Some insight for leaves

Leaves are in charge of respiration in plants. They consist of the main mechanism producing food for plants, using sunlight via photosynthesis through which food for many more organisms, animals, and humans are provided. Having been assigned to convert solar energy to food, which they’ve been doing for millions, perhaps billions, of years, plants have been a significant instrument for sustaining life on or planet. It is as if plants turn their leaves upward in prayer to ask for food on behalf of all living things.

Morphology of a leaf

Leaves are composed of three sections: the base, blade, and petiole. The blade is the most important part of the leaf; it is wide and flat. The exact shape of leaves vary according to climate, geographical conditions, life span and risk of consumption by other organisms. In tropical climates, the blade is often very wide. In drier climates, it is usually smaller, in order to reduce water loss.

Leaves of some plants undergo a transformation called “metamorphosis” to fulfill different tasks. For example, some leaves have a thorny shape and protect the plant form herbivorous animals. Some leaves are designed to store water, and some are converted into a trap in order to capture insects to nourish the plant.

On the cross section of a leaf, one can observe that four layers constitute the inner part. The first one is the epidermis, which covers the leaf from top to bottom. This layer protects the leaf against external elements and is lined with a waterproof, waxy substance.

The palisade parenchyma is located on the upper side of the inner tissue and it houses chloroplast rich cells, which are lined up densely and carry out photosynthesis. The spongy layer under the palisade tissue forms the intercellular air spaces and this layer is responsible for the respiration of the plant.

For photosynthesis to occur, the leaf needs to receive the maximum amount of sunlight. The sun must hit the leaf at a perpendicular angle; thus, the leaf must be amply wide and must sit level. Because the sun hits different latitudes at different angles, plants have branches of different lengths facing different directions, and leaves have different curvatures. Furthermore, leaves are also lined up in a way so as not to block the sun’s rays. For this to happen, it is required for the leaf base to be thin and the leaves to be lined up in a spiral fashion that enables both lower and higher ones to harvest sunlight in the most efficient way. This type of arrangement exemplifies the golden ratio, which is observed among many structures in nature.

Each leaf sprouts at an angle of either 222.5 or 137.5, derived from division of 360 degrees, from the previous leaf under. This spiral leaf growth provides them with the most suitable place to harvest sunlight maximally. This way the gaps around branches are minimized and a maximum number of leaves is positioned without reducing the light capture capacity of the plant.

Seasons and leaves

Plants work like factories during spring and summer, producing a great deal of food through photosynthesis. Some of these foods help the plant grow and some are stored as starch for winter. With the onset of autumn, a majority of plants outside tropical zones go through hibernation, like many organisms do, and enter a dormant period. In order for plants, like trees and bushes, to survive the cold, their leaves are shed to minimize their surface area and conserve energy. Perennial green plants lose their aerial parts, too, including stems and leaves, and hibernate underground as roots, bulbs, and tubers. They sprout back from their roots once spring brings warmer weather.

Many leaves begin to fade and fall once autumn arrives. The leaves of some hardy plants – like cypress, pine, and spruce trees – continue to function through winter. In some of these trees, like the bay tree and the Indian sandalwood, there are protective layers covering the leaves against the cold. Other leaves, like pine needles, are created in a spiny shape to resist the cold.

Colors of autumn

Leaves seem green during the spring and summer months because the chlorophyll found in them absorbs all wavelengths other than green. The other major pigments found in leaves are carotene (orange) and xanthophylls (yellow). These two pigments are the most common pigments in nature.

As autumn approaches, and photosynthesis begins to end, chlorophyll starts to degrade and the other pigments begin to show. Thus, leaves turn yellow and bright red.

As the weather gets colder, the chloroplasts that are near the leaf’s bottom are broken apart, and sugar levels begin to elevate. The sugars produced during this season accumulate in the leaves day by day due to lower photosynthetic speed and reduced transportation to other parts of the plant. These sugars are converted into anthocyanins. At first, leaves appear yellow. A couple weeks before they fall, most leaves shift from yellow to red. Under abundant sunlight, due to concentrated anthocyanins, leaves seem brighter and more colorful – and thus red. Once the live tissues die completely, all leaves turn brown. This is due to the high concentration of tannin.

Leaf color varies not only because of plant genetics and external factors, but also because of climate. Temperature, humidity, soil composition, and levels of sun exposure all affect color. There is a higher degree of color change in the leaves of trees that grow in lower temperatures.

