The process by which plants convert light energy into stored chemical energy, known as photosynthesis, is fundamental to life on Earth. The understanding of this complex process was not the result of a single discovery, but was built slowly over centuries through the meticulous work and innovative experiments of pioneering scientists. These researchers progressively solved the mystery of how plants grow and interact with the atmosphere, revealing the intricate mechanism of energy conversion that sustains nearly all ecosystems.
The Initial Question of Plant Growth
For centuries, the prevailing belief was that plants derived their entire mass directly from the soil. This hypothesis was first challenged in the 17th century by the Flemish chemist Jan Baptista van Helmont.
Van Helmont sought to determine the true source of a plant’s physical mass through one of the earliest quantitative experiments in biology. He planted a five-pound willow sapling in 200 pounds of dried soil. Over a period of five years, he watered the plant only with rainwater, taking steps to prevent airborne dust from entering the soil. The willow tree grew significantly, increasing its weight by 164 pounds to approximately 169 pounds.
When Van Helmont weighed the soil again, he found that its mass had decreased by only about two ounces. Since the massive increase in the tree’s weight could not have come from the minimal loss of soil, he concluded that the entire bulk of the new wood, bark, and roots must have been formed from the water alone. Although he failed to account for gases, his experiment definitively proved that plants do not simply “eat” soil to grow, shifting the scientific focus toward other inputs.
Establishing the Role of Air and Light
Following the realization that plant mass did not come solely from the soil, the focus shifted to the role of the surrounding air. In the 1770s, the English chemist Joseph Priestley conducted experiments demonstrating the exchange between plants and the atmosphere. He observed that a candle burning in a sealed jar would quickly extinguish, and a mouse placed in the same jar would suffocate, concluding that both combustion and respiration depleted the air.
Priestley then placed a sprig of mint plant into the sealed jar. After several days, he found that the plant had the ability to “restore” the air, allowing the candle to burn brightly again and the mouse to survive. This proved that plants were releasing a substance that supported combustion and animal life, a gas later identified as oxygen.
The Dutch physician Jan Ingenhousz built upon Priestley’s findings, recognizing that the results were not always consistent. Ingenhousz discovered that the air-restoring process only occurred when the plant was exposed to light. He observed that submerged aquatic plants released bubbles (oxygen) only in the presence of sunlight, and the process stopped in the dark. Furthermore, Ingenhousz determined that this restorative power belonged only to the green parts of the plant, such as the leaves. These experiments established the requirements of light and green plant matter for the production of oxygen.
Defining the Chemical Inputs and Energy Conversion
The early 19th century brought a more quantitative understanding of the inputs involved in the reaction. The Swiss chemist Nicolas-Théodore de Saussure conducted precise measurements, showing that the increase in a plant’s dry mass could not be accounted for by the water absorbed alone, a finding that corrected Van Helmont’s earlier conclusion. De Saussure demonstrated that plants absorb carbon dioxide from the air, and that this gas was a necessary component for the formation of new plant matter.
His work showed that the total weight increase of the plant was greater than the weight of the carbon dioxide absorbed, confirming that water was also an essential reactant in the process. De Saussure’s quantitative approach established that the plant’s gain in mass was a combination of water from the soil and carbon from the atmosphere.
The final major conceptual piece of the puzzle came from German physicist Julius Robert Mayer in the mid-19th century. Mayer was one of the first to articulate the law of conservation of energy, and he applied this principle to plants. He proposed that the energy captured by plants during photosynthesis was not created but was instead transformed. Mayer correctly stated that plants convert the radiant energy of the sun into stored chemical energy within their tissues.