule opening enlarges to become the egg nucleus. The gametophyte is now ready for fertilization.<\/p>\n<p>The male gametophyte is smaller than the female gametophyte. Inside the anthers, pollen chambers produce microspore cells. The microspore cells ultimately become pollen grains. The pollen grains usually stop growing until they are deposited on a stigma. Eventually, the anther dries out; its pollen chambers split open; and mature pollen grains are released.<\/p>\n<p><center><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/angiospermlifecycle2.jpg\" alt=\"Angiosperm life cycle\" \/><\/center>How do the pollen grains get to the stigma? By pollination, of course. Some plants allow pollen to fall from the anther to the stigma of the same flower. This is <em>self-pollination<\/em>. Most plants, however, transfer their pollen to the stigma of a flower on another plant. This is <em>cross-pollination<\/em>. They do this with the aid of pollinators, such as wind, insects, and birds.<\/p>\n<p>Just like gymnosperms, fertilization begins after the pollen tube has reached the embryo sac and the sperm nuclei enter. Then, <abbr title=\"a fertilization process characteristic of angiosperms, which results in one sperm fusing with the egg cell that develops into a diploid embryo and a second sperm fusing with two polar cells that forms a triploid cell that develops into the endosperm\">double fertilization<\/abbr> occurs. Following this, parts of the ovule toughen to form a seed coat. The ovary wall thickens and joins with other parts of the flower stem to become the fruit that holds the seeds.<\/p>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>The reproductive organs of gymnosperms are<\/p>\n<ol>\n<li>fruits<\/li>\n<li>ferns<\/li>\n<li>cones<\/li>\n<li>flowers<\/li>\n<\/ol>\n<p><a class=\"q-answer button button-primary\">Reveal Answer<\/a><\/p>\n<p class=\"q-reveal\">The correct choice is C. Gymnosperms do not have fruits, ferns, or flowers.<\/p>\n<\/section>\n<h3>Environmental Responses<\/h3>\n<p>Plants respond to their environment in various ways. Obvious examples are when a plant&#8217;s leaves wilt due to a drought or a plant dies due to the presence of herbicide. A plant&#8217;s response to its environment generally falls into one of the following categories:<\/p>\n<ul>\n<li>tropisms<\/li>\n<li>dormancy<\/li>\n<li>photoperiodism<\/li>\n<\/ul>\n<h3>Tropism<\/h3>\n<p>A tropism is a growth response in which the direction of growth is determined by the direction from which the stimulus comes. Auxins are responsible for producing tropisms. If the plant grows toward the stimulus, it is a positive tropism. If the plant grows away from the stimulus, it is a negative tropism. Three common tropisms that plants respond to are phototropism, gravitropism, and thigmotropism.<\/p>\n<table>\n<tbody>\n<tr>\n<td><em>Phototropism<\/em> is a plant&#8217;s growth response to light. The plant&#8217;s shoot tip is the site of the photoreception that triggers this growth response. The growth of a plant&#8217;s shoots toward light is positive phototropism, while the growth of roots away from light is negative phototropism.<\/td>\n<td><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/phototropism.jpg\" alt=\"Phototropism\" \/><\/td>\n<\/tr>\n<tr>\n<td><em>Gravitropism<\/em> is a plant&#8217;s growth response to gravity. Roots display positive gravitropism by curving downward, and shoots display negative gravitropism by bending upward. Gravitropism functions as soon as a seed germinates.<\/td>\n<td><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/gravitropism.jpg\" alt=\"Gravitropism\" \/><\/td>\n<\/tr>\n<tr>\n<td><em>Thigmotropism<\/em> is growth responses to touch. For example, vines and climbing plants have tendrils that coil around supports. They usually grow straight until they touch something; the contact stimulates a response. Mechanical stimulation, such as rubbing stems with a stick, can also cause a response. The response is usually a shorter plant.