Why Do Plants Have Leaves?
The picture at the right shows the International Space Station as it appeared in March of 2001. People live and work only in the thin vertical cylinder. The rest of the station, looking like wings on an old-time airplane, are solar panels. These panels convert sunlight energy into energy usable by the people inside the station.
That's the main function of most leaves on most kinds of green plants -- to convert sunlight energy into something the plant absolutely must have. It's much more than a mere coincidence that the International Space Station is fundamentally similar to a tree stem with leaves spreading in the sunlight.
Therefore, a leaf's main job is usually this: To present its broad surface to the sun so that inside the leaf's' green tissue the almost-magical process of photosynthesis can take place.
Now, during photosynthesis, sunlight energy is used to power chemical reactions that combine water with the air's carbon dioxide to form starchy carbohydrate. In other words, during photosynthesis, sunlight energy is stored in carbohydrate for later use. Here's the simplified chemical formula for photosynthesis:
6CO2 + 12H2O + sunlight ---> 6O2 + C6H12O6 + 6H2O
carbon dioxide + water + sunlight --->
oxygen + carbohydrate + water
Some carbohydrate remains in the leaf but mostly it's transported elsewhere in the plant, perhaps into the stem, or maybe into special underground storage areas, such as the potato plant's potatoes.
Don't lose sight of the fact that according to that wonderful chemical formula above, during photosynthesis leaves such as the Red Maple leaves at the left take three essentially invisible items -- carbon dioxide gas, water and sunlight -- and combine them into the substance making up the largest part of any plant you look at. Look at the biggest tree, and by golly it's mostly carbohydrate and water, and that carbohydrate has been produced by the plant from a gas and water with the process being fueled by sunlight.
Surely this is one of the most amazing facts in the Universe, but we seldom think about it.
Also, notice that when we burn wood, basically we're doing the photosynthesis formula in reverse. We're breaking down the carbohydrate and producing carbon dioxide gas and water, plus energy, which, like sunlight, feels hot and looks bright. In a real sense, first sunlight was captured in the carbohydrate, then the carbohydrate was kept for a while in the plant, and now as the plant burns the sunlight is being released again.
It's worth reflecting on the fact that if we humans were put into a room filled with nothing but carbon dioxide, we'd die pretty quickly. In the same way, if all photosynthesizing plants were removed from the Earth, before long us oxygen-needing animals would die. Therefore, when we speak of leaves photosynthesizing, we're referring to something profoundly important and special. Also, it's worth thinking about the fact that right now humans are destroying Earth's photosynthesizing plant communities, especially rainforests and algae in the oceans (agricultural herbicides and many kinds of pollution drain into the oceans) as if our lives did not depend on them...
We mustn't think that all plants do have leaves, however. Obviously microscopic algae doesn't have them. As the pictures here show, the orange-colored, threadlike, parasitic plant called Dodder (genus Cuscuta of the morning glory family) looks like orange or yellow string and it has no leaves. Dodder twines all over various kinds of plants sinking rootlike haustoria into the host plants' tissues. The host plants' nutrient-rich fluids then flow into the dodder. Since the dodder is actually robbing the host of its fluids and nutrients, dodder doesn't need leaves so that it can make its own food. By the way, the picture at the top right is a closeup clearly showing haustoria penetrating the host-plant's leaves and stem. I found this dodder alongside a local road, so this is not something exotic only to be seen in books. If you look for it, it's easy to find in most of the US.
Some plants have their leaves reduced to such small scales that they at least look leafless. That's practically the case with the Garden Asparagus shown at the right. Many desert plants bear leaves during rainy weather but then drop them when it's dry (most of the time), and this keeps water from evaporating from their leaves' surfaces. This is the case in the US's southwestern desert with several different plants going by the name of Palo Verde (Cercidium floridum, Cercidium microphyllum, Parkinsonia aculeata, of the legume family)
Article courtesy of Jim Conrad of Backyard Nature.