November 26, 2011
AS FALL PROCEEDS and trees and plants prepare for their winter dormancy, a little wind fluttering through the trees causes beautifully colored, dying leaves to float to the ground.
Life on earth depends on the leaves of plants to photosynthesize sunlight into oxygen and to produce the organic material on which many species depend for food. Leaves are literally the powerhouses of the plant world.
Although every leaf shares the same basic structure, the variety in size, shape, texture, color, margin and venation makes each species distinctly different. These differences aid the leaf in its ultimate, all-important purpose of sustaining life on this earth. The shape and structure of leaves can also vary considerably depending on climate, primarily because of the availability of light and potential for water loss because of temperature and humidity.
In botany, a leaf is an above-ground plant organ specialized for the process of photosynthesis. The typical leaf consists of a stalk, petiole and a blade. Veins support the leaf and are filled with vessels (xylem and phloem) that transport food, water and minerals to the plant. The
xylem consists of the vascular tissue that conducts water and dissolved nutrients upward from the root; the phloem is the vascular tissue that conducts sugars and other metabolic products downward from the leaves.
A leaf is made of many layers that are sandwiched between two strata of tough skin cells, the epidermis. Among the epidermal cells are pairs of sausage-shaped guard cells. Each pair of guard cells forms a tiny pore or stoma (plural, stomata) mostly on the underside of leaves for gaseous exchange and the release of water vapor. Between these two layers are large spongy cells, rich in chlorophyll, for food manufacture and permeated with interconnecting air passages leading to the stomata. These epidermal layers protect the leaf from insects, bacteria, and other pests.
Leaves are typically green, flat and thin, a pattern which evolved as a means to maximize the surface area directly exposed to light and the atmosphere. The broader its surface, the more light the leaf can capture, the thinner it is, the more easily it can transport water, sugars and carbon dioxide. Often, leaves in low light are larger, thinner, and darker green (and thus have a higher chlorophyll content) than leaves exposed to bright
sun. To assist in the collection of water for the plant, some leaves are
funnel-shaped in order to actively collect water, which is then directed to the roots.
Leaves also form a canopy of the tree collecting rainwater from a larger area, which is then led via runoff to the root zone of the plant. Likewise, the internal organization of leaves has evolved to maximize exposure of the photosynthetic organelles, the chloroplasts, to absorb light and to metabolize it with the help of carbon dioxide, in the critical process called photosynthesis. The resulting chemical reaction produces sugars that nourish the plant and generate fruits, nuts and legumes for the rest of the food chain. Another important outcome of photosynthesis is the release of
oxygen and water, which the stomata yield or transpire, giving all breathing species the necessary air to breathe and the moisture that becomes clouds and eventually rain.
Leaves also can store food and water, and they are modified in some plants for these purposes. Thus through the process of photosynthesis, leaves are rich in protein, minerals and sugars. Because of their nutritional value, leaves are prominent in the diet of many animals, including humans.
Trees are classified into two types:
•Conifers: The trees are usually evergreen, with leaves that are needlelike or scaly and with cones that contain their seeds, rather than flowers or fruit.
•Broadleaf: The trees can be evergreen or deciduous, shedding leaves seasonally; they have flat leaves and reproduce by flowering. The leaves may be simple (one blade, such as lobed like a maple leaf or unlobed, like a cherry or elm) or compound (divided into leaflets, such as a buckeye leaf). They may attach to the stem singly, in pairs, or in whorls, with
three or more leaves at the node. Leaves provide shade and shelter to the stem and root system below.
As the days shorten and leaves absorb less light, trees prepare for autumn by stopping the food-making process. During the summer, leaves make more glucose than the plant needs for energy and growth. The excess is turned into starch and stored until needed.
The retention or production of three types of pigments — carotenoids, chlorophylls and anthocyanins — determines the color leaves become in fall. Leaves appear green most of the year because they contain an
abundance of chlorophyll, which masks the presence of the other two pigment colors.
In the autumn, when decreasing light causes chlorophyll production to slow, the other pigments begin to reveal themselves. Carotenoids emerge to reveal yellows; anthocyanins come forth to give leaves brilliant shades of red, purple and crimson. Leaves with substantial amounts of both anthocyanins and carotenoids develop orange coloring. In the absence of these pigments, other plant chemicals also can affect leaf color. For example, tannins are responsible for the brownish color of some oak leaves.
Temperature and cloud cover can elicit differences in red leaf color from year to year. When a number of warm, sunny autumn days and cool but not freezing nights come one after the other, the red coloring in autumn leaves is particularly beautiful. The yellow, gold and orange colors created by carotenoid remain fairly constant from year to year because the carotenoid in leaves does not change in response to weather.
The amount of rain in a year also affects autumn leaf color. A severe drought can delay the arrival of fall color by a few weeks. A warm, wet period during fall will lower the intensity, or brightness, of autumn colors. A severe frost will kill the leaves, turning them brown and causing them to drop early. The best autumn colors come when there's been a warm, wet spring, a summer that's not too hot or dry, and a fall like this one with plenty of warm sunny days and cool nights.
When the leaves die and fall back to the ground they provide insulation from winter weather and additional raw materials when they decompose and are turned back into soil. Leaves alone account for 75 percent of our solid waste in the fall.
So, as you enjoy the beauty of autumn, be grateful to leaves and the vital functions they perform to sustain life on this planet. Leaves have protected us from excess carbon dioxide through the millennia, but with the increased use of fossil fuels and the progressively decreasing number and variety of plants, global warming has become a grave concern. Help to preserve life as we know it by caring and protecting the resources of the earth, especially its plants, and by choosing to use alternate forms of energy whenever possible.