3.4.2: Internal Leaf Structure

Learning Objectives

• Describe the microscope internal structure of leaves, including the epidermis, mesophyll, and vascular bundles.
• Compare the adaptations of mesophytic, hydrophytic, and xerophytic leaves.
• Identify the unique features of pine and corn leaves.
• Compare the structures of sun and shade leaves.

Tissue Organization in Leaves

All three tissue types are represented in leaves. The epidermis represents the dermal tissue, the mesophyll that fills the leaf is ground tissue, and the vascular bundles that form the leaf veins represent vascular tissue (Figure \(\PageIndex{1}\)). These three tissues will be discussed using a eudicot leaf that is adapted to a moderate amount of water (mesophytic leaf). Variations in leaf structure are discussed later on this page.

Figure \(\PageIndex{1}\): A cross section through a eudicot leaf. The upper epidermis is a single layer of parenchyma cells. There are no stomata present in the upper epidermis of this leaf. Below the epidermis, cells (appearing pink due to staining of the nuclei and chloroplasts) are arranged in columns, forming the palisade mesophyll. Beneath the palisade mesophyll is the spongy mesophyll. The cells are approximately the same size as the palisade mesophyll, but there are large intercellular spaces between them. The lower epidermis is another single layer of parenchyma cells, but several stomata (flanked by guard cells) are visible in this epidermal layer. A large vascular bundle is in the center of the leaf. The xylem (stained pink) is on the top and the phloem is on the bottom. Image by Maria Morrow (CC-BY-NC).

Epidermis

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 (singular = stoma; Figure \(\PageIndex{2}\)), openings through which the exchange of gases takes place. Two guard cells surround each stoma, regulating its opening and closing, and the guard cells are sometimes flanked by subsidiary cells. Guard cells are the only epidermal cells to contain chloroplasts. In most cases, the lower epidermis contains more stomata than the upper epidermis because the bottom of the leaf is cooler and less prone to water loss.

Figure \(\PageIndex{2}\): Visualized at 500x with a scanning electron microscope, several stomata are clearly visible on (a) the surface of this sumac (Rhus glabra) leaf. The small, oval-like stomata are scattered on the bumpy surface of the leaf. At 5,000x magnification, the thick lip-like guard cells of (b) a single stoma from lyre-leaved sand cress (Arabidopsis lyrata) have the appearance of lips that surround the opening. In this (c) light micrograph cross-section of an A. lyrata leaf, the guard cell pair is visible along with the large, substomatal air space in the leaf. The other cells in the leaf (mesophyll cells) are egg-shaped. (credit: modification of work by Robert R. Wise; part c scale-bar data from Matt Russell)

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 protective layer called the cuticle covers surface of the epidermal cells (Figure \(\PageIndex{3}\)). The cuticle is rich in lignin (which lends some rigidity) and waxes (which function in waterproofing). 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 water loss by blocking air flow across the leaf surface (Figure \(\PageIndex{4}\)). For this reason, trichomes (like stomata) are frequently denser on the lower side of the leaf.

Figure \(\PageIndex{3}\): A cross section through the upper epidermis and the upper portion of the mesophyll (palisade mesophyll). On top of the upper epidermis of this leaf, a transparent layer of cuticle is visible, sealing the top of the leaf. Image by Berkshire Community College Bioscience Image Library (public domain).

Figure \(\PageIndex{4}\): Trichomes give leaves a fuzzy appearance as in this (a) sundew (Drosera sp.). Scanning electron micrographs of (b) branched, tree-like trichomes on the leaf of Arabidopsis lyrata and (c) multibranched trichomes, which resemble sea anenomes, on a mature Quercus marilandica leaf. (credit a: John Freeland; credit b, c: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

Mesophyll

Below the epidermis are layers of cells known as the mesophyll, or “middle leaf.” Mesophyll cells contain many chloroplasts and specialize in photosynthesis. The mesophyll of most leaves typically contains two arrangements of parenchyma cells: the palisade parenchyma and spongy parenchyma (Figure \(\PageIndex{5}\)). The palisade parenchyma (also called the palisade mesophyll) has column-shaped and may be present in one, two, or three layers. The palisade cells specialize in capturing incoming sunlight (including slanted sun rays), rotating chloroplasts to the top of the leaf and then allowing them to regenerate by cycling them toward the leaf's center. They also decrease the intensity of sunlight for the spongy mesophyll. Although palisade cells may appear tightly packed in a cross section because there are many rows of cells behind those in the foreground, there is actually ample space (intercellular air spaces) between them. Below the palisade parenchyma are seemingly loosely arranged cells of an irregular shape. These are the cells of the spongy parenchyma (or spongy mesophyll). The intercellular air spaces found between mesophyll cells facilitate gaseous exchange.

