. E ARLY E XPERIMENTS It is interesting to learn about those simple experiments that led to a gradual development in our understanding of photosynthesis. Joseph Priestley ( - ) in performed a series of experiments that revealed the essential role of air in the growth of green plants. Priestley, you may recall, discovered oxygen in .
Priestley observed that a candle burning in a closed space – a bell jar, soon gets extinguished (Figure . a, b, c, d). Similarly, a mouse would soon suffocate in a closed space. He concluded that a burning candle or an animal that breathe the air, both somehow, damage the air.
But when he placed a mint plant in the same bell jar, he found that the mouse stayed alive and the candle continued to burn. Priestley hypothesised as follows: Plants restore to the air whatever breathing animals and burning candles remove. Can you imagine how Priestley would have conducted the experiment using a candle and a plant? Remember, he would need to rekindle the candle to test whether it burns after a few days.
How many different ways can you think of to light the candle without disturbing the set-up? Using a similar setup as the one used by Priestley, but by placing it once in the dark and once in the sunlight, Jan Ingenhousz ( - ) showed that sunlight is essential to the plant process that somehow purifies the air fouled by burning candles or breathing animals. Ingenhousz in an elegant experiment with an aquatic plant showed that in bright sunlight, small bubbles were formed around the green parts while in the dark they did not. Later he identified these bubbles to be of oxygen.
Hence he showed that it is only the green part of the plants that could release oxygen. (a) (c) (b) (d) Figure . Priestley’s experiment It was not until about that Julius von Sachs provided evidence for production of glucose when plants grow. Glucose is usually stored as starch.
His later studies showed that the green substance in plants (chlorophyll as we know it now) is