{"id":222,"date":"2015-04-06T21:25:49","date_gmt":"2015-04-06T21:25:49","guid":{"rendered":"https:\/\/courses.candelalearning.com\/biology2xmaster\/?post_type=chapter&#038;p=222"},"modified":"2024-04-25T18:55:33","modified_gmt":"2024-04-25T18:55:33","slug":"gymnosperms","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-biology2\/chapter\/gymnosperms\/","title":{"raw":"Gymnosperms","rendered":"Gymnosperms"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Identify the main characteristics of gymnosperms<\/li>\r\n<\/ul>\r\n<\/div>\r\n<p id=\"fs-idp68426896\"><strong><span id=\"term981\" data-type=\"term\">Gymnosperms<\/span><\/strong>, meaning \u201cnaked seeds,\u201d are a diverse group of seed plants. According to the \"anthophyte\" hypothesis, the angiosperms are a sister group of one group of gymnosperms (the Gnetales), which makes the gymnosperms a paraphyletic group. Paraphyletic groups are those in which not all descendants of a single common ancestor are included in the group. However , the \"netifer\" hypothesis suggests that the gnetophytes are sister to the conifers, making the gymnosperms monophyletic and sister to the angiosperms. Further molecular and anatomical studies may clarify these relationships. Characteristics of the gymnosperms include naked seeds, separate female and male gamtophytes, pollen cones and ovulate cones, pollination by wind and insects, and tracheids (which transport water and solutes in the vascular system).<\/p>\r\n<p id=\"fs-idp52182176\">Gymnosperm seeds are not enclosed in an ovary; rather, they are only partially sheltered by modified leaves called\u00a0<strong><span id=\"term982\" data-type=\"term\">sporophylls<\/span><\/strong>. You may recall the term\u00a0<strong><span id=\"term983\" data-type=\"term\">strobilus<\/span><\/strong>\u00a0(plural = strobili) describes a tight arrangement of sporophylls around a central stalk, as seen in pine cones. Some seeds are enveloped by sporophyte tissues upon maturation. The layer of sporophyte tissue that surrounds the megasporangium, and later, the embryo, is called the\u00a0<strong><span id=\"term984\" data-type=\"term\">integument<\/span><\/strong>.<\/p>\r\n<p id=\"fs-idm63613648\">Gymnosperms were the dominant phylum in the Mesozoic era. They are adapted to live where fresh water is scarce during part of the year, or in the nitrogen-poor soil of a bog. Therefore, they are still the prominent phylum in the coniferous biome or\u00a0<em data-effect=\"italics\">taiga<\/em>, where the evergreen conifers have a selective advantage in cold and dry weather. Evergreen conifers continue low levels of photosynthesis during the cold months, and are ready to take advantage of the first sunny days of spring. One disadvantage is that conifers are more susceptible than deciduous trees to leaf infestations because most conifers do not lose their leaves all at once. They cannot, therefore, shed parasites and restart with a fresh supply of leaves in spring.<\/p>\r\n<p id=\"fs-idm200847248\">The life cycle of a gymnosperm involves alternation of generations, with a dominant sporophyte in which reduced male and female gametophytes reside. All gymnosperms are heterosporous. The male and female reproductive organs can form in cones or strobili. Male and female sporangia are produced either on the same plant, described as\u00a0<strong><span id=\"term985\" data-type=\"term\">monoecious<\/span><\/strong>\u00a0(\u201cone home\u201d or bisexual), or on separate plants, referred to as\u00a0<strong><span id=\"term986\" data-type=\"term\">dioecious<\/span><\/strong>\u00a0(\u201ctwo homes\u201d or unisexual) plants. The life cycle of a conifer will serve as our example of reproduction in gymnosperms.<\/p>\r\n<span style=\"color: #077fab; font-size: 1.15em; font-weight: 600;\">Life Cycle of a Conifer<\/span>\r\n<p id=\"fs-idp58000784\">Pine trees are conifers (coniferous = cone bearing) and carry both male and female sporophylls on the same mature sporophyte. Therefore, they are monoecious plants. Like all gymnosperms, pines are heterosporous and generate two different types of spores (male microspores and female megaspores). Male and female spores develop in different strobili, with small male cones and larger female cones. In the male cones, or staminate cones, the\u00a0<strong><span id=\"term987\" data-type=\"term\">microsporocytes<\/span><\/strong>\u00a0undergo meiosis and the resultant haploid microspores give rise to male gametophytes or \u201cpollen grains\u201d by mitosis. Each pollen grain consists of just a few haploid cells enclosed in a tough wall reinforced with sporopollenin. In the spring, large amounts of yellow pollen are released and carried by the wind. Some gametophytes will land on a female cone. Pollination is defined as the initiation of pollen tube growth. The pollen tube develops slowly, and the generative cell in the pollen grain produces two haploid sperm or generative nuclei by mitosis. At fertilization, one of the haploid sperm nuclei will unite with the haploid nucleus of an egg cell.