Figure \(\PageIndex{5}\): The meosphyll often consists of palisade and spongy parenchyma. 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 stoma 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 contain tiny bumps, which are the chloroplasts. Spongy cells smaller and irregularly shaped. (credit b: modification of work by Robert R. Wise)

Vascular Bundles (Veins)

Like the stem, the leaf contains vascular bundles composed of xylem and phloem (Figure \(\PageIndex{6-7}\)). When a typical stem vascular bundle (which has xylem internal to the phloem) enters the leaf, xylem usually faces upwards, whereas phloem faces downwards. The conducting cells of the xylem (tracheids and vessel elements) transport water and minerals to the leaves. The sieve-tube elements of the phloem transports the photosynthetic products from the leaf to the other parts of the plant. The phloem is typically supported by a cluster of fibers (sclerenchyma) that increase structural support for the veins. A single vascular bundle, no matter how large or small, always contains both xylem and phloem tissues.

Figure \(\PageIndex{6}\): This scanning electron micrograph shows xylem (larger cells on top) and phloem (smaller cells on bottom) 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)

Figure \(\PageIndex{7}\): A cross section through the midvein of a eudicot leaf. The xylem tissue within the large vascular bundle is arranged in an arcing semi-circle, with phloem tissue in an arc traveling just below it. The thick cell walls of the large xylem vessels stain reddish, while the cells of the phloem are smaller with thin cell walls and stain blue. There are layers of collenchyma cells under the epidermis both above and below the midvein. The vascular bundle just to the left of the midrib is coming more or less straight at us, so it is easy to distinguish the tissues. In contrast, the vascular bundle to the right of the midrib was moving diagonally and so was caught in an oblique section and looks more like a smear. Image by Berkshire Community College Bioscience Image Library (public domain).

Leaf Adaptations

The broad, flat shape of most leaves increases surface area relative to volume, which helps it capture sunlight; however this also provides more opportunity for water loss. The anatomy of a leaf has everything to do with achieving the balance between photosynthesis and water loss in the environment in which the plant grows. Plants that grow in moist areas can grow large, flat leaves to absorb sunlight like solar panels because sunlight is likely more limiting than water. Plants in dry areas must prevent water loss and adapt a variety of leaf shapes and orientations to accomplish the duel tasks of water retention and sunlight absorption. In general, leaves adapted to dry environments are small and thick with a much lower surface area-to-volume ratio.

In regards to water, there are three main types of plants: mesophytes, hydrophytes, and xerophytes. Mesophytes are typical plants which adapt to moderate amounts of water ("meso" means middle, and "phyte" means plant). Many familiar plants are mesophytes, such as lilac, Ranunculus (buttercup), roses, etc. Hydrophytes grow in water ("hydro" refers to water). Their leaf blades are frequently highly dissected (deeply lobed) to access gases dissolved in water, and their petioles and stems have air canals to supply underwater organs with gases. Hygrophytes (not discussed further) live in constantly wet environment, their leaves adapted to rapidly release water through the stomata. They sometimes even excrete of water drops through the leaf margins (guttation). Xerophytes are adapted to the scarce water ("xero" refers to dryness). Xerophytes are found in deserts and Mediterranean climates (such as in much of California), where summers are hot and dry. The leaves of mesophytes are called mesophytic, hydrophyte leaves are called hydrophytic, and so on. The structure of mesophytic leaves was already described (Figure \(\PageIndex{1}\)). Adaptaions in hydrophytic and xerophytic leaves and discussed below in more detail.

Hydrophytic Leaves

The structure of a hydrophytic leaf differs from a mesophytic leaf due to selective pressures in the environment -- water is plentiful, so the plant is more concerned with staying afloat and preventing herbivory. Hydrophytic leaves have a thin epidermal layer and the absence of stomata in the lower epidermis (Figure \(\PageIndex{8}\)). In the spongy mesophyll, there are large pockets where air can be trapped, helping the leaf float. This type of parenchyma tissue, specialized for trapping gases, is called aerenchyma. Sharp, branched sclereids (astrosclereids) traverse the mesophyll of a hydrophytic leaf. These provide the leaf structural support, as well as prevention of herbivory. Vascular tissue is somewhat reduced in hydrophytic leaves.

Figure \(\PageIndex{8}\): Cross sections through a hydrophytic leaf from Nymphaea (water lily) at two magnifications. The upper epidermis is a thin layer of parenchyma with many stomata. Below each stoma, there is a chamber of air located within the palisade mesophyll (this makes them easier to find). Under the palisade mesophyll is a much larger region of spongy mesophyll than in the mesophytic leaf. Most of the space is taken up by large air pockets, making this tissue aerenchyma. The lower epidermis has no stomata. Within the mesophyll, there are spiky, pink-stained astrosclereids that have been sectioned at different angles during slide preparation. These stain pink due to the lignin in their thick secondary cell walls. Images by Maria Morrow (CC-BY-NC).

