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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.
A leaf is a plant organ with multiple veins (branched or unbranched) and photosynthetic tissue that grow laterally from nodes on the plant stem. Their primary function is to serve as the site of photosynthesis; however, plants have adapted leaves to serve different purposes.
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.
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).
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).
The mesophyll of leaves is the middle layer of tissue. Mesophyll means “middle leaf” in Greek (meso= middle, phyll= leaf). The mesophyll tissue of the leaf is made of parenchyma cells. Parenchyma cells are a variety of living, thin-walled cells and make up parts of the plant that are not epidermal or vascular tissues.
The two different types of parenchyma cells that make up the mesophyll tissue of leaves are:
Palisade parenchyma cells - packed tightly together beneath the epidermal cells
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.
The outer layer covering the leaves is known as the epidermis. The epidermis may only be one layer of cells thick, or it may be multiple layers, depending on the leaf.
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 are openings in the leaf surface, typically on the underside of the leaf. Stomata (stoma= singular) are controlled by elongated- disc-shaped cells in the epidermis known as guard cells.
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.
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.
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 are defined as outgrowths of the epidermal cells in plants (Fig. 4).
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 is the excretion of water and minerals from small openings in the leaves, similar to stomata (called hydathodes). Guttation is caused by a build-up of hydrostatic (water) pressure in the roots of plants.
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.
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.
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.
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).
Leaves are the primary site of photosynthesis in plants. Photosynthesis is the process by which plants can use carbon dioxide and the light energy from the sun to produce sugars (carbohydrates) and an oxygen byproduct. Therefore, leaves produce food in the form of sugars for the plant.
In the autumn months, leaves of deciduous trees break down the chlorophyll, their photosynthetic pigment. This leaves behind other types of pigments, giving the leaves a yellow color before they eventually fall off the trees. The yellow is usually caused by carotenoids and flavonoids.
If a leaf turns uncharacteristically yellow, it may be because of a lack of micronutrients or macronutrients (i.e., nitrogen).
The main function of the leaf is to make food for the plant via photosynthesis.
Leaves also:
Leaves are numerous and vary in shape and size based on which vascular plant they are on. Leaves have mesophyll tissue in their middle layer made of parenchyma cells. The parenchyma cells in leaves are:
Palisade parenchyma cells and,
Spongy parenchyma cells.
The palisade parenchyma is tightly packed, and the spongy parenchyma is loosely packed. Both have chloroplasts, the photosynthetic organelle of plants.
The epidermis is made of a layer or layers of epidermal cells that secrete a waxy covering called a cuticle that helps prevent leaves from drying out. The epidermis also contains stomatal openings, which allow for gas exchange on the leaf surface. Stomata are controlled by the opening and closing of guard cells.
Leaves grow through a combination of both cell division and cell growth (expansion). Several biochemical signaling processes and chemicals are involved in the timing and rate of leaf growth.
Monocots have leaf growth cell division regulated more spatially, while dicots are considered to have leaf growth cell division regulated more temporally (time-based).1
1Nelissen et al., 2018. Leaf growth in dicots and monocots: so different yet so alike. Current Opinion in Plant Biol. Vol. 33, pgs 72-76.
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