<\/p>\r\n<p id=\"fs-idp140324640\">Female cones, or\u00a0<strong><span id=\"term988\" data-type=\"term\">ovulate cones<\/span><\/strong>, contain two ovules per scale. Each ovule has a narrow passage that opens near the base of the sporophyll. This passage is the micropyle, through which a pollen tube will later grow. One megaspore mother cell, or\u00a0<strong><span id=\"term989\" data-type=\"term\">megasporocyte<\/span><\/strong>, undergoes meiosis in each ovule. Three of the four cells break down; only a single surviving cell will develop into a female multicellular gametophyte, which encloses archegonia (an archegonium is a reproductive organ that contains a single large egg). As the female gametophyte begins to develop, a sticky pollination drop traps windblown pollen grains near the opening of the micropyle. A pollen tube is formed and grows toward the developing gametophyte. One of the generative or sperm nuclei from the pollen tube will enter the egg and fuse with the egg nucleus as the egg matures. Upon fertilization, the diploid egg will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant. Although several eggs may be formed and even fertilized, there is usually a single surviving embryo in each ovule. Fertilization and seed development is a long process in pine trees: it may take up to two years after pollination. The seed that is formed contains three generations of tissues: the seed coat that originates from the sporophyte tissue, the gametophyte tissue that will provide nutrients, and the embryo itself.<\/p>\r\nFigure\u00a01 illustrates the life cycle of a conifer. The sporophyte (2<em data-effect=\"italics\">n<\/em>) phase is the longest phase in the life of a gymnosperm. The gametophytes (1<em data-effect=\"italics\">n<\/em>)\u2014produced by microspores and megaspores\u2014are reduced in size. It may take more than a year between pollination and fertilization while the pollen tube grows towards the growing female gametophyte (1<em data-effect=\"italics\">n<\/em>), which develops from a single megaspore. The slow growth of the pollen tube allows the female gametophyte time to produce eggs (1<em data-effect=\"italics\">n<\/em>).\r\n\r\n[caption id=\"attachment_2208\" align=\"aligncenter\" width=\"605\"]<img class=\" wp-image-2208\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/01162753\/Figure_26_02_01.png\" alt=\"The conifer life cycle begins with a mature tree, which is called a sporophyte and is diploid (2n). The tree produces male cones in the lower branches, and female cones in the upper branches. The male cones produce pollen grains that contain two generative (sperm) nuclei and a tube nucleus. When the pollen lands on a female scale, a pollen tube grows toward the female gametophyte, which consists of an ovule containing the megaspore. Upon fertilization, a diploid zygote forms. The resulting seeds are dispersed, and grow into a mature tree, ending the cycle.\" width=\"605\" height=\"622\" \/> Figure\u00a01.\u00a0<span class=\"os-caption\">Conifer life cycle. This image shows the life cycle of a conifer. Pollen from male cones blows up into upper branches, where it fertilizes female cones. The megaspore shown in the image develops into the female gametophyte as the pollen tube slowly grows toward it, eventually fusing with the egg and delivering a male nucleus, which combines with the female nucleus of the mature egg.<\/span>[\/caption]\r\n\r\n<header>\r\n<div class=\"textbox exercises\">\r\n<h3>Practice Question<\/h3>\r\nAt what stage does the diploid zygote form?\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>when the female cone begins to bud from the tree<\/li>\r\n \t<li>at fertilization<\/li>\r\n \t<li>when the seeds drop from the tree<\/li>\r\n \t<li>when the pollen tube begins to grow<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"494399\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"494399\"]The diploid zygote forms after the pollen tube has finished forming, so that the male generative nuclei can fuse with the female gametophyte.[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox\">\r\n\r\nWatch this video to see the process of seed production in gymnosperms.\r\n\r\n<script type=\"text\/javascript\" src=\"\/\/static.3playmedia.com\/p\/projects\/20361\/files\/1593726\/plugins\/11116.js\"><\/script><script src=\"https:\/\/www.youtube.com\/iframe_api\" type=\"text\/javascript\"><\/script>\r\n<iframe id=\"myytplayer2\" src=\"https:\/\/www.youtube.com\/embed\/D9byVQxvMXs?enablejsapi=1\" width=\"440\" height=\"300\" frameborder=\"0\"><\/iframe>\r\n\r\n<\/div>\r\n<\/header>\r\n<h2>Diversity of Gymnosperms<\/h2>\r\n<p id=\"fs-idm71140752\">Modern gymnosperms are classified into four phyla. Coniferophyta, Cycadophyta, and Ginkgophyta are similar in their pattern of seed development and also in their production of\u00a0<em data-effect=\"italics\">secondary cambium<\/em>\u00a0(cells that generate the vascular system of the trunk or stem and are partially specialized for water transportation). However, the