Xerophytic Leaves

Xerophytic leaves (Figure \(\PageIndex{9}\)) have thick cuticles to limit water loss, especially on the upper epidermis (Figure \(\PageIndex{10}\)). Both the upper and lower epidermis consists of several layers (multiple epidermis). Sometimes the additional layers are called the hypodermis ("hypo" meaning under; "dermis" meaning skin). Depressions in the lower epidermis creates a pockets that are lined with trichomes, and the stomata are located at the base of these pockets (called stomatal crypts; figure \(\PageIndex{10}\)). The trichomes help capture evaporating moisture and maintain a relatively humid environment around the stomata. These stomatal crypts are located only on the underside of the leaves, where they experience less sun exposure and therefore less water loss. The upper epidermis is free from stomata.

Figure \(\PageIndex{9}\): Tissue organization through the xerophytic leaf of oleander (Nerium). On the right side of the image, the layers of tissue are labeled from the upper surface of the leaf to the lower. The upper epidermis of the leaf is sealed by a thick, waxy cuticle. There are no stomata present in the upper epidermis. Just below the epidermis are several layers of tightly packed cells called the hypodermis. Beneath the hypodermis, the palisade and spongy mesophylls are arranged as in a mesophytic leaf. There are more layers of hypodermis between the spongy mesophyll and the lower epidermis. There are invaginations in the lower epidermis called stomatal crypts. Stomata are located within these, surrounded by trichomes. Image by Maria Morrow (CC-BY-NC).

Figure \(\PageIndex{10}\): The margin of the xerophytic leaf of oleander (Nerium). Left: There is a thick cuticle on the upper epidermis (it looks like a transparent skin). The cuticle gets thinner as it transitions to the lower epidermis. Right: A stomatal crypt on the lower epidermis is filled with trichomes. Several closed stomata are visible; they are bordered by dark guard cells. Images from Berkshire Community College Bioscience Image Library (public domain).

Pine Leaves

Pines evolved during a period in Earth’s history when conditions were becoming increasingly dry, and pine needles have many adaptations to deal with these conditions. Many of these adaptations are similar the xerophytic leaves of some angiosperms (described above) because pines themselves are xerophytes.

The epidermis of the leaf seems to be more than one cell layer thick (figure \(\PageIndex{11}\)). These subsequent layers of epidermis-like tissue under the single, outer layer of true epidermis are called the hypodermis , which offers a thicker barrier and helps prevent water loss. The epidermis itself is coated on the outside by a thick layer of wax called the cuticle. Because waxes are hydrophobic, this also helps prevent water loss through the epidermis. The stomata are typically sunken, occurring within the hypodermis instead of the epidermis. Sunken stomata create a pocket of air that is protected from the airflow across the leaf and can aid in maintaining a higher moisture content (figure \(\PageIndex{12}\)).

Figure \(\PageIndex{11}\): Cross section of a pine leaf (needle). Much like the Nerium leaf, this leaf is coated in a thick cuticle and there is a hypodermis below the epidermis (because this leaf is so round, there is not really a distinct 'upper' and 'lower'). There are no stomatal crypts, but the stomata are sunken, located in the hypodermis. The leaf has a low surface area to volume ratio (more volume, less surface area), which decreases water loss. In the center of the leaf, There is a large region surrounded by an suberized endodermis (much look in a root). There are two vascular bundles within this region, surrounded by transfusion tissue. Image and caption (modifed) by Maria Morrow (CC-BY-NC).

Figure \(\PageIndex{12}\): A closer view of a sunken stoma and the outermost layers of the pine needle. The thick cuticle is visible as a transparent layer coating the small epidermal cells. Each of the epidermal cells has a thick cell wall. The hypodermis is composed of 3-4 layers of small, tightly packed cells that also have thick walls (sclerenchyma). The guard cells of the stoma are located about three layers below the epidermis, and the cuticle can be seen extending down over them. The stoma is open in this image. Below the stoma, there is a gap of air space, then highly invaginated mesophyll cells. Image by Maria Morrow (CC-BY-NC).

Within the mesophyll, there are several canals that appear as large, open circles in the cross section of the leaf. These are resin canals. The cells lining them secrete resin (the sticky stuff that coniferous trees exude, often called pitch), which contains compounds that are toxic to insects and bacteria. When pines evolved, not only was the Earth becoming drier, but insects were evolving and proliferating. These resin canals are not features that help the plant survive dry conditions, but they do help prevent herbivory. In addition to prevention of herbivory, resin can aid in closing wounds and preventing infection at wound sites.

