<h2><strong>Permanent Tissues</strong></h2>
<p>The meristematic <a data-lesson="complex-tissue-structure" data-subject="biology" data-tutorial="introducing-animal-diversity" href="https://app.ulearngo.com/biology/introducing-animal-diversity/complex-tissue-structure">tissues</a> give rise to cells that perform a specific function. Once cells develop to perform this particular function, they lose their ability to divide. The process of developing a particular structure suited to a specific function is known as <i>cellular differentiation</i>. We will examine two types of permanent tissue:</p>
<h3>1. Simple Permanent Tissues</h3>
<ul>
<li>Epidermis</li>
<li>Parenchyma</li>
<li>Collenchyma</li>
<li>Sclerenchyma</li>
</ul>
<h3>2. Complex Permanent Tissues</h3>
<div>
<ul>
<li>xylem vessels (made up of tracheids and vessels)</li>
<li>phloem vessels (made up of sieve tubes and companion cells)</li>
</ul>
</div>
<h2><strong>Epidermis Tissue</strong></h2>
<p>The epidermis is a single layer of cells that covers plants' leaves, flowers, <a data-lesson="root-modifications" data-subject="biology" data-tutorial="plant-form-and-physiology" href="https://app.ulearngo.com/biology/plant-form-and-physiology/root-modifications">roots</a> and stems. It is the outermost cell layer of the plant body and plays a protective role in the plant. The function of key structural features are listed in the table below.</p>
<table>
<tbody>
<tr>
<td><em>Structure</em></td>
<td><em>Function</em></td>
</tr>
<tr>
<td>Layer of cells covering surface of entire plant.</td>
<td>Acts as a barrier to <a data-lesson="kingdom-fungi" data-subject="biology" data-tutorial="classification-introduction" href="https://app.ulearngo.com/biology/classification-introduction/kingdom-fungi">fungi</a> and other microorganisms and pathogens.</td>
</tr>
<tr>
<td>Layer is thin and transparent.</td>
<td>Allow for <a data-lesson="light" data-subject="biology" data-tutorial="sensory-systems" href="https://app.ulearngo.com/biology/sensory-systems/light">light</a> to pass through, thereby allowing for <a data-lesson="photosynthesis" data-subject="biology" data-tutorial="cell-biology" href="https://app.ulearngo.com/biology/cell-biology/photosynthesis">photosynthesis</a> in the tissues below.</td>
</tr>
<tr>
<td>Epidermal tissues have abundant <em>trichomes</em> which are tiny hairs projecting from surface of epidermis. Trichomes are abundant in some plant leaves.</td>
<td>Leaf trichomes trap water in the area above the stomata and prevent water loss.</td>
</tr>
<tr>
<td>Root hairs are elongations of epidermal cells in the root.</td>
<td>Root hairs maximise the surface area over which absorption of water from the soil can occur.</td>
</tr>
<tr>
<td>Epidermal tissues in <a data-lesson="leaf-arrangement-and-leaf-form" data-subject="biology" data-tutorial="plant-form-and-physiology" href="https://app.ulearngo.com/biology/plant-form-and-physiology/leaf-arrangement-and-leaf-form">leaves</a> are covered with a waxy <em>cuticle</em>.</td>
<td>The waxy outer layer on the epidermis prevents water loss from leaves.</td>
</tr>
<tr>
<td>Epidermal tissues contain <em>guard cells</em> containing chloroplasts.</td>
<td>Guard cells control the opening and closing of the pores known as <em>stomata</em> thus controlling water loss in plants.</td>
</tr>
<tr>
<td>Some plant epidermal cells can secrete poisonous or bad-tasting substances.</td>
<td>The bitter <a data-lesson="summarizing-taste-and-smell" data-subject="biology" data-tutorial="sensory-systems" href="https://app.ulearngo.com/biology/sensory-systems/summarizing-taste-and-smell">taste</a> of the substances deter browsing and grazing by animals.</td>
</tr>
</tbody>
</table>
<div>
<div class="wp-caption alignnone" style="width: 306px"><img alt="Image" height="307" src="https://data.ulearngo.com/assets/cdb0b29c-b2d4-41d8-8997-4186a838f1a5" width="306"/><p class="wp-caption-text">Scanning electron microscope image of Nicotiana alata (tobacco plant) upper leaf surface, showing trichomes (also known as `hairs') and a few stomata.</p></div>
<div class="ns-tutorial">
<div class="ns-school-defn2">
<h3>Fact:</h3>
<p>The chemicals in trichomes make <a data-lesson="nutritional-adaptations-of-plants" data-subject="biology" data-tutorial="soil-plant-nutrition" href="https://app.ulearngo.com/biology/soil-plant-nutrition/nutritional-adaptations-of-plants">plants</a> less easily digested by hungry animals and can also slow down the <a data-lesson="population-growth-patterns" data-subject="biology" data-tutorial="ecology-overview" href="https://app.ulearngo.com/biology/ecology-overview/population-growth-patterns">growth</a> of fungus on the plant. As such they act as a form of protection for the plant against predation.</p>