three phyla are not closely related phylogenetically to each other. Gnetophyta are considered the closest group to angiosperms because they produce true xylem tissue, with vessels as well as the tracheids found in the rest of the gymnosperms. It is possible that vessel elements arose independently in the two groups<\/p>\r\n\r\n<section id=\"fs-idm31842544\" data-depth=\"2\"><\/section>\r\n<h3>Conifers (Coniferophyta)<\/h3>\r\n<p id=\"fs-idm5823056\"><span id=\"term990\" data-type=\"term\">Conifers<\/span>\u00a0are the dominant phylum of gymnosperms, with the greatest variety of species (Figure 2). Typical conifers are tall trees that bear scale-like or needle-like leaves. Water evaporation from leaves is reduced by their narrow shape and a thick cuticle. Snow easily slides off needle-shaped leaves, keeping the snow load light, thus reducing broken branches. Such adaptations to cold and dry weather explain the predominance of conifers at high altitudes and in cold climates. Conifers include familiar evergreen trees such as pines, spruces, firs, cedars, sequoias, and yews. A few species are deciduous and lose their leaves in fall. The bald cypress, dawn redwood, European larch and the tamarack (Figure 2c) are examples of deciduous conifers. Many coniferous trees are harvested for paper pulp and timber. The wood of conifers is more primitive than the wood of angiosperms; it contains\u00a0<em data-effect=\"italics\">tracheids<\/em>, but no vessel elements, and is therefore referred to as \u201csoft wood.\u201d<\/p>\r\n\r\n<div id=\"fig-ch26_02_02\" class=\"os-figure\"><\/div>\r\n\r\n[caption id=\"attachment_5474\" align=\"alignnone\" width=\"2269\"]<img class=\"size-full wp-image-5474\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2015\/04\/05162847\/Figure_26_02_021_new.jpg\" alt=\" Photo A shows a juniper tree with a gnarled trunk. Photo B shows a sequoia with a tall, broad trunk and branches starting high up the trunk. Photo C shows a forest of tamarack with yellow needles. Photo D shows a tall spruce tree covered in pine cones. Photo B. Photo C Part D\" width=\"2269\" height=\"556\" \/> Figure\u00a02.\u00a0Conifers are the dominant form of vegetation in cold or arid environments and at high altitudes. Shown here are the (a) evergreen spruce <em>Picea sp<\/em>., (b) juniper <em>Juniperus sp<\/em>., (c) sequoia <em>Sequoia Semervirens<\/em>, which is a deciduous gymnosperm, and (d) the tamarack <em>Larix larcinia<\/em>. Notice the yellow leaves of the tamarack. (credit a: modification of work by Rosendahl; credit b: modification of work by Alan Levine; credit c: modification of work by Wendy McCormic; credit d: modification of work by NOAA)[\/caption]\r\n<h3>Cycads<\/h3>\r\n[caption id=\"attachment_2210\" align=\"alignright\" width=\"351\"]<img class=\" wp-image-2210\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/01163114\/Figure_26_02_03.jpg\" alt=\"Photo shows a cycad with leaves resembling those of a fern, with thin leaves branching from a thick stem. Two very large cones sit in the middle of the leaves, close to the ground.\" width=\"351\" height=\"263\" \/> Figure\u00a03.\u00a0This <em>Encephalartos ferox<\/em> cycad has large cones and broad, fern-like leaves. (credit: Wendy Cutler)[\/caption]\r\n<p id=\"fs-idp135537280\"><span id=\"term991\" data-type=\"term\">Cycads<\/span>\u00a0thrive in mild climates, and are often mistaken for palms because of the shape of their large, compound leaves. Cycads bear large strobili or cones (Figure 3), and may be pollinated by beetles rather than wind, which is unusual for a gymnosperm. Large cycads dominated the landscape during the age of dinosaurs in the Mesozoic, but only a hundred or so smaller species persisted to modern times. They face possible extinction, and several species are protected through international conventions. Because of their attractive shape, they are often used as ornamental plants in gardens in the tropics and subtropics.<\/p>\r\n\r\n<h3>Gingkophytes<\/h3>\r\n<p id=\"fs-idp82268768\">The single surviving species of the\u00a0<span id=\"term992\" data-type=\"term\">ginkgophytes<\/span>\u00a0group is\u00a0<em data-effect=\"italics\">Ginkgo biloba<\/em>\u00a0(Figure 4). Its fan-shaped leaves\u2014unique among seed plants because they feature a dichotomous venation pattern\u2014turn yellow in autumn and fall from the tree. For centuries,\u00a0<em data-effect=\"italics\">G. biloba<\/em>\u00a0was cultivated by Chinese Buddhist monks in monasteries, which ensured its preservation. It is planted in public spaces because it is unusually resistant to pollution. Male and female organs are produced on separate plants. Typically, gardeners plant only male trees because the seeds produced by the female plant have an off-putting smell of rancid butter.<\/p>\r\n\r\n\r\n[caption id=\"attachment_2212\" align=\"aligncenter\" width=\"544\"]<img class=\"size-full wp-image-2212\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/01163516\/Figure_26_02_04.jpg\" alt=\"Illustration shows the green, fan-shaped leaves of Ginkgo biloba.