Green plants are "autotrophic" (self-nourishing). With energy from the sun, water and nutrients from the soil, carbon dioxide from the air, and the help of "chlorophyll" (the green coloring matter in leaves and other parts of plants) a leaf produces "food" consisting of sugars and starches. The food then circulates throughout the plant in the sap. Some of the food is used by the plant right away for energy. Some is sent off to the roots for storage. And some is used to create more of the plant itself (i.e. roots, leaves, stems, fruit). The food manufacturing process is called photosynthesis ("photo" means light and "synthesis" means putting together).
If you tear a leaf into two parts and look closely along the tear, you should be able to see a thin, film-like layer on the underside of the leaf. If you look at a piece of this layer under a microscope, you'll see tiny openings called "stomata". Plants breathe through the stomata. During photosynthesis, plants take in carbon dioxide and give off a waste product, oxygen. Much of the oxygen animals and humans require is made by green plants. At night, when there is no light and photosynthesis cannot take place, plants give off small amounts of carbon dioxide. Humans, of course, continually breathe in oxygen and give off carbon dioxide.
Part of the problem contributing to global warming is that there is too much carbon dioxide from cars, planes, and factories going into our air. The carbon dioxide cannot be recycled quickly enough by natural cycles, so it builds up in the earth's atmosphere. More green plants will use up more carbon dioxide. So, planting trees and other plants helps our planet.
To start your explorations, collect some green leaves. Arrange them according to their shade of green (light to dark). Why are different leaves different shades? Is there a relationship between where leaves are found (e.g. top branches of a tree, on a shrub in the shade) and their shade of green?
Next, extract some chlorophyll. Place a fresh leaf in a container with a small amount of rubbing alcohol (note: dipping the leaf in boiling water first speeds the extraction). Examine the leaf after a few hours. The leaf's green coloring (chlorophyll) is in the alcohol. Try a variety of leaves.
Now, cut pieces of cardboard, each just large enough to make a "patch" on a leaf of a shrub or a tree. Use paper clips to attach patches to several leaves. After four days to a week, remove the patches and examine the lighter-colored spots on the leaves. The patches prevent light from reaching the spots and photosynthesis cannot take place.
A plant gets much of its carbon dioxide through the stomata. Another important part of a leaf's surface is the cuticle, a waxy coating. The cuticle is waterproof, keeps moisture inside the leaf, and is transparent so that the sun can shine through. Cover a leaf with petroleum jelly. The jelly blocks the stomata but, like the cuticle, allows sunlight through. What happens after a few days? Does this happen to leaves at other times of the year?