Soil gets too much credit. Although roots disappear into the ground, most of the material in a plant's dried leaves, stems and trunk originates in carbon dioxide drawn from the air, rather than earth swallowed underground. The distinction matters enormously. A growing tree is less an excavation project than an accumulation of atmospheric carbon.
Leaves do the building. Through pores called stomata, carbon dioxide enters tissues where photosynthesis uses light energy to support the manufacture of sugars, with water supplying electrons and hydrogen. Carbon fixation makes this possible. In the Calvin cycle, the enzyme Rubisco helps incorporate carbon dioxide into organic molecules that can become sugars, which supply material for cellulose, starch and other compounds. Sunlight supplies energy, not matter. As these compounds accumulate, the plant gains dry mass, even though much of its fresh weight consists of water that drying removes.
Roots deserve a narrower credit. They absorb water and mineral nutrients, including nitrogen, phosphorus and potassium, without which the machinery that captures carbon and builds tissues cannot keep working. Small contributions can constrain growth. Carbon typically accounts for roughly forty-five percent of plant dry mass, while oxygen and hydrogen make up much of the remainder; water also contributes atoms, so air alone cannot claim every gram. Growth has its deductions. Plants release some captured carbon through cellular respiration, and only the balance retained in tissues adds to their standing biomass. The trunk holds the evidence. Beneath its bark, a tree stores carbon that once drifted past its leaves.
