Anatomy of a leaf
Summary
The outermost layer of a plant’s leaf is its epidermis, covered by a waxy cuticle. The epidermis has specialised cells called trichomes, which can secrete substances such as aromatic oils and toxins; and stomata, which allow the exchange of gases and water vapour between the plant and its environment. Inside the leaf, the mesophyll consists of two layers: the palisade mesophyll, which is optimised for photosynthesis, and the spongy mesophyll, within which gaseous exchange takes place. The structure of a leaf is supported by its veins, which also contain vascular bundles to transport water, minerals, sugars and other substances. The leaf is attached to the plant’s stem by a petiole, which positions the leaf to capture the optimal amount of available sunlight.
Leaves are one of the main large scale structures of a plant (the others being roots, stems and flowers - see this article). To understand the critical processes that keep plants alive (photosynthesis, respiration, and transpiration), and some of the potential problems plants can face in their environment, we need to understand the anatomy of these structures in a little more detail than we have so far. This is the first of four articles in which we will dig just deep enough into these each of them.
Epidermis
The upper and lower surfaces of a leaf both consist of dermal cells (see this article for more on the types of cells and tissues within plants). These cells contain a waxy substance, cutin (a lipid, see this article for more on the types of substances produced by plants). Together, they form the cuticle, a waxy layer that helps to protect the plant from potentially harsh conditions in its environment (such as strong sunlight, drying winds, or herbivorous insects). In some plants, these cells may exude cutin to form an additional protective layer. These plants, such as Ilex aquifolium (common holly) are easily recognised by the waxy feel of their leaves. Cells in the leaf’s cuticle and epidermis allow light to pass through (at least in part) to the interior of the leaf.
In addition to the cuticle, the epidermis of a leaf contains two other structures, trichomes (often known as leaf hairs, though they are also found on plants’ stems) and stomata (also found on both leaves and stems, but to a much greater extent on leaves). Trichomes may be “secretory” (also known as “glandular” trichomes), adapted to secrete substances into the plant’s environment. These substances include aromatic oils to attract pollinators or deter herbivores (for example, Lavandula angustifolia, English lavender) and toxins to defend the plant (for example, Urtica dioica, stinging nettle). Trichomes may also help deter some herbivores from eating a plant by irritating their palate.
Trichomes may also be adapted by plants to serve other purposes. Some plants (for example, Cistus salviifolius) have trichomes that contain chemicals that can absorb ultraviolet radiation, helping to protect the plant from strong sunlight. Some plants (for example, Stachys byzantina, lamb’s ear) may have densely distributed trichomes, giving their leaves a hairy feel; these reflect sunlight, protecting the cells underneath, and break up the flow of air across the surface of the leaf, reducing water loss through evaporation. These may also protect plants from light frost, with the trichomes frosting rather than the cells beneath. And some carnivorous plants (for example, Drosera capensis) have evolved sticky trichomes to trap small insects.
Stomata consist of an opening (a “pore”) in the epidermis, together with a air of specialised “guard cells”. These guard cells are each shaped like half of a rubber tyre, and are ‘inflated’ or ‘deflated’ by the movement of water in and out of the cells respectively. When water flows out of the guard cells, they ‘deflate’, and the pressure of surrounding cells pushes them together, closing the pore. When water flows into them, they ‘inflate’, allowing them to resist the pressure of surrounding cells, opening the pore (the ‘hole’ in the ‘tyre’). The flow of water in and out of these cells is regulated by a critical nutrient, potassium, though the details of this are beyond the scope of this article.
Stomata allow a plant to control the flow of water vapour and gases in and out of its leaves. The release of water vapour is an essential part of the process of transpiration, creating the water potential difference (see this article for more on movement of substances within plants) that draws moisture up through the plant from the soil. the process of photosynthesis needs carbon dioxide, which must be brought into the plant from the air; and produces oxygen, which must be released by the plant into the air. The process of respiration needs oxygen, which is taken into the plant in the form of water vapour (the O in H₂O), and releases carbon dioxide.
Plants are unable to use the oxygen produced by photosynthesis in respiration because oxygen is needed at different times, in different quantities and in different parts of the plant to where it is produced. Similarly, they are unable to use the carbon dioxide produced through respiration in photosynthesis. As a result, managing the exchange of water vapour, oxygen and carbon dioxide between the plant and is environment (“gaseous exchange”) is a critical activity.
Mesophyll
The interior of a leaf is known as the “mesophyll” (this word is derived from the Latin for middle, “meso”, and the Greek for leaf, “phyllon) and consists of two layers, the “palisade mesophyll” and the “spongy mesophyll”. Both of these layers consist of chlorenchyma cells; parenchyma cells (the least specialised basic cells of a plant) that contain chloroplasts (the plant structures within which photosynthesis takes place).
In the palisade mesophyll layer, these chlorenchyma cells are tightly packed and aligned vertically (like palisade fencing, hence the name), thereby exposing as many cells as possible to sunlight hitting the surface of the leaf, and hence maximising the use of the leaf’s surface for photosynthesis.
