{"id":1952,"date":"2017-01-31T18:35:27","date_gmt":"2017-01-31T18:35:27","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-biology2\/?post_type=chapter&#038;p=1952"},"modified":"2024-04-25T18:57:30","modified_gmt":"2024-04-25T18:57:30","slug":"leaves","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-biology2\/chapter\/leaves\/","title":{"raw":"Leaves","rendered":"Leaves"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Identify the structure and function of a typical leaf<\/li>\r\n<\/ul>\r\n<\/div>\r\nLeaves are the main sites for photosynthesis: the process by which plants synthesize food. Most leaves are usually green, due to the presence of chlorophyll in the leaf cells. However, some leaves may have different colors, caused by other plant pigments that mask the green chlorophyll.\r\n\r\nThe thickness, shape, and size of leaves are adapted to the environment. Each variation helps a plant species maximize its chances of survival in a particular habitat. Usually, the leaves of plants growing in tropical rainforests have larger surface areas than those of plants growing in deserts or very cold conditions, which are likely to have a smaller surface area to minimize water loss.\r\n<h2>Structure of a Typical Leaf<\/h2>\r\n[caption id=\"attachment_1979\" align=\"alignright\" width=\"300\"]<img class=\" wp-image-1979\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31181848\/Figure_30_04_01.jpg\" alt=\" Illustration shows the parts of a leaf. The petiole is the stem of the leaf. The midrib is a vessel that extends from the petiole to the leaf tip. Veins branch from the midrib. The lamina is the wide, flat part of the leaf. The margin is the edge of the leaf.\" width=\"300\" height=\"306\" \/> Figure\u00a01. Deceptively simple in appearance, a leaf is a highly efficient structure.[\/caption]\r\n\r\nEach leaf typically has a leaf blade called the <strong>lamina<\/strong>, which is also the widest part of the leaf. Some leaves are attached to the plant stem by a <strong>petiole<\/strong>. Leaves that do not have a petiole and are directly attached to the plant stem are called <strong>sessile<\/strong> leaves. Small green appendages usually found at the base of the petiole are known as <strong>stipules<\/strong>. Most leaves have a midrib, which travels the length of the leaf and branches to each side to produce veins of vascular tissue. The edge of the leaf is called the margin. Figure\u00a01 shows the structure of a typical eudicot leaf.\r\n\r\nWithin each leaf, the vascular tissue forms veins. The arrangement of veins in a leaf is called the <strong>venation<\/strong> pattern. Monocots and dicots differ in their patterns of venation (Figure\u00a02). Monocots have parallel venation; the veins run in straight lines across the length of the leaf without converging at a point. In dicots, however, the veins of the leaf have a net-like appearance, forming a pattern known as reticulate venation. One extant plant, the <em>Ginkgo biloba<\/em>, has dichotomous venation where the veins fork.\r\n\r\n[caption id=\"attachment_1980\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-1980\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31181944\/Figure_30_04_02abc-1024x644.jpg\" alt=\"Part A photo shows the broad, sword-shaped leaves of a tulip. Parallel veins run up the leaves. Part B photo shows a teardrop-shaped linden leaf that has veins radiating out from the midrib. Smaller veins radiate out from these. Right photo shows a fan-shaped ginkgo leaf, which has veins radiating out from the petiole.\" width=\"1024\" height=\"644\" \/> Figure\u00a02. (a) Tulip (<em>Tulipa<\/em>), a monocot, has leaves with parallel venation. The netlike venation in this (b) linden (<em>Tilia cordata<\/em>) leaf distinguishes it as a dicot. The (c) <em>Ginkgo biloba<\/em> tree has dichotomous venation. (credit a photo: modification of work by \u201cDrewboy64\u201d\/Wikimedia Commons; credit b photo: modification of work by Roger Griffith; credit c photo: modification of work by \"geishaboy500\"\/Flickr; credit abc illustrations: modification of work by Agnieszka Kwiecie\u0144)[\/caption]\r\n<h2>Leaf Arrangement<\/h2>\r\nThe arrangement of leaves on a stem is known as <strong>phyllotaxy<\/strong>. The number and placement of a plant\u2019s leaves will vary depending on the species, with each species exhibiting a characteristic leaf arrangement. Leaves are classified as either alternate, spiral, or opposite. Plants that have only one leaf per node have leaves that are said to be either alternate\u2014meaning the leaves alternate on each side of the stem in a flat plane\u2014or spiral, meaning the leaves are arrayed in a spiral along the stem. In an opposite leaf arrangement, two leaves arise at the same point, with the leaves connecting opposite each other along the branch. If there are three or more leaves connected at a node, the leaf arrangement is classified as <strong>whorled<\/strong>.