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Plant Leaves

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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

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.

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

Plant Leaves, Leaf Cross-Section Diagram, StudySmarterFigure 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 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:

  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

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

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.

Plant Leaves, Image of Stomata under a microscope, StudySmarter 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.

Plant Leaves, Image of Leave Parts, StudySmarterFigure 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

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).

Plant leaves, Image of Arabidopsis trichomes, StudySmarterFigure 4: Trichomes (the trident-like projections) of an Arabidopsis sp. leaf. Source: Frost Museum, via Flickr.com.

Guttation

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.

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.

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).

Plant Leaves - Key takeaways

  • A leaf is a plant organ that grows laterally from the stem, containing veins, branched or unbranched, and photosynthetic tissue.
  • The leaf is the site of photosynthesis in plants and has special cells which contain chloroplasts.
  • Parts of the leaf include the epidermis (outer layer) and the mesophyll (middle layer).
  • The mesophyll is made of parenchyma cells, tightly packed palisade parenchyma, and loosely packed spongy parenchyma cells, both of which photosynthesize.
  • The epidermal cells secret a waxy cuticle to help prevent water loss.
  • Stomata are openings in the epidermis controlled by guard cells that let gas exchange happen at the leaf surface.
  • Leaves have many other structures and functions, including trichomes (epidermal outgrowths), guttation (releasing excess water), storage (of water in arid climates), and known reproduction (floral additions known as bracts or vegetative propagation).

Frequently Asked Questions about Plant Leaves

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: 

  • 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.

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:

  1. Palisade parenchyma cells and,

  2. 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.

Final Plant Leaves Quiz

Question

Which of the following is not a characteristic of a leaf? 

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Answer

A leaf absorbs water and nutrients from the soil.

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Question

Which of the following statements about stomata are true?

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Answer

They are openings in the leaf surface.

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Question

The palisade parenchyma cells of the mesophyll tissue are:

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Answer

Tightly packed beneath the upper epidermal layer.

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Question

The spongy parenchyma cells of the mesophyll tissue are:

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Answer

Loosely packed and closer to the bottom epidermal layer.

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Question

When the guard cells are filled with water, they curve, allowing the stomata to _____.

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Answer

Open

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Question

Which of the following types of cells of the leaf contain chloroplasts and photosynthesize?

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Answer

Spongy parenchyma cells

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Question

The epidermal cells of leaves secrete a waxy covering known as a ______.

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Answer

Cuticle

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Question

The basic structure of an angiosperm leaf includes:

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Answer

Lamina: the photosynthetic tissue and veins.

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Question

Stomata allow for:

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Answer

Gas exchange

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Question

______ are defined as outgrowths of epidermal cells in plant leaves.

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Answer

Trichomes

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Question

Which of the following is not a function of trichomes?

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Answer

Help open stomata

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Question

______ is the excretion of water and minerals from small openings in the leaves similar to stomata


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Answer

Guttation

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Question

Guttation usually occurs in humid climates where the plants have a ______ rate of transpiration.

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Answer

Low

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Question

Guttation helps to relieve _________ in the roots of plants. 

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Answer

Water (hydrostatic) pressure

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Question

True or False: Leaves can be used to store water for plants that live in arid climates. 

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Answer

True

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Question

Asexual reproduction where a part of the plant capable of growing into a new plant gets separated from the parent plant is known as ________ __________.


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Answer

Vegetative propagation

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Question

Bracts are....

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Answer

modified leaves that may appear to be flowers in some plant species, increasing the showiness of a flower to attract pollinators. 

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