\" width=\"544\" height=\"727\" \/> Figure\u00a04.\u00a0This plate from the 1870 book Flora Japonica, Sectio Prima (Tafelband) depicts the leaves and fruit of <em>Gingko biloba<\/em>, as drawn by Philipp Franz von Siebold and Joseph Gerhard Zuccarini.[\/caption]\r\n<h3>Gnetophytes<\/h3>\r\n<p id=\"fs-idm72638668\">The phylogenetic position of the\u00a0<span id=\"term993\" data-type=\"term\">gnetophytes<\/span>\u00a0is not currently resolved. Their possession of vessel elements suggests they are the closest relative to modern angiosperms. However, molecular analysis places them closer to the conifers. The three living genera are quite dissimilar:\u00a0<em data-effect=\"italics\">Ephedra<\/em>,\u00a0<em data-effect=\"italics\">Gnetum<\/em>, and\u00a0<em data-effect=\"italics\">Welwitschia<\/em>\u00a0(Figure 5), which may indicate that the group is not monophyletic. Like angiosperms, they have broad leaves.\u00a0<em data-effect=\"italics\">Ephedra<\/em>\u00a0occurs in dry areas of the West Coast of the United States and Mexico.\u00a0<em data-effect=\"italics\">Ephedra<\/em>\u2019s small, scale-like leaves are the source of the compound\u00a0<em data-effect=\"italics\">ephedrine<\/em>, which is used in medicine as a potent decongestant. Because ephedrine is similar to amphetamines, both in chemical structure and neurological effects, its use is restricted to prescription drugs.\u00a0<em data-effect=\"italics\">Gnetum<\/em>\u00a0species\u00a0are found in some parts of Africa, South America, and Southeast Asia, and include trees, shrubs and vines.\u00a0<em data-effect=\"italics\">Welwitschia<\/em>\u00a0is found in the Namib desert, and is possibly the oddest member of the group. It produces only two leaves, which grow continuously throughout the life of the plant (some plants are hundreds of years old). Like the ginkgos,\u00a0<em data-effect=\"italics\">Welwitschia<\/em>\u00a0produces male and female gametes on separate plants.<\/p>\r\n\r\n\r\n[caption id=\"attachment_2213\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-2213\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/01163616\/Figure_26_02_05-1024x272.jpg\" alt=\" Photo A shows Mormon tea, a short, scrubby plant with yellow branches radiating out from a central bundle. Photo B shows a plant with large, teardrop-shaped green leaves. Photo C shows a plant with long, flat leaves radiating along the ground from a central part with pink buds.\" width=\"1024\" height=\"272\" \/> Figure\u00a05.\u00a0(a) <em>Ephedra viridis<\/em>, known by the common name Mormon tea, grows on the West Coast of the United States and Mexico. (b)<em>Gnetum gnemon<\/em> grows in Malaysia. (c) The large <em>Welwitschia mirabilis<\/em> can be found in the Namibian desert. (credit a: modification of work by USDA; credit b: modification of work by Malcolm Manners; credit c: modification of work by Derek Keats)[\/caption]\r\n\r\n<div class=\"textbox\">\r\n\r\nWatch this video describing the amazing strangeness of Welwitschia.\r\n\r\n<script type=\"text\/javascript\" src=\"\/\/static.3playmedia.com\/p\/projects\/20361\/files\/1593729\/plugins\/11085.js\"><\/script><script src=\"https:\/\/www.youtube.com\/iframe_api\" type=\"text\/javascript\"><\/script>\r\n<iframe id=\"myytplayer\" src=\"https:\/\/www.youtube.com\/embed\/PVLACJsoGjk?enablejsapi=1\" width=\"440\" height=\"300\" frameborder=\"0\"><\/iframe>\r\n\r\n<\/div>\r\n<div class=\"textbox learning-objectives\">\r\n<h3>In Summary:\u00a0Gymnosperms<\/h3>\r\nGymnosperms are heterosporous seed plants that produce naked seeds. They appeared in the Paleozoic period and were the dominant plant life during the Mesozoic. Modern-day gymnosperms belong to four phyla. The largest phylum, Coniferophyta, is represented by conifers, the predominant plants at high altitude and latitude. Cycads (phylum Cycadophyta) resemble palm trees and grow in tropical climates. <em>Gingko biloba<\/em> is the only representative of the phylum Gingkophyta. The last phylum, Gnetophyta, is a diverse group of shrubs that produce vessel elements in their wood.\r\n\r\n<\/div>\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/607f5e94-67b1-4561-8b46-a1e6ca89652a\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Identify the main characteristics of gymnosperms<\/li>\n<\/ul>\n<\/div>\n<p id=\"fs-idp68426896\"><strong><span id=\"term981\" data-type=\"term\">Gymnosperms<\/span><\/strong>, meaning \u201cnaked seeds,\u201d are a diverse group of seed plants. According to the &#8220;anthophyte&#8221; hypothesis, the angiosperms are a sister group of one group of gymnosperms (the Gnetales), which makes the gymnosperms a paraphyletic group. Paraphyletic groups are those in which not all descendants of a single common ancestor are included in the group. However , the &#8220;netifer&#8221; hypothesis suggests that the gnetophytes are sister to the conifers, making the gymnosperms monophyletic and sister to the angiosperms. Further molecular and anatomical studies may clarify these relationships. Characteristics of the gymnosperms include naked seeds, separate female and male gamtophytes, pollen cones and ovulate cones, pollination by wind and insects, and tracheids (which transport water and solutes in the vascular system).