The Living Lamppost

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Tuvia Cohen 

Come, we are going to visit a small Jewish village in rural Russia. Hop into the time machine, and back we travel to the last century.  

Warily, you step out of the machine and look around. The short winter day draws to a close, and the early dusk envelops the brown street with its huddled low-built wooden houses. Nighttime falls, and the whole village descends into darkness. Here and there a pool of light from an oil lamp spills into the street, but otherwise, there is not a glimmer.  

Gingerly you walk, not knowing where you are going, your feet slipping and unsure in the grooved path. As night deepens, the blackness intensifies, and you wonder how the inhabitants manage to get about.  

The answer is that they don’t. The blackness of the night imprisons everyone in their homes. Not a gas lamp, not a glimmer or a spark to break the barrier of blackness. Suddenly you feel a great wave of yearning – a longing to be back in a society that is illuminated and bright, where electricity brightens your night and guarantees your freedom of movement. It’s time to come back to today, to the age of brightness.  

What could be more modern than streetlamps? If you can remember the lamplighter who went around with his ladder to “windup” the gas lamps, tell nobody your age! Today, no one winds up, no one sets a time switch, and no one pours in the oil. At dusk and dawn, millions of streetlights turn themselves on and off with no human intervention at all. Do you know how it works? With a pecu!  

What’s a PECU? 

“Pecu” is the acronym for a photo electric control unit, which operates a switch in the electrical supply to the lights. There, up above, on the streetlamp, lies a photocell. The photocell contains a compound which is sensitive to light. As dawn rises, light falling on the photocell causes electrons to flow from one atom to the other, conducting electricity to the switch and turning it off. At the other end of the day, as darkness falls, the electrons in the compound become immobile, the current stops, and the lights are turned on. Brilliant! No matter how early darkness falls, the lights will faithfully switch on, thanks to the advanced technology of our modern times. As far as street lighting is concerned, the “good old days” were not so good. 

Shh! I hear someone laughing. Shh! I hear it again. Who is it? A leaf! Why are you laughing, what are you saying? I don’t believe it! The leaf is saying that its technology is so complex that it makes our most modern streetlamps appear primitive in comparison. Could you explain? 

The Trees’ Bedtime 

The autumn season is often called “fall,” and the reason is obvious. When the days become shorter and the temperature begins to drop, millions of trees shed their leaves. The falling of the leaves of the deciduous trees (trees that lose their leaves in autumn, forming new ones in spring) gives the season its distinctive name. It is a spectacular process. The leaves of these trees turn a brilliant red and gold, providing a festival of color that has become a major tourist attraction in many parts of America. Beautiful it certainly is.  

But what makes the leaves fall from the tree? How is it that the twigs do not descend with the leaves? And why don’t evergreen trees see the need to drop their leaves like other trees do?  

And then we come to the most enigmatic question of all. How do the trees know when it is time to dispense with their leaves? What is the timing device, the pecu, that triggers the mechanism and starts the process? Prepare to hear some answers that will amaze you. 

Colorful Farwell  

First the “why,” and then the “how.” The enormous surface area of leaves on a deciduous tree converts sunshine into energy. They also draw water from the roots (by a brilliant process called osmosis). A great deal of water evaporates into the atmosphere from the leaves, which is fine as long as the supply of water in the soil is plentiful. In winter, however, the ground freezes, and the roots cannot take up water from the soil.  

A plant whose roots are in frozen soil is as short of water as a plant in a dry desert. The last thing that the tree needs is a drain of water through evaporation via the leaves, with no replenishment through the roots. Dropping the leaves in autumn enables the tree to survive the winter. And thus, as winter sets in, the tree becomes dormant. It simply goes to sleep. Good night, tree! 

The Great Leaf Exodus 

How, though, does the leaf actually fall off? At a given signal, special cells begin to grow across the leafstalk at the point where it is attached to the twig. This creates an area of weakness across the base of the leafstalk from the outside inwards.  

The natural “glue” by which the packing cells are normally stuck together dissolves. The biochemical processes that normally take place within the leaf stop, and the chlorophyll that gives the leaf its distinctive green color breaks down and disappears, leaving the other pigments of yellow and orange that were there all the time. Eventually, only the veins are running through, and the leaf loosens.  

But before it falls off, something incredible happens. The leaf contains many desirable minerals. Before dropping to the ground, the leaf transfers those minerals into the tree, where they are stored for the new generation of leaves, due to arrive in the spring. At the same time, the tree contains many undesirable toxins that it does not require or want. These toxins are shunted to the leaves prior to their downfall, giving the tree the perfect process of elimination! So much wisdom in a “simple” leaf!  

Will the departure of the leaf leave an open abrasion on the twig? Not at all. As the leaf loosens, a layer of cork develops under the area of weakness, effectively sealing the injury. The demonstrations of intelligence stagger the imagination! 

The Primitive Streetlamp 

But how does the tree know when to begin all these many complex processes? Who whispers into its bark that autumn is approaching? Hundreds of millions of dollars have been spent on trying to answer this question, so far without complete success. It is simply too complex!  

There are, however, certain things that we do know. Every leaf – whether on a plant or on a tree – contains a chemical called phytochrome. This chemical is sensitive to light, and is crucial in activating the numerous processes that occur within the plant. Every species makes a slightly different use of the chemical messages that it receives from its phytochrome.  

The experts suspect that the phytochrome in the leaves of the tree, being the photosensitive cells of the tree, react to the lessening of light as the days shorten, and it is their chemical message that triggers the mechanism that results in the dropping of the leaves. Indeed, it has been discovered that trees that stand next to lampposts retain their leaves for longer periods than trees of the same species that stand away from the extra source of light.  

The Wise Old Tree 

So there you have it. We walk along the street and consider our lampposts the last word in sophisticated technology. There they stand proudly, the result of decades of research and scientific development. They even have photosensitive cells. Well may the humble tree laugh. It has been endowed with these sophisticated skills since its creation, and much more! To quote the words of one expert botanist, “Placed next to a tree, the streetlamp appears primitive, almost naive, by comparison!” 

But there’s more. As autumn approaches, our humble tree produces a special gaseous hormone called ethylene, which, in its complex chemical way, breaks down starches and produces sugars, encourages leaf-drop, and enhances the cork layer which forms at the base of the leaf. And you thought leaves falling in autumn happened by itself – just pretty colors!  

Evergreen Enigma: Why Some Trees Keep Their Leaves 

How do the evergreens manage? They have small, thick-skinned leaves, whose relatively small surface area prevents significant loss of moisture, and which are designed with a waxy upper surface that locks the moisture in. Furthermore, the pressure with which the pine tree draws up its water by the process of osmosis (called the osmotic pressure) is higher than that of an oak tree. This has the effect of lowering the freezing point, so that when the oak tree is already forming ice crystals in its sap (which are obviously harmful, thus necessitating the elimination of the leaves to prevent osmosis), the pine, at the same temperature, because of the increased pressure, does not form any ice crystals. Therefore, pine leaves, you can stay on!  

Who gave the pine all this knowledge – knowledge so advanced and complex that not even the human brain can fathom it all, but knowledge so vital that without it the tree could not have survived until now? Here they are, in their millions, living testimony to the wisdom of their Designer and Creator.