It's not surprising that early scientists hypothesized that plants ate dirt. They didn't know, as we now do, how energy-rich sunlight is. Still, it seems remarkable that plants have evolved photosynthesis--the ability to harness the sun's energy to produce their own food.
Photosynthesis is the process by which plants transform water and carbon dioxide (a gas that's plentiful in the air) into carbohydrates (sugars and starches), using the energy of sunlight. While sunlight provides the energy needed to drive this reaction, a chemical in the leaves of plants makes the reaction possible. That chemical is a green pigment called chlorophyll. Chlorophyll is found inside the photosynthetic cells of plants, attached to the membranes of small, round structures called chloroplasts. Chlorophyll absorbs light in the red and blue-violet portions of the visible spectrum, and reflects the green portion of the spectrum; this is what gives chlorophyll its characteristic green color.
As remarkable as photosynthesis is, the process is not very efficient. Studies show that prairie grasses in the western United States are some of the most efficient plants at harnessing the sun's energy, but even they capture little more than about 3 percent of the energy that reaches the prairie surface. The rest of that energy is reflected away, absorbed by humidity in the air or by the ground, or simply lost in myriad other ways before the plants can use it.
One of the most critical factors influencing the efficiency of photosynthesis is the amount (intensity and duration) of light that hits a leaf. Generally, the more light that strikes a leaf, the greater the rate of photosynthesis in that leaf. For example, a leaf that is exposed to direct sunlight will photosynthesize at the highest rate, while a leaf directly beneath it and in its shadow will photosynthesize at a much lower rate. Because of this, many plants have evolved leaf and branch structures that minimize overlap and shading, and thus maximize the plant's overall rate of photosynthesis.