The composition of soil plays a major role in the color of leaves. Leaves that turn yellow early indicate a nitrogen shortage; on the other hand, the presence of a strong red color indicates very acidic soil. A high alkaline ratio is present in places where leaves are purple.

Walking among fallen leaves and the colorful scenery in a forest in autumn can trigger unique emotions. Depending on the psychological state of a person, the colors of autumn sometimes remind us about the briefness of this world, but they can also hint at the infinite life to come.

The motifs and patterns of leaves

Receiving sufficient sunlight is a significant matter for leaves. Therefore, they are created differently. No two plant leaves are the same.

Some of the leaves are simple and some are compounds. According to their arrangements, opposite, alternate, whorled, and rowed forms exist. Leaf blades can be ovals, kidneys, triangles, or even hearts. Edges can be smooth, serrated, toothed, or lobed. Leaf veins can also have many different motifs.

Each plant species has its own leaf motif. The alfalfa leaf has a triple pattern of specific angles; walnut leaves have an opposite arrangement of eight to ten. A hand-like motif, like the fingers of a praying hand, formed of seven leaves, can be observed on chestnut trees.

In order to answer these questions, let’s first take a closer look at the nature of leaves and how they function. Simply put, leaves are nature’s food factories. During spring and summer the leaves serve as factories where most of the food necessary for tree growth is manufactured. The process of transforming water and carbon dioxide into sugar is called photosynthesis, which means literally “putting together with light” (Figure 1). This food-making process is performed by the chlorophyll molecules that are present in the leaf cells. Chlorophyll absorbs energy from sunlight. While the water is sucked in from the soil by the roots and carbon dioxide is inhaled through the pores of the leaves, chlorophyll synthesizes carbohydrates, such as sugars and starch. During winter, when this process cannot continue due to lack of light or water, the trees rest and live off the food they stored during the summer.

What is most astonishing is that it is again chlorophyll that gives the leaves their green color. Along with the chlorophylls (green pigment), spectrums of pigments, such as carotenoid, anthocyanin, and xanthophyll, give different colors to the leaves (Figure 2). When chlorophyll is abundant in the leaf cells, as it is during the growing season, the green color of chlorophyll dominates and masks the colors of any other coloring pigments that may be present in the leaf. Thus, the leaves of summer are characteristically green. When for some reason the number of chlorophylls decreases significantly, the color of other pigments paint the leaves, such as in the fall. For example, maple and acer leaves turn to bright red because of the carotenoid and anthocyanin pigments while sugar maple leaves turn yellow due to xanthophyll pigments.

Time to change colors

At the end of summer and the beginning of autumn, the length of days and the average temperature begin to decrease. Then the trees “know” that it is time to get ready for winter, their sleep-time, and the leaves stop making food. Since there is no longer a need for them, the chlorophyll molecules break down, and so the green color begins to fade. As we, the grieving viewers of this process, say farewell to the color of life, we are surprised by the splendor demonstrated by vibrant colors ranging from yellow to orange.

All these colors are due to the mixing of varying amounts of the chlorophyll residue and other pigments in the leaf becoming visible during the fall season. For the realization of this beauty, mixtures of pigments give rise to the reddish and purplish fall colors of trees such as dogwoods and sumacs, while others offer the sugar maple its brilliant orange or yellow. In some trees, like the maple, a red pigment will be formed in the fall if the days are warm and the nights cold. These trees produce sugar in the leaves during the day, but this sugar cannot move out when the nights are cold, and the leaf’s connections to the tree begin to break down. After that the high sugar concentration favors the formation of a class of pigments called anthocyanin, which is red. Thus, the leaves left out in the sunlight turn red through this process. The brown color of trees (like oaks) that appears after chlorophyll breaks down comes from plant wastes left in the leaves. The autumn foliage of some trees is only yellow, so it is the combination of all these things that makes the beautiful colors we enjoy in the fall.

Thanks to scientific research, today we know the details of leaf color change. The approximate size of these pigments is so small that hundreds of millions of them, put edge to edge, could measure only 1 meter. Isn’t it amazing how the beauty of autumn is exhibited through the hands of such small and blind painters?

Abdullah Akpinar is a graduate student in Planning and Landscape Architecture at Clemson University, Southern Carolina.