Leaves – Background

LEAVES: NATURE’S SUNCATCHERS

Leaves are all around us. They are a welcome sign of spring, brightening the landscape with fresh green. They give us shade in summer and color fields and hillsides emerald. Every fall they dazzle us with their fiery display of color. Leaves come in a variety of shapes and sizes, yet a close-up look reveals patterns that help us identify them. Despite differences in form, all leaves perform the same vital function – capturing sunlight and turning it into food for the plant.

At its most fundamental, a leaf is a flattened green surface, or blade, supported by a slender stalk, or petiole. It is made up of layers of cells sandwiched between outer waxy coverings that help keep water in and harmful bacteria and fungi out. These surface layers are dotted with tiny pores called stomata, which can open and close to regulate gas exchange, moisture, and temperature within the leaf. A network of veins carries water and nutrients into and out of the leaf and provides a supportive framework for these delicate tissues. A simple leaf has a single blade, while a compound leaf, like clover, is made of several smaller blades, called leaflets, attached to the petiole. Leaves are arranged on the stem to maximize their exposure to sunlight.

Just as we can recognize people by small facial differences, we can identify plants by studying their leaf shapes and arrangement. Each leaf has a unique set of features, and there are many variations in leaf designs. Leaves can be broad like maple leaves, narrow like blades of grass, or even needle-shaped like the leaves of evergreens. Leaf edges, also called leaf margins, can vary, too. Lilac leaves have smooth margins, elm leaves are toothed like a saw blade, dandelion leaves are jagged, and oak leaves are divided into lobes. Some leaves, like those of carrot, are so finely dissected they look like lace.

Another distinctive identifying feature of leaves is the pattern of veins that run throughout the blade. These act as the leaf’s plumbing system, carrying water and nutrients in and food – in the form of sugars – out of the leaf. There are three main types of leaf venation: pinnate, palmate and parallel. Pinnate venation consists of one main vein, called a midrib, running through the middle of the leaf with side veins extending to the leaf margins, similar to the structure of a feather. In palmate leaves, such as maple leaves, the main veins radiate from the petiole attachment and fan out through the leaf blade. In the grass, lily, and related families, parallel veins are the rule, running side by side down the length of the leaf blade.

Environmental conditions influence leaf features in many ways. Shaded leaves tend to be larger, thinner, and darker green than those in full sunlight. Quaking aspen grows in open sunny places and has many small leaves with flattened petioles that let each leaf tremble in the slightest breeze, allowing sun to filter to the leaves below and keeping the leaf from overheating. Leaves of tropical plants often have “drip tips” that allow water to drain from leaf surfaces, thus preventing mold growth that could damage the leaf. Common mullein grows in sunny open places, and their fuzzy leaves are thought to trap moisture, reflect sunlight, and reduce water loss.

Leaves come in all shapes and sizes, yet they all have the same function: to produce food for the plant. For unlike animals, plants don’t raid the cookie jar or go hunting for food when they are hungry; they make their own food. And their green color is key to this function. The green comes from the pigment, chlorophyll, which is contained in the upper surface of the leaf. To understand this food-making process, think of it as a recipe – the plant’s version of making a batch of brownies. Along with chlorophyll, the main ingredients are solar energy from the sun, water absorbed by plant roots, and carbon dioxide, a gas absorbed from air through the leaves. Chlorophyll absorbs sunlight and acts as a catalyst, transforming solar energy into chemical energy used to make plant food. This energy is then used to split water into its component parts, hydrogen and oxygen. The hydrogen joins with carbon dioxide to form a simple sugar called glucose. This sugar is carried in the veins to other parts of the plant and used to help the plant grow, making new leaves, flowers, fruits, and seeds. As a byproduct, plants give off oxygen, an essential element needed by living things. This chemical reaction, called photosynthesis, is thought to be one of the most important processes on Earth, for it provides food, not only for plants, but ultimately for most animals who depend on plants for energy to live and grow.

All summer, leaves are busy making and storing food. But the shortening days of fall signal a change in the seasons. Tree leaves have to cope with less light, cold temperatures, and frozen water. Tough, waxy evergreen needles are resistant to the cold and drought-like conditions of winter and remain on the tree. But broad-leaved trees seal off and shed their leaves. Special cork cells grow over the veins into the leaves, cutting off their water supply. Without water, chlorophyll production starts to dwindle. Now the other pigments that have been there all along start to show through. The orange colors come from carotene and the yellow from xanthophylls, two pigments common in familiar fruits and vegetables, like carrots, corn, and bananas. While yellow and orange colors are simply unmasked, red and purple colors are produced in response to bright light and sugar in leaves. As veins get sealed off, sugars get trapped in the leaves. On cool nights, a chemical reaction converts sugars in the sap to a new pigment, anthocyanin, tinting leaves red to purple.

Although not completely understood, temperature, light, and water supply are thought to influence the degree and duration of fall color. An early frost weakens the leaf, turning it brown before the best color is developed. Drought tends to delay or halt color, causing pigments to fade, leaves to dry out and turn brown. A series of rainy or overcast days limits light, hence photosynthesis, resulting in less sugar production and fewer red leaves. The best colors are produced when a warm, wet spring is followed by a summer that is not too hot or dry. These combined with sunny fall days and cool nights above freezing produce the best display of colors. Fall leaf color can also be used to identify trees, as each type of tree has it own special range of color.

Leaves make up our natural world, and without them there would be no forests, no grassy meadows, no gardens of leafy greens. Through the amazing process of photosynthesis, they provide the oxygen we breathe and the food we eat. We couldn’t live without them!

Suggested Reading

Bang, Molly and Chisholm, Penny. Living Sunlight—How Plants Bring the Earth to Life. New York: Blue Sky Press, 2009.

Maestro, Betsy. Why Do Leaves Change Color? New York: Harper Collins, 1994.

Silverstein, A. et al. Photosynthesis. Minneapolis, MN: Twenty-First Century Books, 2008.

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