\r\n<h2>Leaf Form<\/h2>\r\nLeaves may be simple or compound (Figure\u00a03). In a <strong>simple leaf<\/strong>, the blade is either completely undivided\u2014as in the banana leaf\u2014or it has lobes, but the separation does not reach the midrib, as in the maple leaf. In a <strong>compound leaf<\/strong>, the leaf blade is completely divided, forming leaflets, as in the locust tree. Each leaflet may have its own stalk, but is attached to the rachis. A <strong>palmately compound leaf<\/strong> resembles the palm of a hand, with leaflets radiating outwards from one point Examples include the leaves of poison ivy, the buckeye tree, or the familiar houseplant <em>Schefflera<\/em> sp. (common name \u201cumbrella plant\u201d). <strong>Pinnately compound leaves<\/strong> take their name from their feather-like appearance; the leaflets are arranged along the midrib, as in rose leaves (<em>Rosa<\/em> sp.), or the leaves of hickory, pecan, ash, or walnut trees.\r\n\r\n[caption id=\"attachment_1981\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-1981\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31182211\/Figure_30_04_03a-1024x251.jpg\" alt=\"Photo (a) shows the large-leaves of a potted banana plant growing from a single stem; (b) shows a horse chestnut plant, which has five leaves radiating from the petiole as fingers radiate from the palm of a hand; (c) shows a scrub hickory plant with feather-shaped leaves opposing each other along the stem, and a single leaf at the end of the stem. (d) shows a honey locust with five pairs of stem-like veins connected to the midrib. Tiny leaflets grow from the veins.\" width=\"1024\" height=\"251\" \/> Figure\u00a03. Leaves may be simple or compound. In simple leaves, the lamina is continuous. The (a) banana plant (<em>Musa <\/em>sp.) has simple leaves. In compound leaves, the lamina is separated into leaflets. Compound leaves may be palmate or pinnate. In (b) palmately compound leaves, such as those of the horse chestnut (<em>Aesculus hippocastanum<\/em>), the leaflets branch from the petiole. In (c) pinnately compound leaves, the leaflets branch from the midrib, as on a scrub hickory (<em>Carya floridana<\/em>). The (d) honey locust has double compound leaves, in which leaflets branch from the veins. (credit a: modification of work by \"BazzaDaRambler\"\/Flickr; credit b: modification of work by Roberto Verzo; credit c: modification of work by Eric Dion; credit d: modification of work by Valerie Lykes)[\/caption]\r\n<h2>Leaf Structure and Function<\/h2>\r\nThe outermost layer of the leaf is the epidermis; it is present on both sides of the leaf and is called the upper and lower epidermis, respectively. Botanists call the upper side the adaxial surface (or adaxis) and the lower side the abaxial surface (or abaxis). The epidermis helps in the regulation of gas exchange. It contains stomata (Figure\u00a04): openings through which the exchange of gases takes place. Two guard cells surround each stoma, regulating its opening and closing.\r\n\r\n[caption id=\"attachment_1982\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-1982\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31182304\/Figure_30_04_04abc-1024x265.jpg\" alt=\"Photo (a) shows small oval-like stomata scattered on the bumpy surface of a leaf that is magnified 500 times; (b) is a close-up of a stoma showing the thick lip-like guard cells either side of an opening. Photo (a) and (b) are scanning electron micrographs. Photo (c) is a light micrograph of a leaf cross section that shows a large air space underneath two guard cells. The air space is surrounded by large oval and egg-shaped cells.\" width=\"1024\" height=\"265\" \/> Figure\u00a04. Visualized at 500x with a scanning electron microscope, several stomata are clearly visible on (a) the surface of this sumac (<em>Rhus glabra<\/em>) leaf. At 5,000x magnification, the guard cells of (b) a single stoma from lyre-leaved sand cress (<em>Arabidopsis lyrata)<\/em> have the appearance of lips that surround the opening. In this (c) light micrograph cross-section of an <em>A. lyrata<\/em> leaf, the guard cell pair is visible along with the large, sub-stomatal air space in the leaf. (credit: modification of work by Robert R. Wise; part c scale-bar data from Matt Russell)[\/caption]\r\n\r\nThe epidermis is usually one cell layer thick; however, in plants that grow in very hot or very cold conditions, the epidermis may be several layers thick to protect against excessive water loss from transpiration. A waxy layer known as the <strong>cuticle<\/strong> covers the leaves of all plant species. The cuticle reduces the rate of water loss from the leaf surface. Other leaves may have small hairs (trichomes) on the leaf surface. Trichomes help to deter herbivory by restricting insect movements, or by storing toxic or bad-tasting compounds; they can also reduce the rate of transpiration by blocking air flow across the leaf surface (Figure\u00a05).