Plant Leaves

We see leaves everywhere, on the trees in forests, on the shrubs in gardens, and in the fields and lawns of grass that dot our landscapes. Leaves vary in size, shape, and quantity, depending on which plant you look. But why are they so numerous?

Leaves are organs that have several special functions. They are designed to minimize the loss of water from a plant and make sugars through photosynthesis.

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 are often green because they contain chlorophyll, a chemical important to photosynthesis.

Leaf Structure and Function

Having vascular tissue running through them, leaves act as a food source for the rest of the plant. 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).

Diagram of the Cells and Tissues in a Plant Leaf

Figure 1: Cross-section of a leaf, showing the different tissues and cells. Source: Zephyris original work, via Wikimedia.

Besides vascular tissue, leaves also have several tissues with different functions. These tissues include the mesophyll, the photosynthetic tissue, the epidermis, or the outer layer of leaf cells (Fig. 1).

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

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

Stomata

Unlike other epidermal cells, guard cells contain chloroplasts and photosynthesize (Fig. 2). 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.

Even though stomata are adapted to prevent water loss and allow for gas exchange, they are the source of 90 percent of the water loss in a plant, and the stomates are only about 1 percent of a leaf’s surface!

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

Figure 2: Stomata on the underside of a Ligustrum leaf. Source: Fayette A. Reynolds M.S., Berkeley Community College Bioscience Image Library.

Basic Leaf Structure

Although all leaves vary in size, shape, number, and adaptations, plant biologists have classified the basic external leaf structure to make it easier to compare and categorize plants. It is also important to establish terminology scientists can use while describing leaves.

The basic leaf of a flowering plant (angiosperms) includes the below parts (Fig. 3).

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

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

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

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

Figure 3: The external anatomy of a yellow willow leaf. Source: Matt Lavin, via Flickr.com, edited.

Other Structures and Functions of Leaves in Plants

In most vascular plants, leaves serve to produce food, but 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 4: 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 adapted 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.