</div>
</div>
</div>
<h3>Guard Cells and Stomata</h3>
<p>A <em>stoma</em> is a pore found in the leaf and stem epidermis that allows for gaseous exchange. The stoma is bordered on either side by a pair of specialised cells known as <em>guard cells</em>. Guard cells are bean shaped specialised epidermal cells found mainly on the lower surface of leaves which are responsible for regulating the size of the stoma opening. Together, the stoma and the guard cells are referred to as <em>stomata</em>.</p>
<p>The stomata in the epidermis allow oxygen, <a data-lesson="carbon-cycle" data-subject="biology" data-tutorial="ecology-overview" href="https://app.ulearngo.com/biology/ecology-overview/carbon-cycle">carbon dioxide</a> and water vapour to enter and leave the leaf. The guard cells also contain chloroplasts for photosynthesis. Opening and closing of the guard cells is determined by the turgor pressure of the two guard cells. The turgor pressure is controlled by movements of large quantities of ions and sugar into the guard cells. When guard cells take up these solutes, the <a data-lesson="water-potential" data-subject="biology" data-tutorial="plant-form-and-physiology" href="https://app.ulearngo.com/biology/plant-form-and-physiology/water-potential">water potential</a> decreases causing osmotic water to flow into the guard cells. This leads to an increase in the swelling of the guard cells and the stomatal pores open.</p>
<table>
<tbody>
<tr>
<td colspan="2"><em>Structure</em></td>
</tr>
<tr>
<td>
<div class="wp-caption alignnone" style="width: 306px"><img alt="Image" height="313" src="https://data.ulearngo.com/assets/a8948274-6a2d-4633-9f49-356333130594" width="306"/><p class="wp-caption-text">Stomata in a tomato leaf as seen under a scanning electron microscope.</p></div>
</td>
<td>
<div class="wp-caption alignnone" style="width: 306px"><img alt="Image" height="305" src="https://data.ulearngo.com/assets/722d84ac-1f13-451d-ba88-017f0cc396aa" width="306"/><p class="wp-caption-text">The above is a microscopic image of an Arabidopsis thaliana (commonly known as `Thale cress' or `mouse ear') stoma showing two guard cells exhibiting green fluorescence, with chloroplasts staining red.</p></div>
</td>
</tr>
</tbody>
</table>
<div class="ns-school-emp">
<h2>Investigation: <strong>Practical Investigation of Leaf Epidermis</strong></h2>
<div class="ns-school-defn">
<h3><b>Materials</b></h3>
<div>
<ul>
<li>leaves of Agapanthus, Wandering Jew <i>(Tradescantia</i> ) or similar plants that have epidermis that strips off easily</li>
<li>microscopes</li>
<li>microscope slides and cover slips</li>
<li>dissecting needles</li>
<li>scissors</li>
</ul>
</div>
<h3><b>Instructions</b></h3>
<div>
<ol>
<li>Rip a piece of leaf lengthwise and check for "thinner bits" near the edges, which will be epidermal tissue (ensure that you have LOWER epidermis because this is where the guard cells are found.)</li>
<li>Use the scissors to cut off a small section of epidermis and mount it in water on a microscope slide. Cover with a cover slip.</li>
<li>Focus the slide on low power and search for a section of the sample that does not have air bubbles over the stomata.</li>
<li>Enlarge the part of the specimen you chose and focus on high power.</li>
<li>Adjust lighting if necessary and draw one stoma and its guard cells. Label all parts.</li>
</ol>
</div>
<h3><b>Questions</b></h3>
<div>
<ol>
<li>Describe the shape of the guard cells and normal epidermal cells.</li>
<li>Which epidermal cells have chloroplasts?</li>
<li>Describe the <a data-lesson="cell-wall" data-subject="biology" data-tutorial="introducing-the-cell" href="https://app.ulearngo.com/biology/introducing-the-cell/cell-wall">wall</a> thickness around the guard cells and account for any visible differences.</li>
</ol>
</div>
</div>
</div>

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Epidermis Tissue

Biology studies life and living organisms. Biology can be divided into different sections that cover the morphology, physiology, anatomy, behavior, and so on of living organisms.

Biology

Discover the characteristics and functions of different plant and animal tissues, such as meristematic and nervous tissue, as well as learn about traditional medicines and ethical issues in stem cell research in this tutorial.


Epidermis Tissue

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