There are two bundles of vascular tissue embedded within a region of cells called transfusion tissue, which functions in transporting materials to and from the mesophyll cells. The transfusion tissue and vascular bundles are surrounded by a distinct layer of cells called the endodermis. This is similar to the tissue of the same name in the root, but the cells are not impregnated with the water-repelling compound suberin.

Finally, the overall shape of the leaf allows for as little water loss as possible by decreasing the relative surface area, taking a rounder shape as opposed to a flatter one. This low surface area-to-volume ratio is characteristic of xerophytes.

Corn Leaves

The model organism for monocots in botany is usually corn (Zea mays). In corn, there are approximately the same number of stomata on both the upper and lower epidermis. The mesophyll is not divided into two distinct types. The vascular bundles all face the same directly (appearing circular in cross section) because they run parallel to each other.

Corn is not necessarily a xerophyte, but it is adapted to deal with high temperatures. One of these adaptations, C₄ type photosynthesis is discussed in Photorespiration and Photosynthetic Pathways and results in a cell arrangement called Kranz anatomy. The vascular bundles are surrounded by obviously inflated parenchyma cells that form a structure called a bundle sheath, and these are packed with chloroplasts (Figure \(\PageIndex{13}\)). (Bundle sheaths surround vascular bundles of other types of leaves as well, but the bundle sheath cells are much smaller). Mesophyll cells encircle the bundle sheath cells. In C₄ photosynthesis, carbon dioxide is first gathered by the mesophyll cells and temporarily stored as a four-carbon sugar. This four-carbon sugar is transferred to the bundle sheath cells, where it is broken down to release carbon dioxide. It is in the bundle sheath cells where a process called the Calvin cycle, and glucose is ultimately produced. C₄ photosynthesis concentrates carbon dioxide inside the bundle sheath cells, reducing the need to frequently open stomata for gas exchange. This helps conserve water.

Figure \(\PageIndex{13}\): A vascular bundle of a corn (Zea mays) leaf. There are two vascular bundles in this image. The one on the left is difficult to distinguish and most of what you see are the enlarged bundle sheath cells. The larger vascular bundle on the right has less prominent bundle sheath cells, though they still form a distinct border between the vascular tissue and the mesophyll. The xylem tissue is located closer to the upper epidermis. You can locate it by searching for the large, open cells (vessel elements) with red-stained secondary walls. Below the xylem is the phloem tissue, which encompasses a smaller area. The larger cells in the phloem are sieve-tube elements, and the smaller ones are companion cells. Images from Berkshire Community College Bioscience Image Library (public domain).

When moisture is plentiful, the corn leaves are fully expanded and able to maximize photosynthesis. When moisture is limited, the leaves roll inward, limiting both moisture loss and photosynthetic capacity. This is accomplished by the presence of bulliform cells in the upper epidermis (Figure \(\PageIndex{14}\)). These clusters of enlarged cells are swollen with water when there is abundant water available. As the water content in the plant decreases, these cells shrivel, causing the upper epidermis to curl or fold inward at these points. This adaptation to sun exposure can be found in many other grasses, as well (corn is a member of the Poaceae, the grass family).

Figure \(\PageIndex{14}\): Cross section of Zea mays (corn). The bulliform cells of are the group of tall cells along the upper epidermis, just to the left of the opening (stoma).

Sun and Shade Leaves

The light intensity experienced by a developing leaf influences its structure. Leaves that develop when consistently exposed to direct sunlight (sun leaves) thus differ from leaves exposed to low light intensities (shade leaves) in several ways (Figure \(\PageIndex{15}\)). Relative to shade leaves, sun leaves are smaller and thicker. This reduces surface area relative to volume, conserving water, which would otherwise be easily lost under bright sunlight and resultantly warmer temperatures. In contrast, the broad, thin shape of shade leaves helps capture sufficient light when light intensity is low. The thicker cuticle of sun leaves also limits water loss. They have more palisade parenchyma and more vascular tissue. Sun leaves can maintain a high photosynthetic rate at high light intensities, but shade leaves cannot.

Figure \(\PageIndex{15}\): Cross sections of a sun leaf (left) and shade leaf (right). The palisade parenchyma of the sun leaf consists of several layers, but there is only a single layer in the shade leaf. The chloroplasts (red dots) are also packed more densely in both the palisade and spongy mesophyll cells of the sun leaf compared to the shade leaf. The sun leaf is overall thicker. Images by Melissa Ha (CC-BY).

​Attributions

Curated and authored by Melissa Ha using the following sources:

• 30.2 Stems and 30.4 Leaves from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
• 5.3 The Leaf from Introduction to Botany by Alexey Shipunov (public domain).
• 9.2 Introduction to Leaf Anatomy and 9.3 Leaf Anatomy from Botany Lab Manual by Maria Morrow (licensed CC-BY)
• 13 Leaves from A Photographic Atlas for Botany by Maria Morrow (licensed CC-BY-NC).