\r\n\r\n[caption id=\"attachment_1983\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-1983\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31182402\/Figure_30_04_05-1024x297.jpg\" alt=\"Photo (a) shows a plant with many fuzzy white hairs growing from its surface. Scanning electron micrograph (b) shows branched tree-like hairs emerging from the surface of a leaf. The trunk of each hair is about 250 microns tall. Branches are somewhat shorter. Scanning electron micrograph (c) shows many multi-pronged hairs about 100 microns long that look like sea anemones scattered across a leaf surface.\" width=\"1024\" height=\"297\" \/> Figure\u00a05. Trichomes give leaves a fuzzy appearance as in this (a) sundew (<em>Drosera<\/em> sp.). Leaf trichomes include (b) branched trichomes on the leaf of <em>Arabidopsis lyrata<\/em> and (c) multibranched trichomes on a mature <em>Quercus marilandica<\/em> leaf. (credit a: John Freeland; credit b, c: modification of work by Robert R. Wise; scale-bar data from Matt Russell)[\/caption]\r\n\r\nBelow the epidermis of dicot leaves are layers of cells known as the mesophyll, or \u201cmiddle leaf.\u201d The mesophyll of most leaves typically contains two arrangements of parenchyma cells: the palisade parenchyma and spongy parenchyma (Figure\u00a06). The palisade parenchyma (also called the palisade mesophyll) has column-shaped, tightly packed cells, and may be present in one, two, or three layers. Below the palisade parenchyma are loosely arranged cells of an irregular shape. These are the cells of the spongy parenchyma (or spongy mesophyll). The air space found between the spongy parenchyma cells allows gaseous exchange between the leaf and the outside atmosphere through the stomata. In aquatic plants, the intercellular spaces in the spongy parenchyma help the leaf float. Both layers of the mesophyll contain many chloroplasts. Guard cells are the only epidermal cells to contain chloroplasts.\r\n\r\nIn the leaf drawing (Figure\u00a06a), the central mesophyll is sandwiched between an upper and lower epidermis. The mesophyll has two layers: an upper palisade layer comprised of tightly packed, columnar cells, and a lower spongy layer, comprised of loosely packed, irregularly shaped cells. Stomata on the leaf underside allow gas exchange. A waxy cuticle covers all aerial surfaces of land plants to minimize water loss. These leaf layers are clearly visible in the scanning electron micrograph (Figure\u00a06b). The numerous small bumps in the palisade parenchyma cells are chloroplasts. Chloroplasts are also present in the spongy parenchyma, but are not as obvious. The bumps protruding from the lower surface of the leave are glandular trichomes, which differ in structure from the stalked trichomes in Figure\u00a05.\r\n\r\n[caption id=\"attachment_3759\" align=\"aligncenter\" width=\"800\"]<img class=\" wp-image-3759\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/03\/15184533\/Figure_30_04_06ab.jpg\" alt=\" Part A is a leaf cross section illustration. A flat layer of rectangular cells make up the upper and lower epidermis. A cuticle layer protects the outside of both epidermal layers. A stomatal pore in the lower epidermis allows carbon dioxide to enter and oxygen to leave. Oval guard cells surround the pore. Sandwiched between the upper and lower epidermis is the mesophyll. The upper part of the mesophyll is comprised of columnar cells called palisade parenchyma. The lower part of the mesophyll is made up of loosely packed spongy parenchyma. Part B is a scanning electron micrograph of a leaf in which all the layers described above are visible. Palisade cells are about 50 microns tall and 10 microns wide and are covered with tiny bumps, which are the chloroplasts. Spongy cells smaller and irregularly shaped. Several large bumps about 20 microns across project from the lower surface of the leaf.