Plant leaves are all around us, from the trees in the forest to the shrubs in our gardens. They come in different shapes, sizes, and numbers depending on the plant. But have you ever wondered why there are so many leaves? Well, leaves are actually special organs with a few key functions. One of these functions is to help plants conserve water. Another important role leaves play is making food for the plant through a process called photosynthesis. So the next time you see a leaf, remember how important it is for the plant's survival!

The Definition of a Plant Leaf

Leaves are important plant organs that grow from nodes on the stem. They have multiple veins, either branched or unbranched, and contain photosynthetic tissue. The primary function of leaves is to provide a site for photosynthesis. However, leaves have evolved to serve different purposes for plants.

They are typically flat and thin, which allows for a large surface area to absorb light for photosynthesis. Chlorophyll is a chemical found in leaves that gives them their green color and plays a crucial role in photosynthesis. So, the next time you see a leaf, remember how crucial it is for the plant's survival!

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

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 mesophyll layer of a leaf is the middle layer of tissue. In fact, the word mesophyll means "middle leaf" in Greek. It's made up of a type of living cell called parenchyma cells. These cells are thin-walled and are found in parts of the plant that aren't epidermal or vascular tissues.

There are two types of parenchyma cells that make up the mesophyll tissue of leaves. Palisade parenchyma cells are tightly packed beneath the epidermal cells, while spongy parenchyma cells are loosely packed under the palisade parenchyma layer. The spaces between the spongy parenchyma cells allow for greater gas diffusion in this part of the mesophyll tissue. Both types of cells contain chloroplasts and can undergo photosynthesis.

The mesophyll layer also includes vascular bundles, which contain xylem and phloem veins. These veins help transport products necessary for photosynthesis to the leaves and transport the sugars made in the leaves elsewhere. So, the mesophyll layer plays a crucial role in the overall function of leaves.

Epidermis

The epidermis is the outermost layer that covers the leaves. It may consist of a single layer of cells or multiple layers, depending on the leaf. Epidermal cells do not contain chloroplasts and do not undergo photosynthesis. Instead, their primary function is to protect the plant by secreting a cuticle, a waxy covering that prevents water loss through evaporation from the leaf surfaces.

However, the cuticle also presents a problem for the leaves. It blocks gases from diffusing through the leaf into the photosynthetic tissues. This is where stomata come in. Stomata are small pores that allow for the exchange of gases, such as carbon dioxide and oxygen, between the leaf and the atmosphere. The stomata are usually found in the lower epidermis of the leaf, with a higher density in leaves that require more gas exchange.

The stomata open and close in response to environmental cues, such as light and humidity, to regulate gas exchange and water loss. The presence of stomata in the epidermis is crucial for the survival of the plant, as it allows for the exchange of gases necessary for photosynthesis while also preventing excessive water loss.

Stomata

Stomata are small openings found on the underside of leaves that allow for the exchange of gases between the leaf and the atmosphere. They are controlled by guard cells, which are elongated-disc-shaped cells in the epidermis. Unlike other epidermal cells, guard cells contain chloroplasts and can undergo photosynthesis.

The opening and closing of stomata is controlled by the presence and absence of water in the leaf. When guard cells are filled with water, they become turgid and expand, causing the stomata to open and gas exchange to occur. When they are not filled with water, they become flaccid, and the stomatal opening closes.

Even though stomata are adapted to prevent water loss and allow for gas exchange, they are responsible for 90 percent of the water loss in a plant, despite only covering about 1 percent of a leaf's surface area. This water loss through the stomata is known as transpiration.

Transpiration is important for the plant's survival, as it helps to "pull" water and nutrients up from the roots through the xylem. The process of transpiration is regulated by environmental factors such as light and humidity, as well as by the plant's internal regulation of water balance.

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Basic Leaf Structure

Plant biologists have classified the basic external leaf structure to facilitate comparison and categorization of plants. The basic leaf of a flowering plant (angiosperms) consists of several parts.

The lamina, also known as the leaf blade, is the thin, flat surface of the leaf that contains veins for transport and photosynthetic tissue. The petiole is the part of the leaf that attaches it to the stem. Stipules are small structures at the leaf node that help to protect the developing leaf. The midrib, also known as the central vein, runs through the middle of the leaf blade and provides structural support.

Additionally, there are smaller veins that branch out from the midrib called secondary veins. These secondary veins further divide into tertiary veins, forming a complex network that delivers water, nutrients, and sugars throughout the leaf. The arrangement of veins in a leaf can be used to help identify the plant species.

Understanding the basic external leaf structure and its terminology is important for plant biologists to accurately describe and compare plant species.

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Other Structures and Functions of Leaves in Plants

While leaves in most vascular plants primarily serve to produce food via photosynthesis, many species of plants have adapted their leaves for specific purposes. The form and function of leaves can vary based on environmental pressures on the plant, including climate and herbivory.

Trichomes are outgrowths of the epidermal cells in plants that occur on plant organs such as leaves and stems. They vary in cell number, shape, size, and function. One function of trichomes is to deter herbivory by making it physically harder for insects or other pests to eat the leaves or by secreting chemicals that make the leaves toxic to pests. Trichomes can also help thicken the leaves' epidermis and prevent excessive transpiration, which can lead to drying out.

In addition to deterring herbivores and reducing water loss, trichomes can also serve other functions such as reflecting excess light or reducing the impact of wind on the plant. The presence or absence of trichomes on leaves can be an important identifying characteristic for different plant species.

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Leaves play a vital role in the growth and survival of plants, serving as the site of photosynthesis and other important functions. They contain veins for transport, photosynthetic tissue, and specialized cells containing chloroplasts.

The leaf is composed of the epidermis, which is the outer layer, and the mesophyll, the middle layer. The mesophyll is made up of parenchyma cells and tightly packed palisade parenchyma cells that photosynthesize, while the spongy parenchyma cells are loosely packed and also photosynthesize. The epidermal cells secrete a waxy cuticle to help prevent water loss, and stomata are openings in the epidermis that allow for gas exchange. Plants have adapted their leaves to perform additional functions, such as trichomes that deter herbivory, guttation to release excess water, and storage of water in arid climates. Some leaves have even evolved into bracts that attract pollinators, and plants can undergo asexual reproduction through vegetative propagation from leaf margins. Understanding the various structures and functions of leaves is crucial for plant biologists to accurately identify and classify plant species, as well as to understand their role in the ecosystem.

Plant Leaves

What do leaves produce for plants? 

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.

Why do plant leaves turn yellow? 

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).  

What are the four functions of a leaf?

The main function of the leaf is to make food for the plant via photosynthesis.  Leaves also: Help prevent water loss through their waxy cuticle.Allow gas exchange through their stomata.And help the movement of the xylem by the loss of water through transpiration or evaporation from the leaves.

What are the parts of the leaf? 

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. 

How do leaves grow?

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).11Nelissen 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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