The dense packing of the cells in the palisade mesophyll also serves another purpose. Provided sufficient moisture is available within the leaf, the internal water pressure within these cells (their “turgor pressure”) pushes them strongly against each other. The effect of this is to flatten out the leaf’s surface (the leaf is said to be “turgid”), which in turn allows the plant to orient the entire leaf towards the sun, again making full use of available light for photosynthesis. Should this internal water pressure drop, the palisade mesophyll cells are no longer able to maintain the turgidity of the leaf, which then wilts. This can be due to insufficient water being available to the plant, but can also be a defensive mechanism against too strong sunlight; by allowing water to evaporate from the leaf, turgor pressure drops, and the rate of photosynthesis can be slowed. This may seem undesirable; however, photosynthesis produces byproducts that can be harmful to a plant, so controlling its rate is an important balancing act, facilitated in part by the cells of the palisade mesophyll.
Below the palisade mesophyll lies the spongy mesophyll layer. In this layer, the chlorenchyma cells are far more loosely structured, with large air gaps between them. The vast majority of light reaching the leaf is absorbed by the palisade mesophyll, so cells within the spongy mesophyll need not be optimised for photosynthesis. Instead, they are arranged to facilitate gaseous exchange.
The air gaps within the spongy mesophyll layer serve two key purposes. The first of these is to allow rapid diffusion of gases and water vapour through the leaf. Diffusion can take place within liquids; this is however significantly slower than diffusion in air (which is approximately 10,000 times faster than in water). The air gaps in the spongy mesophyll mean that, once the stomata open, gases and water vapour can quickly move in and out of the leaf.
The second fundamental purpose of these air gaps is to allow the evaporation of water within the leaf. Water travels in liquid form through the plant’s xylem, before it reaches the leaves. Evaporation of water within the leaf, followed by rapid diffusion and escape to the environment through the leaves’ stomata, reduces the water pressure within the leaf (creating a water potential gradient), which in turn draws water through the xylem.
Veins
The remaining structure within a plant’s leaves (not shown in the diagram above) are the leaves’ veins. These consist of tightly packed vascular bundles (xylem and phloem cells), wrapped in sheaths of parenchyma and collenchyma cells. Leaf veins serve to transport water and minerals to the leaves (through the xylem), and to transport sugars and other substances to and from the leaves (through the phloem).
The presence of collenchyma cells in the sheaths of leaf veins lends them a degree of rigidity, but without becoming brittle (as would be the case if the sheaths instead consisted of schlerenchyma cells). As a result, leaf veins are able to provide flexible support to the leaf overall, preventing it from collapsing under its own weight (turgor pressure within the palisade mesophyll is unable to support the leaf over large distances, since the forces generated by the leaf’s weight would exceed those generated by turgor pressure, instead only providing relative rigidity between the veins of the leaf).
Petiole
The final leaf structure is the petiole, the ‘stalk’ that attaches the leaf to the stem of the plant (except in sessile plans, where the leaves are attached directly to the stem). The vascular bundles of the leaf veins emanate from the junction of the petiole and the leaf lamina; these continue through the petiole into the stem of the plant.
In addition to providing this vascular connection between the leaves and the rest of the plant, the petiole enables a plant to best position its leaves to take advantage of available light. It does this in three main ways; one evolutionary, one dependent on growing conditions, and one related to the changing availability of light throughout the day.
From an evolutionary perspective, plants have evolved differing petioles to take advantage of the light available in the areas in which they evolved, positioning their leaves further from or closer to the main stem of the plant. Some plants have very long petioles (the record being that of Victoria amazonica, the royal waterlily, which can reach up to 7m in length); others are very short (such as Veronica spicata, whose petioles are typically under 10mm in length). Different plants have also evolved different positioning of leaf petioles on their stems, allowing their leaves to be distributed around the plant to maximise their exposure to available light (without shading one another out).
As well as the length and position of the petiole, plants have evolved their petioles to hold their leaves at different angles. Where a plant has many layers of leaves, horizontally held leaves near the top of the plant would shade out those below. Accordingly, the petioles of some plants, particularly taller plants, hold their leaves at steeper angles the closer they are to the top of the plant.
The amount of light available to any given plant as it grows will vary depending on the location of the plant (including the extent of competition for light with other plants). As the plant grows, it can allow some cells to elongate more than others (this is achieved by varying the levels of a plant hormone called auxin across different cells, something that is out of scope for this article, but something that we will come back to another time). If all the cells in a petiole are elongated, the petiole will itself become longer. If just those on one side are elongated, it will bend away from that side. As a result, the plant is able to adjust the size and shape of its petioles to capture as much available light as possible.
Once a plant is fully grown, it can no longer adjust the position of its leaves by varying the size and shape of its petioles in this manner. However, petioles are able to bend and twist to some extent throughout the course of a day to optimise the position of leaves to capture sunlight. The mechanism by which this is achieved is complex and beyond our scope, but involves the use of enzymes to soften cell walls in some parts of the petiole, and the movement of water in other parts of the petiole to alter turgor pressure; the forces generated by these actions cause the petiole to bend and twist towards the light.