\" width=\"800\" height=\"251\" \/> Figure\u00a06. (a) Leaf drawing (b) Scanning electron micrograph of a leaf. (credit b: modification of work by Robert R. Wise)[\/caption]\r\n\r\nLike the stem, the leaf contains vascular bundles composed of xylem and phloem (Figure\u00a07). The xylem consists of tracheids and vessels, which transport water and minerals to the leaves. The phloem transports the photosynthetic products from the leaf to the other parts of the plant. A single vascular bundle, no matter how large or small, always contains both xylem and phloem tissues.\r\n\r\n[caption id=\"attachment_1985\" align=\"aligncenter\" width=\"544\"]<img class=\"size-full wp-image-1985\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31182756\/Figure_30_04_07.jpg\" alt=\" The scanning electron micrograph shows an oval vascular bundle. Small phloem cells make up the bottom of the bundle, and larger xylem cells make up the top. The bundle is surrounded by a ring of larger cells.\" width=\"544\" height=\"404\" \/> Figure\u00a07. This scanning electron micrograph shows xylem and phloem in the leaf vascular bundle from the lyre-leaved sand cress (Arabidopsis lyrata). (credit: modification of work by Robert R. Wise; scale-bar data from Matt Russell)[\/caption]\r\n<h2>Leaf Adaptations<\/h2>\r\nConiferous plant species that thrive in cold environments, like spruce, fir, and pine, have leaves that are reduced in size and needle-like in appearance. These needle-like leaves have sunken stomata and a smaller surface area: two attributes that aid in reducing water loss. In hot climates, plants such as cacti have leaves that are reduced to spines, which in combination with their succulent stems, help to conserve water. Many aquatic plants have leaves with wide lamina that can float on the surface of the water, and a thick waxy cuticle on the leaf surface that repels water.\r\n<div class=\"textbox\">\r\n\r\nWatch \u201cThe Pale Pitcher Plant\u201d episode of the video series <em>Plants Are Cool, Too,<\/em> a Botanical Society of America video about a carnivorous plant species found in Louisiana.\r\n\r\n<script type=\"text\/javascript\" src=\"\/\/static.3playmedia.com\/p\/projects\/20361\/files\/1593742\/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\/uak3m_q-HDo?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:\u00a0Leaves<\/h3>\r\nLeaves are the main site of photosynthesis. A typical leaf consists of a lamina (the broad part of the leaf, also called the blade) and a petiole (the stalk that attaches the leaf to a stem). The arrangement of leaves on a stem, known as phyllotaxy, enables maximum exposure to sunlight. Each plant species has a characteristic leaf arrangement and form. The pattern of leaf arrangement may be alternate, opposite, or spiral, while leaf form may be simple or compound. Leaf tissue consists of the epidermis, which forms the outermost cell layer, and mesophyll and vascular tissue, which make up the inner portion of the leaf. In some plant species, leaf form is modified to form structures such as tendrils, spines, bud scales, and needles.\r\n\r\n<\/div>\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/edc1844b-3ec4-48d6-affb-72a976adb7f9\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Identify the structure and function of a typical leaf<\/li>\n<\/ul>\n<\/div>\n<p>Leaves are the main sites for photosynthesis: the process by which plants synthesize food. Most leaves are usually green, due to the presence of chlorophyll in the leaf cells. However, some leaves may have different colors, caused by other plant pigments that mask the green chlorophyll.<\/p>\n<p>The thickness, shape, and size of leaves are adapted to the environment. Each variation helps a plant species maximize its chances of survival in a particular habitat. Usually, the leaves of plants growing in tropical rainforests have larger surface areas than those of plants growing in deserts or very cold conditions, which are likely to have a smaller surface area to minimize water loss.<\/p>\n<h2>Structure of a Typical Leaf<\/h2>\n<div id=\"attachment_1979\" style=\"width: 310px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1979\" class=\"wp-image-1979\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31181848\/Figure_30_04_01.jpg\" alt=\"Illustration shows the parts of a leaf. The petiole is the stem of the leaf. The midrib is a vessel that extends from the petiole to the leaf tip. Veins branch from the midrib. The lamina is the wide, flat part of the leaf. The margin is the edge of the leaf.\" width=\"300\" height=\"306\" \/><\/p>\n<p id=\"caption-attachment-1979\" class=\"wp-caption-text\">Figure\u00a01. Deceptively simple in appearance, a leaf is a highly efficient structure.