Over 300 years ago, Jan-Baptista van Helmont, a Dutch physician, studied how a plant gains weight. Up to the time of his experiments, people believed that plants obtained their weight from the soil itself. Van Helmont measured the growth of a willow tree for several years. That tree gained more than 70 kilograms, yet reduced the weight of the soil by less than 60 grams. Van Helmont thus disproved the idea that plants get their weight from soil, but he attributed the growth of the willow tree to the water he added to the soil. This conclusion was partially correct because most of a plant is water. However, only later did botanists discover that plant growth also depends on carbon dioxide from the air and several other minerals that, like water, are absorbed from the soil and are essential for growth.
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