<\/p>\n<p id=\"fs-idp52182176\">Gymnosperm seeds are not enclosed in an ovary; rather, they are only partially sheltered by modified leaves called\u00a0<strong><span id=\"term982\" data-type=\"term\">sporophylls<\/span><\/strong>. You may recall the term\u00a0<strong><span id=\"term983\" data-type=\"term\">strobilus<\/span><\/strong>\u00a0(plural = strobili) describes a tight arrangement of sporophylls around a central stalk, as seen in pine cones. Some seeds are enveloped by sporophyte tissues upon maturation. The layer of sporophyte tissue that surrounds the megasporangium, and later, the embryo, is called the\u00a0<strong><span id=\"term984\" data-type=\"term\">integument<\/span><\/strong>.<\/p>\n<p id=\"fs-idm63613648\">Gymnosperms were the dominant phylum in the Mesozoic era. They are adapted to live where fresh water is scarce during part of the year, or in the nitrogen-poor soil of a bog. Therefore, they are still the prominent phylum in the coniferous biome or\u00a0<em data-effect=\"italics\">taiga<\/em>, where the evergreen conifers have a selective advantage in cold and dry weather. Evergreen conifers continue low levels of photosynthesis during the cold months, and are ready to take advantage of the first sunny days of spring. One disadvantage is that conifers are more susceptible than deciduous trees to leaf infestations because most conifers do not lose their leaves all at once. They cannot, therefore, shed parasites and restart with a fresh supply of leaves in spring.<\/p>\n<p id=\"fs-idm200847248\">The life cycle of a gymnosperm involves alternation of generations, with a dominant sporophyte in which reduced male and female gametophytes reside. All gymnosperms are heterosporous. The male and female reproductive organs can form in cones or strobili. Male and female sporangia are produced either on the same plant, described as\u00a0<strong><span id=\"term985\" data-type=\"term\">monoecious<\/span><\/strong>\u00a0(\u201cone home\u201d or bisexual), or on separate plants, referred to as\u00a0<strong><span id=\"term986\" data-type=\"term\">dioecious<\/span><\/strong>\u00a0(\u201ctwo homes\u201d or unisexual) plants. The life cycle of a conifer will serve as our example of reproduction in gymnosperms.<\/p>\n<p><span style=\"color: #077fab; font-size: 1.15em; font-weight: 600;\">Life Cycle of a Conifer<\/span><\/p>\n<p id=\"fs-idp58000784\">Pine trees are conifers (coniferous = cone bearing) and carry both male and female sporophylls on the same mature sporophyte. Therefore, they are monoecious plants. Like all gymnosperms, pines are heterosporous and generate two different types of spores (male microspores and female megaspores). Male and female spores develop in different strobili, with small male cones and larger female cones. In the male cones, or staminate cones, the\u00a0<strong><span id=\"term987\" data-type=\"term\">microsporocytes<\/span><\/strong>\u00a0undergo meiosis and the resultant haploid microspores give rise to male gametophytes or \u201cpollen grains\u201d by mitosis. Each pollen grain consists of just a few haploid cells enclosed in a tough wall reinforced with sporopollenin. In the spring, large amounts of yellow pollen are released and carried by the wind. Some gametophytes will land on a female cone. Pollination is defined as the initiation of pollen tube growth. The pollen tube develops slowly, and the generative cell in the pollen grain produces two haploid sperm or generative nuclei by mitosis. At fertilization, one of the haploid sperm nuclei will unite with the haploid nucleus of an egg cell.<\/p>\n<p id=\"fs-idp140324640\">Female cones, or\u00a0<strong><span id=\"term988\" data-type=\"term\">ovulate cones<\/span><\/strong>, contain two ovules per scale. Each ovule has a narrow passage that opens near the base of the sporophyll. This passage is the micropyle, through which a pollen tube will later grow. One megaspore mother cell, or\u00a0<strong><span id=\"term989\" data-type=\"term\">megasporocyte<\/span><\/strong>, undergoes meiosis in each ovule. Three of the four cells break down; only a single surviving cell will develop into a female multicellular gametophyte, which encloses archegonia (an archegonium is a reproductive organ that contains a single large egg). As the female gametophyte begins to develop, a sticky pollination drop traps windblown pollen grains near the opening of the micropyle. A pollen tube is formed and grows toward the developing gametophyte. One of the generative or sperm nuclei from the pollen tube will enter the egg and fuse with the egg nucleus as the egg matures. Upon fertilization, the diploid egg will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant. Although several eggs may be formed and even fertilized, there is usually a single surviving embryo in each ovule. Fertilization and seed development is a long process in pine trees: it may take up to two years after pollination. The seed that is formed contains three generations of tissues: the seed coat that originates from the sporophyte tissue, the gametophyte tissue that will provide nutrients, and the embryo itself.