<\/p>\n<\/div>\n<p>Each leaf typically has a leaf blade called the <strong>lamina<\/strong>, which is also the widest part of the leaf. Some leaves are attached to the plant stem by a <strong>petiole<\/strong>. Leaves that do not have a petiole and are directly attached to the plant stem are called <strong>sessile<\/strong> leaves. Small green appendages usually found at the base of the petiole are known as <strong>stipules<\/strong>. Most leaves have a midrib, which travels the length of the leaf and branches to each side to produce veins of vascular tissue. The edge of the leaf is called the margin. Figure\u00a01 shows the structure of a typical eudicot leaf.<\/p>\n<p>Within each leaf, the vascular tissue forms veins. The arrangement of veins in a leaf is called the <strong>venation<\/strong> pattern. Monocots and dicots differ in their patterns of venation (Figure\u00a02). Monocots have parallel venation; the veins run in straight lines across the length of the leaf without converging at a point. In dicots, however, the veins of the leaf have a net-like appearance, forming a pattern known as reticulate venation. One extant plant, the <em>Ginkgo biloba<\/em>, has dichotomous venation where the veins fork.<\/p>\n<div id=\"attachment_1980\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1980\" class=\"size-large wp-image-1980\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31181944\/Figure_30_04_02abc-1024x644.jpg\" alt=\"Part A photo shows the broad, sword-shaped leaves of a tulip. Parallel veins run up the leaves. Part B photo shows a teardrop-shaped linden leaf that has veins radiating out from the midrib. Smaller veins radiate out from these. Right photo shows a fan-shaped ginkgo leaf, which has veins radiating out from the petiole.\" width=\"1024\" height=\"644\" \/><\/p>\n<p id=\"caption-attachment-1980\" class=\"wp-caption-text\">Figure\u00a02. (a) Tulip (<em>Tulipa<\/em>), a monocot, has leaves with parallel venation. The netlike venation in this (b) linden (<em>Tilia cordata<\/em>) leaf distinguishes it as a dicot. The (c) <em>Ginkgo biloba<\/em> tree has dichotomous venation. (credit a photo: modification of work by \u201cDrewboy64\u201d\/Wikimedia Commons; credit b photo: modification of work by Roger Griffith; credit c photo: modification of work by &#8220;geishaboy500&#8243;\/Flickr; credit abc illustrations: modification of work by Agnieszka Kwiecie\u0144)<\/p>\n<\/div>\n<h2>Leaf Arrangement<\/h2>\n<p>The arrangement of leaves on a stem is known as <strong>phyllotaxy<\/strong>. The number and placement of a plant\u2019s leaves will vary depending on the species, with each species exhibiting a characteristic leaf arrangement. Leaves are classified as either alternate, spiral, or opposite. Plants that have only one leaf per node have leaves that are said to be either alternate\u2014meaning the leaves alternate on each side of the stem in a flat plane\u2014or spiral, meaning the leaves are arrayed in a spiral along the stem. In an opposite leaf arrangement, two leaves arise at the same point, with the leaves connecting opposite each other along the branch. If there are three or more leaves connected at a node, the leaf arrangement is classified as <strong>whorled<\/strong>.<\/p>\n<h2>Leaf Form<\/h2>\n<p>Leaves may be simple or compound (Figure\u00a03). In a <strong>simple leaf<\/strong>, the blade is either completely undivided\u2014as in the banana leaf\u2014or it has lobes, but the separation does not reach the midrib, as in the maple leaf. In a <strong>compound leaf<\/strong>, the leaf blade is completely divided, forming leaflets, as in the locust tree. Each leaflet may have its own stalk, but is attached to the rachis. A <strong>palmately compound leaf<\/strong> resembles the palm of a hand, with leaflets radiating outwards from one point Examples include the leaves of poison ivy, the buckeye tree, or the familiar houseplant <em>Schefflera<\/em> sp. (common name \u201cumbrella plant\u201d). <strong>Pinnately compound leaves<\/strong> take their name from their feather-like appearance; the leaflets are arranged along the midrib, as in rose leaves (<em>Rosa<\/em> sp.), or the leaves of hickory, pecan, ash, or walnut trees.<\/p>\n<div id=\"attachment_1981\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1981\" class=\"size-large wp-image-1981\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31182211\/Figure_30_04_03a-1024x251.jpg\" alt=\"Photo (a) shows the large-leaves of a potted banana plant growing from a single stem; (b) shows a horse chestnut plant, which has five leaves radiating from the petiole as fingers radiate from the palm of a hand; (c) shows a scrub hickory plant with feather-shaped leaves opposing each other along the stem, and a single leaf at the end of the stem. (d) shows a honey locust with five pairs of stem-like veins connected to the midrib. Tiny leaflets grow from the veins.