According to: http://www.appalachianwood.org/forestry/treegrow.htm
"In order for any seed to sprout and develop it must have food, water, sunlight and warmth. Until it gets roots, a stem above ground, and some leaves or needles, the seed uses the food stored in its shell to develop growth. As roots go deeper into the soil they absorb water and minerals form the soil and send these up into the stem.
The leaves act as a chemical laboratory. In a process called photosynthesis, which takes place in the green leaves of a tree in the presence of chlorophyll and sunlight, the tree takes carbon dioxide from the air and manufactures starches and sugars and gives off oxygen as a waste product. The formula is simple but the process has never been artificially duplicated.