<\/p>\n<p>Figure\u00a01 illustrates the life cycle of a conifer. The sporophyte (2<em data-effect=\"italics\">n<\/em>) phase is the longest phase in the life of a gymnosperm. The gametophytes (1<em data-effect=\"italics\">n<\/em>)\u2014produced by microspores and megaspores\u2014are reduced in size. It may take more than a year between pollination and fertilization while the pollen tube grows towards the growing female gametophyte (1<em data-effect=\"italics\">n<\/em>), which develops from a single megaspore. The slow growth of the pollen tube allows the female gametophyte time to produce eggs (1<em data-effect=\"italics\">n<\/em>).<\/p>\n<div id=\"attachment_2208\" style=\"width: 615px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2208\" class=\"wp-image-2208\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/01162753\/Figure_26_02_01.png\" alt=\"The conifer life cycle begins with a mature tree, which is called a sporophyte and is diploid (2n). The tree produces male cones in the lower branches, and female cones in the upper branches. The male cones produce pollen grains that contain two generative (sperm) nuclei and a tube nucleus. When the pollen lands on a female scale, a pollen tube grows toward the female gametophyte, which consists of an ovule containing the megaspore. Upon fertilization, a diploid zygote forms. The resulting seeds are dispersed, and grow into a mature tree, ending the cycle.\" width=\"605\" height=\"622\" \/><\/p>\n<p id=\"caption-attachment-2208\" class=\"wp-caption-text\">Figure\u00a01.\u00a0<span class=\"os-caption\">Conifer life cycle. This image shows the life cycle of a conifer. Pollen from male cones blows up into upper branches, where it fertilizes female cones. The megaspore shown in the image develops into the female gametophyte as the pollen tube slowly grows toward it, eventually fusing with the egg and delivering a male nucleus, which combines with the female nucleus of the mature egg.<\/span><\/p>\n<\/div>\n<header>\n<div class=\"textbox exercises\">\n<h3>Practice Question<\/h3>\n<p>At what stage does the diploid zygote form?<\/p>\n<ol style=\"list-style-type: lower-alpha;\">\n<li>when the female cone begins to bud from the tree<\/li>\n<li>at fertilization<\/li>\n<li>when the seeds drop from the tree<\/li>\n<li>when the pollen tube begins to grow<\/li>\n<\/ol>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q494399\">Show Answer<\/span><\/p>\n<div id=\"q494399\" class=\"hidden-answer\" style=\"display: none\">The diploid zygote forms after the pollen tube has finished forming, so that the male generative nuclei can fuse with the female gametophyte.<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox\">\n<p>Watch this video to see the process of seed production in gymnosperms.<\/p>\n<p><script type=\"text\/javascript\" src=\"\/\/static.3playmedia.com\/p\/projects\/20361\/files\/1593726\/plugins\/11116.js\"><\/script><script src=\"https:\/\/www.youtube.com\/iframe_api\" type=\"text\/javascript\"><\/script><br \/>\n<iframe loading=\"lazy\" id=\"myytplayer2\" src=\"https:\/\/www.youtube.com\/embed\/D9byVQxvMXs?enablejsapi=1\" width=\"440\" height=\"300\" frameborder=\"0\"><\/iframe><\/p>\n<\/div>\n<\/header>\n<h2>Diversity of Gymnosperms<\/h2>\n<p id=\"fs-idm71140752\">Modern gymnosperms are classified into four phyla. Coniferophyta, Cycadophyta, and Ginkgophyta are similar in their pattern of seed development and also in their production of\u00a0<em data-effect=\"italics\">secondary cambium<\/em>\u00a0(cells that generate the vascular system of the trunk or stem and are partially specialized for water transportation). However, the three phyla are not closely related phylogenetically to each other. Gnetophyta are considered the closest group to angiosperms because they produce true xylem tissue, with vessels as well as the tracheids found in the rest of the gymnosperms. It is possible that vessel elements arose independently in the two groups<\/p>\n<section id=\"fs-idm31842544\" data-depth=\"2\"><\/section>\n<h3>Conifers (Coniferophyta)<\/h3>\n<p id=\"fs-idm5823056\"><span id=\"term990\" data-type=\"term\">Conifers<\/span>\u00a0are the dominant phylum of gymnosperms, with the greatest variety of species (Figure 2). Typical conifers are tall trees that bear scale-like or needle-like leaves. Water evaporation from leaves is reduced by their narrow shape and a thick cuticle. Snow easily slides off needle-shaped leaves, keeping the snow load light, thus reducing broken branches. Such adaptations to cold and dry weather explain the predominance of conifers at high altitudes and in cold climates. Conifers include familiar evergreen trees such as pines, spruces, firs, cedars, sequoias, and yews. A few species are deciduous and lose their leaves in fall. The bald cypress, dawn redwood, European larch and the tamarack (Figure 2c) are examples of deciduous conifers. Many coniferous trees are harvested for paper pulp and timber. The wood of conifers is more primitive than the wood of angiosperms; it contains\u00a0<em data-effect=\"italics\">tracheids<\/em>, but no vessel elements, and is therefore referred to as \u201csoft wood.