\" width=\"1024\" height=\"251\" \/><\/p>\n<p id=\"caption-attachment-1981\" class=\"wp-caption-text\">Figure\u00a03. Leaves may be simple or compound. In simple leaves, the lamina is continuous. The (a) banana plant (<em>Musa <\/em>sp.) has simple leaves. In compound leaves, the lamina is separated into leaflets. Compound leaves may be palmate or pinnate. In (b) palmately compound leaves, such as those of the horse chestnut (<em>Aesculus hippocastanum<\/em>), the leaflets branch from the petiole. In (c) pinnately compound leaves, the leaflets branch from the midrib, as on a scrub hickory (<em>Carya floridana<\/em>). The (d) honey locust has double compound leaves, in which leaflets branch from the veins. (credit a: modification of work by &#8220;BazzaDaRambler&#8221;\/Flickr; credit b: modification of work by Roberto Verzo; credit c: modification of work by Eric Dion; credit d: modification of work by Valerie Lykes)<\/p>\n<\/div>\n<h2>Leaf Structure and Function<\/h2>\n<p>The outermost layer of the leaf is the epidermis; it is present on both sides of the leaf and is called the upper and lower epidermis, respectively. Botanists call the upper side the adaxial surface (or adaxis) and the lower side the abaxial surface (or abaxis). The epidermis helps in the regulation of gas exchange. It contains stomata (Figure\u00a04): openings through which the exchange of gases takes place. Two guard cells surround each stoma, regulating its opening and closing.<\/p>\n<div id=\"attachment_1982\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1982\" class=\"size-large wp-image-1982\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31182304\/Figure_30_04_04abc-1024x265.jpg\" alt=\"Photo (a) shows small oval-like stomata scattered on the bumpy surface of a leaf that is magnified 500 times; (b) is a close-up of a stoma showing the thick lip-like guard cells either side of an opening. Photo (a) and (b) are scanning electron micrographs. Photo (c) is a light micrograph of a leaf cross section that shows a large air space underneath two guard cells. The air space is surrounded by large oval and egg-shaped cells.\" width=\"1024\" height=\"265\" \/><\/p>\n<p id=\"caption-attachment-1982\" class=\"wp-caption-text\">Figure\u00a04. Visualized at 500x with a scanning electron microscope, several stomata are clearly visible on (a) the surface of this sumac (<em>Rhus glabra<\/em>) leaf. At 5,000x magnification, the guard cells of (b) a single stoma from lyre-leaved sand cress (<em>Arabidopsis lyrata)<\/em> have the appearance of lips that surround the opening. In this (c) light micrograph cross-section of an <em>A. lyrata<\/em> leaf, the guard cell pair is visible along with the large, sub-stomatal air space in the leaf. (credit: modification of work by Robert R. Wise; part c scale-bar data from Matt Russell)<\/p>\n<\/div>\n<p>The epidermis is usually one cell layer thick; however, in plants that grow in very hot or very cold conditions, the epidermis may be several layers thick to protect against excessive water loss from transpiration. A waxy layer known as the <strong>cuticle<\/strong> covers the leaves of all plant species. The cuticle reduces the rate of water loss from the leaf surface. Other leaves may have small hairs (trichomes) on the leaf surface. Trichomes help to deter herbivory by restricting insect movements, or by storing toxic or bad-tasting compounds; they can also reduce the rate of transpiration by blocking air flow across the leaf surface (Figure\u00a05).<\/p>\n<div id=\"attachment_1983\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1983\" class=\"size-large wp-image-1983\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31182402\/Figure_30_04_05-1024x297.jpg\" alt=\"Photo (a) shows a plant with many fuzzy white hairs growing from its surface. Scanning electron micrograph (b) shows branched tree-like hairs emerging from the surface of a leaf. The trunk of each hair is about 250 microns tall. Branches are somewhat shorter. Scanning electron micrograph (c) shows many multi-pronged hairs about 100 microns long that look like sea anemones scattered across a leaf surface.\" width=\"1024\" height=\"297\" \/><\/p>\n<p id=\"caption-attachment-1983\" class=\"wp-caption-text\">Figure\u00a05. Trichomes give leaves a fuzzy appearance as in this (a) sundew (<em>Drosera<\/em> sp.). Leaf trichomes include (b) branched trichomes on the leaf of <em>Arabidopsis lyrata<\/em> and (c) multibranched trichomes on a mature <em>Quercus marilandica<\/em> leaf. (credit a: John Freeland; credit b, c: modification of work by Robert R. Wise; scale-bar data from Matt Russell)<\/p>\n<\/div>\n<p>Below the epidermis of dicot leaves are layers of cells known as the mesophyll, or \u201cmiddle leaf.