Leaf Function and Anatomy: A Conversation

Leaf Function and Anatomy:  A Conversation

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What is the function of a leaf? Leaves perform three essential functions, and the most critical one is producing food for the plant.

By Mark Hall I have been fascinated with leaves since childhood. The old sugar maples back home were ablaze with spectacular colors every October. The sight of falling leaves was always a pleasure, as was the time-honored practice of bounding headfirst into tall piles. Those early days fueled an appreciation for leaves and a desire to learn more.  

Granted, leaves are pretty and can generate a sense of nostalgia, but how important are they?  

The answer is an emphatic “Very!” Leaves perform three essential functions, and the most critical one is producing food for the plant. As you may remember from a science class long ago, this is accomplished through a process known as photosynthesis. Here, energy from sunlight is used to transform water and carbon dioxide into glucose and oxygen, and this glucose provides the plant with the energy it needs to survive. Now, how is that for serving an essential purpose?  

Well, providing energy for its own survival is undoubtedly very crucial.  

Another vital function of leaves is the release of surplus water from the plant. On hot, dry days, all plants cool themselves by purging a great deal of water in the form of vapor through microscopic pores on the leaf surface, called stomata. Interestingly, this process, known as transpiration, releases more water than you might guess. The weight of water given off is often higher than the weight of the plants themselves and amounts to 99% of the water taken in by the roots. An oak tree can transpire 40,000 gallons of water annually, and an acre of corn can transpire 3,000 to 4,000 gallons per day.  

An additional form of water displacement is called guttation. Unlike transpiration, this mode takes place at low temperatures and involves removing water in the form of a liquid from the interior of the leaf through its outer edges. In contrast to transpiration, guttation is experienced only by herbaceous plants or those lacking a woody stem.  

The third important function of leaves is gas exchange, which involves air change between a plant and its environment. During photosynthesis, plants need carbon dioxide from the atmosphere around them, releasing oxygen when that process is complete. This exchange of carbon dioxide and oxygen is carried out through the stomata, which are the microscopic pores that also release water vapor during transpiration. This exchange of gases helps replenish oxygen and controls the amount of carbon dioxide in the air.  

Leaves indeed serve several important purposes, but what about their anatomy? They appear to be so thin and simplistic, and their interior must be practically nondescript, right?  

Wrong! A study of leaf anatomy reveals that there is much more than meets the eye. Inside every thin, delicate leaf are multiple cell layers. Together, these layers comprise three main tissues found within the leaf: the epidermis, the mesophyll, and vascular tissue.  

The peripheral tissue at the top and bottom of the leaf is called the epidermis. This layer contains the stomata, the microscopic pores which release water vapor and control the exchange of oxygen and carbon dioxide. Scattered throughout the epidermis, these elliptical-shaped stomata are each surrounded by guard cells, one on each side of the opening. As these guard cells change shape, they open and close the stomata in the center. Covering the epidermis is an extremely fine, protective coating called the cuticle, which helps prevent excessive water loss, as well as injury and infections.  

The layer at the center of the leaf, called the mesophyll, is composed of two parts. The upper part is called the palisade mesophyll. These cells are very tightly packed and column-shaped. The lower mesophyll leaf layer is called the spongy mesophyll. Unlike those of palisade mesophyll, spongy mesophyll cells are dissimilar in shape. This variety in cell shape means that the cells are not packed tightly together, creating the air space necessary for oxygen and carbon dioxide movement. Both the upper and lower mesophyll layers contain an abundance of chloroplasts – organelles within the cells that contain the green pigment chlorophyll, which absorbs the sunlight for photosynthesis.  


The final main type of leaf tissue is vascular tissue. Spreading throughout the spongy mesophyll as veins, this extensive, cylindrical tissue crisscrosses not only the entire leaf, but also the entire plant. Inside the vascular tissue, two tubular formations called xylem and phloem transport nutrients and water throughout the plant. In addition to transportation, these veins also provide structure and support to the leaves and to the plant as a whole.  

I am now fully convinced that leaves are truly captivating. After a look into the interior of the leaf, I am captivated by a marvelous world of intricate detail.  


Originally published in the March/April 2023 issue of Countryside and Small Stock Journal and regularly vetted for accuracy.

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