\u201d<\/p>\n<div id=\"fig-ch26_02_02\" class=\"os-figure\"><\/div>\n<div id=\"attachment_5474\" style=\"width: 2279px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-5474\" class=\"size-full wp-image-5474\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2015\/04\/05162847\/Figure_26_02_021_new.jpg\" alt=\"Photo A shows a juniper tree with a gnarled trunk. Photo B shows a sequoia with a tall, broad trunk and branches starting high up the trunk. Photo C shows a forest of tamarack with yellow needles. Photo D shows a tall spruce tree covered in pine cones. Photo B. Photo C Part D\" width=\"2269\" height=\"556\" \/><\/p>\n<p id=\"caption-attachment-5474\" class=\"wp-caption-text\">Figure\u00a02.\u00a0Conifers are the dominant form of vegetation in cold or arid environments and at high altitudes. Shown here are the (a) evergreen spruce <em>Picea sp<\/em>., (b) juniper <em>Juniperus sp<\/em>., (c) sequoia <em>Sequoia Semervirens<\/em>, which is a deciduous gymnosperm, and (d) the tamarack <em>Larix larcinia<\/em>. Notice the yellow leaves of the tamarack. (credit a: modification of work by Rosendahl; credit b: modification of work by Alan Levine; credit c: modification of work by Wendy McCormic; credit d: modification of work by NOAA)<\/p>\n<\/div>\n<h3>Cycads<\/h3>\n<div id=\"attachment_2210\" style=\"width: 361px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2210\" class=\"wp-image-2210\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/01163114\/Figure_26_02_03.jpg\" alt=\"Photo shows a cycad with leaves resembling those of a fern, with thin leaves branching from a thick stem. Two very large cones sit in the middle of the leaves, close to the ground.\" width=\"351\" height=\"263\" \/><\/p>\n<p id=\"caption-attachment-2210\" class=\"wp-caption-text\">Figure\u00a03.\u00a0This <em>Encephalartos ferox<\/em> cycad has large cones and broad, fern-like leaves. (credit: Wendy Cutler)<\/p>\n<\/div>\n<p id=\"fs-idp135537280\"><span id=\"term991\" data-type=\"term\">Cycads<\/span>\u00a0thrive in mild climates, and are often mistaken for palms because of the shape of their large, compound leaves. Cycads bear large strobili or cones (Figure 3), and may be pollinated by beetles rather than wind, which is unusual for a gymnosperm. Large cycads dominated the landscape during the age of dinosaurs in the Mesozoic, but only a hundred or so smaller species persisted to modern times. They face possible extinction, and several species are protected through international conventions. Because of their attractive shape, they are often used as ornamental plants in gardens in the tropics and subtropics.<\/p>\n<h3>Gingkophytes<\/h3>\n<p id=\"fs-idp82268768\">The single surviving species of the\u00a0<span id=\"term992\" data-type=\"term\">ginkgophytes<\/span>\u00a0group is\u00a0<em data-effect=\"italics\">Ginkgo biloba<\/em>\u00a0(Figure 4). Its fan-shaped leaves\u2014unique among seed plants because they feature a dichotomous venation pattern\u2014turn yellow in autumn and fall from the tree. For centuries,\u00a0<em data-effect=\"italics\">G. biloba<\/em>\u00a0was cultivated by Chinese Buddhist monks in monasteries, which ensured its preservation. It is planted in public spaces because it is unusually resistant to pollution. Male and female organs are produced on separate plants. Typically, gardeners plant only male trees because the seeds produced by the female plant have an off-putting smell of rancid butter.<\/p>\n<div id=\"attachment_2212\" style=\"width: 554px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2212\" class=\"size-full wp-image-2212\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/01163516\/Figure_26_02_04.jpg\" alt=\"Illustration shows the green, fan-shaped leaves of Ginkgo biloba.\" width=\"544\" height=\"727\" \/><\/p>\n<p id=\"caption-attachment-2212\" class=\"wp-caption-text\">Figure\u00a04.\u00a0This plate from the 1870 book Flora Japonica, Sectio Prima (Tafelband) depicts the leaves and fruit of <em>Gingko biloba<\/em>, as drawn by Philipp Franz von Siebold and Joseph Gerhard Zuccarini.<\/p>\n<\/div>\n<h3>Gnetophytes<\/h3>\n<p id=\"fs-idm72638668\">The phylogenetic position of the\u00a0<span id=\"term993\" data-type=\"term\">gnetophytes<\/span>\u00a0is not currently resolved. Their possession of vessel elements suggests they are the closest relative to modern angiosperms. However, molecular analysis places them closer to the conifers. The three living genera are quite dissimilar:\u00a0<em data-effect=\"italics\">Ephedra<\/em>,\u00a0<em data-effect=\"italics\">Gnetum<\/em>, and\u00a0<em data-effect=\"italics\">Welwitschia<\/em>\u00a0(Figure 5), which may indicate that the group is not monophyletic. Like angiosperms, they have broad leaves.