\u201d The mesophyll of most leaves typically contains two arrangements of parenchyma cells: the palisade parenchyma and spongy parenchyma (Figure\u00a06). The palisade parenchyma (also called the palisade mesophyll) has column-shaped, tightly packed cells, and may be present in one, two, or three layers. Below the palisade parenchyma are loosely arranged cells of an irregular shape. These are the cells of the spongy parenchyma (or spongy mesophyll). The air space found between the spongy parenchyma cells allows gaseous exchange between the leaf and the outside atmosphere through the stomata. In aquatic plants, the intercellular spaces in the spongy parenchyma help the leaf float. Both layers of the mesophyll contain many chloroplasts. Guard cells are the only epidermal cells to contain chloroplasts.<\/p>\n<p>In the leaf drawing (Figure\u00a06a), the central mesophyll is sandwiched between an upper and lower epidermis. The mesophyll has two layers: an upper palisade layer comprised of tightly packed, columnar cells, and a lower spongy layer, comprised of loosely packed, irregularly shaped cells. Stomata on the leaf underside allow gas exchange. A waxy cuticle covers all aerial surfaces of land plants to minimize water loss. These leaf layers are clearly visible in the scanning electron micrograph (Figure\u00a06b). The numerous small bumps in the palisade parenchyma cells are chloroplasts. Chloroplasts are also present in the spongy parenchyma, but are not as obvious. The bumps protruding from the lower surface of the leave are glandular trichomes, which differ in structure from the stalked trichomes in Figure\u00a05.<\/p>\n<div id=\"attachment_3759\" style=\"width: 810px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-3759\" class=\"wp-image-3759\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/03\/15184533\/Figure_30_04_06ab.jpg\" alt=\"Part A is a leaf cross section illustration. A flat layer of rectangular cells make up the upper and lower epidermis. A cuticle layer protects the outside of both epidermal layers. A stomatal pore in the lower epidermis allows carbon dioxide to enter and oxygen to leave. Oval guard cells surround the pore. Sandwiched between the upper and lower epidermis is the mesophyll. The upper part of the mesophyll is comprised of columnar cells called palisade parenchyma. The lower part of the mesophyll is made up of loosely packed spongy parenchyma. Part B is a scanning electron micrograph of a leaf in which all the layers described above are visible. Palisade cells are about 50 microns tall and 10 microns wide and are covered with tiny bumps, which are the chloroplasts. Spongy cells smaller and irregularly shaped. Several large bumps about 20 microns across project from the lower surface of the leaf.\" width=\"800\" height=\"251\" \/><\/p>\n<p id=\"caption-attachment-3759\" class=\"wp-caption-text\">Figure\u00a06. (a) Leaf drawing (b) Scanning electron micrograph of a leaf. (credit b: modification of work by Robert R. Wise)<\/p>\n<\/div>\n<p>Like the stem, the leaf contains vascular bundles composed of xylem and phloem (Figure\u00a07). The xylem consists of tracheids and vessels, which transport water and minerals to the leaves. The phloem transports the photosynthetic products from the leaf to the other parts of the plant. A single vascular bundle, no matter how large or small, always contains both xylem and phloem tissues.<\/p>\n<div id=\"attachment_1985\" style=\"width: 554px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1985\" class=\"size-full wp-image-1985\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31182756\/Figure_30_04_07.jpg\" alt=\"The scanning electron micrograph shows an oval vascular bundle. Small phloem cells make up the bottom of the bundle, and larger xylem cells make up the top. The bundle is surrounded by a ring of larger cells.\" width=\"544\" height=\"404\" \/><\/p>\n<p id=\"caption-attachment-1985\" class=\"wp-caption-text\">Figure\u00a07. This scanning electron micrograph shows xylem and phloem in the leaf vascular bundle from the lyre-leaved sand cress (Arabidopsis lyrata). (credit: modification of work by Robert R. Wise; scale-bar data from Matt Russell)<\/p>\n<\/div>\n<h2>Leaf Adaptations<\/h2>\n<p>Coniferous plant species that thrive in cold environments, like spruce, fir, and pine, have leaves that are reduced in size and needle-like in appearance. These needle-like leaves have sunken stomata and a smaller surface area: two attributes that aid in reducing water loss. In hot climates, plants such as cacti have leaves that are reduced to spines, which in combination with their succulent stems, help to conserve water. Many aquatic plants have leaves with wide lamina that can float on the surface of the water, and a thick waxy cuticle on the leaf surface that repels water.