The Colors of Autumn
With the coming of autumn, we witness a most interesting phenomenon. Trees' green leaves change color in a matter of a few days. Shortly afterward, all the leaves fall off and the tree branches are left bare. The tree now appears lifeless until the return of spring, because all vital functions have now been reduced to a minimum. Leaf shedding and regeneration, which reminds one of human death and of resurrection after death, as stated in a verse, is a transition during which several miraculous events take place. Allah reveals this in the Qur'an:
He brings forth the living from the dead and brings forth the dead from the living and brings the earth to life after it was dead. In the same way you too will be brought forth. (Surat ar-Rum:19)
Before finally dying and dropping off the twig, a leaf undergoes various stages. A great many chemical compounds combine together and put different systems into action to separate the leaf's stem from where it grew. No substance is wasted as this occurs, and leaf shedding becomes a very beneficial process for both the plant and its surroundings. Not only do autumn leaves remind us of death and resurrection, they also once again reveal the infinite knowledge and might of Allah.
How Leaves Become Colored
We do not pay much attention to the green color of leaves in the summer; but when autumn comes, we suddenly become aware of color change. As leaves change their colors and fall from the trees, we find ourselves faced by the brightest of pictures. Bright green trees turn yellow, red, and brown within a few short days. But what causes this color change, and why do leaves fall from the trees?
All leaves, be they yellow, red, purple or green, are colored by the various pigments they contain. The best-known among plant pigments is without doubt chlorophyll, which gives leaves their characteristic green color and also plays a very important role in photosynthesis, as you have seen throughout this book. In moderate climates, leaves begin to change color with the coming of autumn. The yellows, oranges, reds and finally browns that replace the green in leaves are the product of the yellow and orange pigment carotene. A pigment called anthocyanin also plays a part. Together, these three pigments give leaves and summer flowers their colors.
The green of chlorophyll in leaves is so strong that it masks the existing yellows and oranges of the leaves. Before plants shed their leaves in the fall, they re-absorb the useful substances contained in them. One of the consequences of this is that chlorophyll begins to break down. Since chlorophyll no longer predominates at this stage, the yellow and orange pigments, which had not been apparent before, now begin to be seen.
When leaves come to the end of their life spans, the pigment anthocyanin starts to increase, turning some leaves from their normal green to a light reddish-purple. Anthocyanin pigments vary from red to purple, and the red, blue and purple regions of a plant are entirely their responsibility. When temperatures are low and plants are exposed to extremely bright light, a great many of them tend to increase their anthocyanin levels. That is why the color red increases in many plants in autumn. These pigments generally turn color, from yellow to orange and red. In addition to weather conditions, color change in autumn is to a large extent related to the individual plant species. The strikingly beautiful scenes we see in autumn are thus the work of these pigments.64
Is there any benefit to the falling of the leaves?
Millions of leaves fall every year, and reappear with the coming of spring. At first sight, it might appear as if all these leaves fall to earth for nothing. Yet this is a misconception, because the fall of leaves occupies an important place in the ecological system. Nothing has been created for no reason. Whichever system or living thing we examine, we find a purpose and wisdom in its creation. Falling leaves are also a part of this perfect system. The largest falling leaves replenish the soil with nutrients. In addition, falling leaves help with the retention and absorption of rain by forming a humus layer on the forest floor, and many tiny living things find shelter under them. Finally, fallen leaves become a source of food for many organisms living in forests.
Every year, as the leaves fall, some 300 million tons of chlorophyll enter the soil worldwide. Every year, 900 million tons of chlorophyll are broken down in the seas, since seaweeds and algae containing chlorophyll have short life spans. Were it not for this annual loss of chlorophyll, the consequences would be disastrous. Increasing levels of chlorophyll would lead to seaweeds and algae cells able to use less sunlight. As a result, cells would receive insufficient quantities of light, resulting in less photosynthesis. Life would come to an end in the oceans, and thus in the whole world.
One of the most interesting phenomena that take place in leaves about to be shed is the exceedingly purposeful break-down/separation process. Before the leaf falls, useable substances such as protein and carbohydrate are stored in the branches or stems of the plant. Thus the leaf that is due to fall does not take these substances with it, and an important part of the materials necessary to form leaves in the future is retained. As can be seen from these examples, it is ecologically essential for the survival of life on Earth for chlorophyll to be eliminated at the correct time and for the substances a plant needs to be stored inside while it goes dormant. One of the first signs of aging in leaves is that the cells in the base of the leaves beginning to produce the gas ethylene. Shortly afterwards, the ethylene spreads to all parts of the leaf, and when it reaches the leaf stalk, small cells where the leaf stem joins to the main body of the plant, the small cells here begin to swell up, leading to a tension in the stem. The number of cells in the section where the leaf is bound to the stem raises and they begin to produce special enzymes. First of all, cellulose enzymes break down membranes consisting of cellulose, then pectinase enzymes break down the pectin layer that binds the cells together. The leaf becomes unable to withstand this increasing weakness, and the stalk starts to separate from the body of the plant.
Long before the leaf falls, a detachment zone forms where the leaf stem joins the main body. The membranes of special impermeable tissue cells known as the parenchyma begin to soften and assume a jelly-like state by passing through various chemical changes. This causes the cells to divide from one another. The leaf is soon joined to the plant's main body only by vessels that permit only fluids to pass through. Very rapid changes take place around the fissure as it continues to widen, and the cells begin to produce some kind of fungus essence. This substance gradually places itself in the cellulose in the membrane and stiffens it. All the cells producing fungus essence die, leaving a large empty space behind them. As a result, the leaf breaks off in a light breeze. However, at this point a layer of fungus develops to cover the leaf scar that has opened. These physical and chemical changes are a very carefully planned process that takes place not just in one leaf but in all of them. This system has been created to ensure that leaves fall off when the proper time comes.
64. "Leaves of green, yellow & red," http://botany.about.com/science/botany/library/weekly/aa120797.htm.