LET us CONSIDER first the process of photosynthesis—that is, the building with the energy of light. In this process, organic matter is formed literally from thin air and water. The air contains minute amounts of carbon dioxide (0.03 percent by volume or three parts in 10,000 parts of air). Through millions of small pores, or stomata, on the leaf surfaces, air penetrates the leaves and gives up about 10 percent of its meager supply of precious carbon dioxide to the tree. In the leaf cells are found small particles called chloroplasts; these contain a green substance, chlorophyll, similar in structure to the hemoglobin of the blood. In fact, in reflected light chlorophyll appears not green but blood red.
Carbon dioxide unites with the chlorophyll and in a chain of reactions, regulated by the enzymes, it combines with oxygen and hydrogen of water to form sugar. An excess of oxygen is released in this process. The energy that is needed for transformation of carbon dioxide and water into the organic substance (sugar) is supplied by sunlight. Only about 1 percent of the solar energy that falls on a leaf is used for photosynthesis. The sugar formed in the process of photosynthesis is dextrose. From it 95 percent of the body of the tree is ultimately made by a series of complicated reactions. Dextrose may be converted into other sugars or it may be combined with nitrogen to form the amino acids, the building blocks from which proteins are made and on which all life, both plant and animal, depends. Part of the dextrose is also used for other purposes, such as conversion into starch, fats, and other substances.
The most favorable conditions for photosynthesis are mild temperatures (about 70° F.) and diffused, moderate light. On hot, bright, summer days the efficiency of photosynthesis goes down. An ample supply of water is essential. When the soil is dry and not enough water is delivered to the crown, the rate of photosynthesis declines. Fertility of the soil is also important, for the building of the tree body requires an ample supply of mineral elements.