<\/p>\n<div class=\"textbox\">\n<p>Watch \u201cThe Pale Pitcher Plant\u201d episode of the video series <em>Plants Are Cool, Too,<\/em> a Botanical Society of America video about a carnivorous plant species found in Louisiana.<\/p>\n<p><script type=\"text\/javascript\" src=\"\/\/static.3playmedia.com\/p\/projects\/20361\/files\/1593742\/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\/uak3m_q-HDo?enablejsapi=1\" width=\"440\" height=\"300\" frameborder=\"0\"><\/iframe><\/p>\n<\/div>\n<div class=\"textbox learning-objectives\">\n<h3>In Summary:\u00a0Leaves<\/h3>\n<p>Leaves are the main site of photosynthesis. A typical leaf consists of a lamina (the broad part of the leaf, also called the blade) and a petiole (the stalk that attaches the leaf to a stem). The arrangement of leaves on a stem, known as phyllotaxy, enables maximum exposure to sunlight. Each plant species has a characteristic leaf arrangement and form. The pattern of leaf arrangement may be alternate, opposite, or spiral, while leaf form may be simple or compound. Leaf tissue consists of the epidermis, which forms the outermost cell layer, and mesophyll and vascular tissue, which make up the inner portion of the leaf. In some plant species, leaf form is modified to form structures such as tendrils, spines, bud scales, and needles.<\/p>\n<\/div>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_edc1844b-3ec4-48d6-affb-72a976adb7f9\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/edc1844b-3ec4-48d6-affb-72a976adb7f9?iframe_resize_id=assessment_practice_id_edc1844b-3ec4-48d6-affb-72a976adb7f9\" 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-1952\">\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><li><strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\">CC BY-SA: Attribution-ShareAlike<\/a><\/em><\/li><\/ul><div class=\"license-attribution-dropdown-subheading\">All rights reserved content<\/div><ul class=\"citation-list\"><li>Plants Are Cool Too! The Pale Pitcher Plant (Episode 1 - Sarracenia alata. <strong>Authored by<\/strong>: BotanicalSociety. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/youtu.be\/uak3m_q-HDo\">https:\/\/youtu.be\/uak3m_q-HDo<\/a>. <strong>License<\/strong>: <em>All Rights Reserved<\/em>. <strong>License Terms<\/strong>: Standard YouTube License<\/li><li>Life - Venus Flytraps: Jaws of Death - BBC One. <strong>Authored by<\/strong>: BBC. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/youtu.be\/O7eQKSf0LmY\">https:\/\/youtu.be\/O7eQKSf0LmY<\/a>. <strong>License<\/strong>: <em>All Rights Reserved<\/em>. <strong>License Terms<\/strong>: Standard YouTube License<\/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":17,"menu_order":6,"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\"},{\"type\":\"cc\",\"description\":\"\",\"author\":\"\",\"organization\":\"\",\"url\":\"\",\"project\":\"\",\"license\":\"cc-by-sa\",\"license_terms\":\"\"},{\"type\":\"copyrighted_video\",\"description\":\"Plants Are Cool Too! The Pale Pitcher Plant (Episode 1 - Sarracenia alata\",\"author\":\"BotanicalSociety\",\"organization\":\"\",\"url\":\"https:\/\/youtu.be\/uak3m_q-HDo\",\"project\":\"\",\"license\":\"arr\",\"license_terms\":\"Standard YouTube License\"},{\"type\":\"copyrighted_video\",\"description\":\"Life - Venus Flytraps: Jaws of Death - BBC One\",\"author\":\"BBC\",\"organization\":\"\",\"url\":\"https:\/\/youtu.be\/O7eQKSf0LmY\",\"project\":\"\",\"license\":\"arr\",\"license_terms\":\"Standard YouTube License\"}]","CANDELA_OUTCOMES_GUID":"07c106e8-b21e-43b0-8786-2fd6c4f02982, efc99d64-af2a-4b3a-b5ea-ac0e40e5886e","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1952","chapter","type-chapter","status-publish","hentry"],"part":145,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1952","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\/17"}],"version-history":[{"count":18,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1952\/revisions"}],"predecessor-version":[{"id":8369,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1952\/revisions\/8369"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/parts\/145"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1952\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/media?parent=1952"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapter-type?post=1952"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/contributor?post=1952"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/license?post=1952"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}