Plant Leaves

Definition of Plant leaf

Let's start by looking at the definition of a plant leaf.

Often, they are flat and thin, allowing for a larger surface area to enhance their ability to absorb light (for photosynthesis). Leaves of a plant are often green because they contain chlorophyll, a chemical important to photosynthesis.

Leaf Structure

As with anything in biology, structure and function always go together. This is why plant leaf structure varies widely: each plant's leaves are adapted to the surrounding environment.

However, there are some parts of the plant leaf that are a necessary requirement. The leaves of a plant are an integral part of the stem system. Having vascular tissue running through them, leaves on plants play a role in the free exchange of nutrients, water, and end products of photosynthesis. For instance, when sugars are produced, they will be transported via the phloem veins from the leaves (the source) to the parts of the plant that cannot produce their own food (the sinks). Additionally, plants need cells with chloroplasts that can photosynthesize, and structures to allow gas exchange during that process.

Figure 2: can you imagine being a tiny plant starting to grow and having to compete for sunlight with the tall trees that are already well-established in your neighbourhood?

To optimise the balance between photosynthesis and gas exchange, each plant has a differently shaped leaf. This means that, depending on the environment, the leaves on a plant will have a particular shape to try to have a large enough surface exposed to the sun to photosynthesize as much as the plant needs while losing as little water as possible during the gas exchange process. On the other hand, water evaporation on larger leaves cools down the plant the same way that sweat cools down animals. In summary, plants have to reach a compromise for each factor.

Figure 3: as you can see, this cactus has no competition for sunlight, but it's probably been ages since the last rainfall.

Another factor that conditions plant shape is the fact that herbivores eat plants. Each plant has adapted to survive despite that, and one way to do so is to protect the plant from herbivores by having prickly leaves or stems, like thistles.

Plant Leaf Cells

So what are leaves made of? Like all organs and systems in any living organism, plant leaves are composed of different types of cells that work with each other to help in the plant leaf function. The main types of plant leaf cells are:

Table 1: The type of cells that make up plant leaves.

Figure 4: Micrographs of palisade mesophyll cells, a type of plant ground tissue with many chloroplasts) in leaves.

Plant Leaf Diagram

Besides vascular tissue, leaves also have several tissues with different functions. This diagram of a plant leaf show these tissues that include the mesophyll, the photosynthetic tissue, the epidermis, or the outer layer of leaf cells.

Figure 5: Micrographs of palisade mesophyll cells, a type of plant ground tissue with many chloroplasts) in leaves.

Mesophyll

The two different types of parenchyma cells that make up the mesophyll tissue of leaves are:

1. Palisade parenchyma cells - packed tightly together beneath the epidermal cells. They are located right below the epidermis and cuticle, which are the outermost layers of the leaves. These cells are commonly referred to as leaf cells.

2. Spongy parenchyma cells - loosely packed under the layer of palisade parenchyma. The space between the spongy parenchyma cells allows for greater gas diffusion in this part of the mesophyll tissue.

Both types of cells have chloroplasts and photosynthesize. Within the mesophyll, there are vascular bundles containing both xylem and phloem veins. This helps bring products necessary for photosynthesis to the leaves and transport the sugars made in the leaves elsewhere.

Epidermis

The epidermal cells do not have chloroplasts and do not photosynthesize. Instead, they protect the plant by secreting a cuticle, a waxy covering. The cuticle protects from water loss via evaporation from leaf surfaces. But at the same time, it also blocks gases from diffusing through the leaf into the photosynthetic tissues. This presents a problem for the leaves: how can they allow for the exchange of gases so that they may obtain carbon dioxide for photosynthesis and expel oxygen, the byproduct of the process? A result of this problem is the stomata.

Stomata

Unlike other epidermal cells, guard cells contain chloroplasts and photosynthesize (Fig. 6). Guard cells are controlled by the presence and absence of water in the leaf. When guard cells are filled with water, they are said to be turgid. At this stage, the expansion of disc-shaped cells causes them to curve, allowing the stomata to open and gas exchange to occur. When they are not filled with water, they are said to be flaccid, and the relaxation of the guard cells causes the stomatal opening to close.

The loss of water through the leaves (aka the stomates) is known as transpiration. The transpiration of water from leaves helps to “pull” the column water inside the xylem up the plant.

Figure 6: Stomata on the underside of a Ligustrum leaf. Source: Fayette A. Reynolds M.S., Berkeley Community College Bioscience Image Library.

What are the four main components of plant leaves?

Although all leaves vary in size, shape, number, and adaptations, they all have the same components. The four main components of plant leaves are:

• The lamina (leaf blade): the thin leaf surface that contains veins for transport and photosynthetic tissue.

• The petiole: the part attaching the leaf to the stem.

• Stipules: small structures at the leaf node that help to protect the developing leaf.

• The midrib: the vein that runs through the middle of the leaf blade.

A leaf blade consists of multiple plant cell layers enclosed within a cell wall. Each leaf cell contains chloroplasts, which contain pigments called chlorophylls. The chlorophyll in plants absorbs light, allowing them to capture solar energy.

Figure 7: The external anatomy of a yellow willow leaf. Source: Matt Lavin, via Flickr.com, edited.

Parts of the Leaf

Although we just looked at the main components of a leaf, let's talk about the other parts of the leaf.

• The apex is the tip of the leaf.

• The margin is the edge of the leaf

• The leaf veins carry food/water throughout leaf; they also act as structural support.

• The base is the bottom of the leaf.

Function of Leaves in Plants

Leaves are organs that have several special functions, but what do leaves do for a plant?. The main function of leaves is to produce food for the plant by photosynthesis, and also minimize the loss of water from a plant. Other leaf functions might include storage and reproduction.

Many species of plants have adapted their leaves for specific purposes. Often, leaves will differ based on the environmental pressures on the plant, including the climate and herbivory.

Trichomes

They occur on plant organs, including both the leaves and the stem. They vary in cell number (unicellular or multicellular), shape, size, and function. One function of trichomes is to deter herbivory, making it physically harder for insects or other pests to eat the leaves or secreting chemicals that make the leaves toxic to pests. Another function is to help thicken the leaves’ epidermis and prevent too much transpiration (that could lead to drying out).

Figure 8: Trichomes (the trident-like projections) of an Arabidopsis sp. leaf. Source: Frost Museum, via Flickr.com.

Guttation

This excretion of water helps relieve the pressure in the roots of plants with a slow rate of transpiration (water evaporation from leaves). Plants with slow transpiration rates are typically found in areas with warm soils and lots of humidity, like tropical rainforests.

Storage

Some leaves are even adapted to help not only conserve water but store it as well. Succulent plants can store water in their leaves, stems, and roots to help them survive in arid (dry) climates. The leaves of these plants are often thicker and have a thicker cuticle to help combat drying out.

Reproduction

Plant leaves in some angiosperm species have evolved to form bracts, which look like flowers but are actually just modified leaves. These may help draw pollinators' attention to species with smaller flowers. One example is the bracts of dogwood tree flowers, which are white and showy.

Plant leaves may also be the site of asexual reproduction. Asexual reproduction, where a part of the plant capable of growing into a new one gets separated from the parent plant, is known as vegetative propagation. Some species can grow new plants on the edges of their leaf margins (e.g., mother of thousands).