\u00a0<em data-effect=\"italics\">Ephedra<\/em>\u00a0occurs in dry areas of the West Coast of the United States and Mexico.\u00a0<em data-effect=\"italics\">Ephedra<\/em>\u2019s small, scale-like leaves are the source of the compound\u00a0<em data-effect=\"italics\">ephedrine<\/em>, which is used in medicine as a potent decongestant. Because ephedrine is similar to amphetamines, both in chemical structure and neurological effects, its use is restricted to prescription drugs.\u00a0<em data-effect=\"italics\">Gnetum<\/em>\u00a0species\u00a0are found in some parts of Africa, South America, and Southeast Asia, and include trees, shrubs and vines.\u00a0<em data-effect=\"italics\">Welwitschia<\/em>\u00a0is found in the Namib desert, and is possibly the oddest member of the group. It produces only two leaves, which grow continuously throughout the life of the plant (some plants are hundreds of years old). Like the ginkgos,\u00a0<em data-effect=\"italics\">Welwitschia<\/em>\u00a0produces male and female gametes on separate plants.<\/p>\n<div id=\"attachment_2213\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2213\" class=\"size-large wp-image-2213\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/01163616\/Figure_26_02_05-1024x272.jpg\" alt=\"Photo A shows Mormon tea, a short, scrubby plant with yellow branches radiating out from a central bundle. Photo B shows a plant with large, teardrop-shaped green leaves. Photo C shows a plant with long, flat leaves radiating along the ground from a central part with pink buds.\" width=\"1024\" height=\"272\" \/><\/p>\n<p id=\"caption-attachment-2213\" class=\"wp-caption-text\">Figure\u00a05.\u00a0(a) <em>Ephedra viridis<\/em>, known by the common name Mormon tea, grows on the West Coast of the United States and Mexico. (b)<em>Gnetum gnemon<\/em> grows in Malaysia. (c) The large <em>Welwitschia mirabilis<\/em> can be found in the Namibian desert. (credit a: modification of work by USDA; credit b: modification of work by Malcolm Manners; credit c: modification of work by Derek Keats)<\/p>\n<\/div>\n<div class=\"textbox\">\n<p>Watch this video describing the amazing strangeness of Welwitschia.<\/p>\n<p><script type=\"text\/javascript\" src=\"\/\/static.3playmedia.com\/p\/projects\/20361\/files\/1593729\/plugins\/11085.js\"><\/script><script src=\"https:\/\/www.youtube.com\/iframe_api\" type=\"text\/javascript\"><\/script><br \/>\n<iframe loading=\"lazy\" id=\"myytplayer\" src=\"https:\/\/www.youtube.com\/embed\/PVLACJsoGjk?enablejsapi=1\" width=\"440\" height=\"300\" frameborder=\"0\"><\/iframe><\/p>\n<\/div>\n<div class=\"textbox learning-objectives\">\n<h3>In Summary:\u00a0Gymnosperms<\/h3>\n<p>Gymnosperms are heterosporous seed plants that produce naked seeds. They appeared in the Paleozoic period and were the dominant plant life during the Mesozoic. Modern-day gymnosperms belong to four phyla. The largest phylum, Coniferophyta, is represented by conifers, the predominant plants at high altitude and latitude. Cycads (phylum Cycadophyta) resemble palm trees and grow in tropical climates. <em>Gingko biloba<\/em> is the only representative of the phylum Gingkophyta. The last phylum, Gnetophyta, is a diverse group of shrubs that produce vessel elements in their wood.<\/p>\n<\/div>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_607f5e94-67b1-4561-8b46-a1e6ca89652a\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/607f5e94-67b1-4561-8b46-a1e6ca89652a?iframe_resize_id=assessment_practice_id_607f5e94-67b1-4561-8b46-a1e6ca89652a\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-222\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology 2e. <strong>Provided by<\/strong>: OpenStax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":78,"menu_order":9,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology 2e\",\"author\":\"\",\"organization\":\"OpenStax\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction\"}]","CANDELA_OUTCOMES_GUID":"27b39656-0646-4c95-b5c6-4ac20738d801, 257f5c7a-6654-48c2-a460-e1713191b3e4","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-222","chapter","type-chapter","status-publish","hentry"],"part":2222,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/222","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/users\/78"}],"version-history":[{"count":26,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/222\/revisions"}],"predecessor-version":[{"id":8360,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/222\/revisions\/8360"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/parts\/2222"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/222\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/media?parent=222"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapter-type?post=222"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/contributor?post=222"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/license?post=222"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}