<\/td>\n<td><img decoding=\"async\" src=\"http:\/\/americanboard.org\/Subjects\/biology\/wp-content\/uploads\/sites\/2\/2017\/08\/thigmotropism.jpg\" alt=\"Thigmotropism\" \/><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h3>Dormancy<\/h3>\n<p><em>Dormancy<\/em> is a condition in which a seed or a plant remains inactive for a period of time. It is thought that plant hormones play a role in initiating dormancy. Seeds often remain dormant for a period before germination. Plants also exhibit winter dormancy by their leaves changing to autumn colors and eventually falling off.<\/p>\n<h3>Photoperiodism<\/h3>\n<p><em>Photoperiodism<\/em> is the response of plants to the lengths of days and nights. Plants can be categorized as one of three types, depending on how they respond to day length. Short-day plants, such as the goldenrod, need long periods of darkness for flowering. Long-day plants, such as the iris, need short periods of darkness to induce flowering. Day-neutral plants, such as corn, are plants that are not affected by day length. This results in some plants flowering in spring, some in summer, and others in the fall.<\/p>\n<section class=\"question\">\n<h4>Question<\/h4>\n<p>The part of most terrestrial plants that shows positive phototropism and negative gravitropism is a<\/p>\n<ol>\n<li>vascular bundle<\/li>\n<li>root<\/li>\n<li>flower<\/li>\n<li>stem<\/li>\n<\/ol>\n<p><a class=\"q-answer button button-primary\">Reveal Answer<\/a><\/p>\n<p class=\"q-reveal\">The correct choice is D, because the hormone auxin causes stems to grow upward (away from gravity) and toward light. Vascular bundle, roots, and flowers are not subject to this hormonal control.<\/p>\n<\/section>\n<h3>Summary<\/h3>\n<ul>\n<li>Plant life begins with double fertilization occurring within the ovary of a flower.<\/li>\n<li>After a mature seed forms, germination begins as the plant embryo begins growing.<\/li>\n<li>The first sign of germination is the plant root.<\/li>\n<li>Plants grow by cell division that occurs in meristems located at the tips of the shoots and roots.<\/li>\n<li>Most plants undergo secondary growth, in which the plant increases in width and length. This is most dramatic in woody plants.<\/li>\n<li>Depending on how long it takes to produce flowers and how long plants live, plants are classified into three main groups: annuals, biennials, and perennials.<\/li>\n<li>Hormones control the division, growth, maturation, and differentiation of plant cells.<\/li>\n<li>Photosynthesis is the process that plants use to convert the energy of sunlight into the energy in the chemical bonds of carbohydrates.<\/li>\n<li>Photosynthesis is divided into two parts: light reactions and dark reactions (Calvin cycle).<\/li>\n<li>Photosynthesis and cellular respiration form a continuous cycle.<\/li>\n<li>Plants and other organisms form symbiotic relationships, such as lichens, mycorrhizae, pollination, and nitrogen fixation.<\/li>\n<li>All plants reproduce by alternation of generations.<\/li>\n<li>Plants respond to their environment with tropisms, dormancy, and\/or photoperiodism.<\/li>\n<\/ul>\n<\/section>\n<p><!-- CONTENT ENDS HERE --><\/p>\n<div class=\"advance\"><a class=\"button\" href=\"http:\/\/americanboard.org\/Subjects\/biology\/plant-biology-and-physiology\">Workshop Index<\/a>\u00a0<a class=\"button button-primary\" href=\"http:\/\/americanboard.org\/Subjects\/biology\/further-reading-7\">Further Reading \u27a1<\/a><\/div>\n<p><a class=\"backtotop\" href=\"#title\">Back to Top<\/a><\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"<p>Workshop Index\u00a0Further Reading \u27a1 How Do Plants Grow? Objective In this lesson, you will review plant growth, mechanisms, relationships, reproduction, and environmental responses. Previously Covered: Plants evolved from water to land and adapted to different environments. Plants are grouped as either nonvascular or vascular. Plants have an alternation of generations. Beginning of Growth A plant [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-263","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/pages\/263","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/comments?post=263"}],"version-history":[{"count":14,"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/pages\/263\/revisions"}],"predecessor-version":[{"id":1470,"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/pages\/263\/revisions\/1470"}],"wp:attachment":[{"href":"https:\/\/americanboard.org\/Subjects\/biology\/wp-json\/wp\/v2\/media?parent=263"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}