PHOTOSYNTHESIS PART-3 ( JOSEPH PRIESTLEY'S BELL JAR EXPERIMENT, & JAN INGENHOUSZ EXPERIMENT)

NEET-BIOLOGY , NCERT-BIOLOGY,
EARLY EXPERIMENTS
It is interesting to learn about those simple
experiments that led to a gradual development in
our understanding of photosynthesis.
Joseph Priestley (1733-1804) in 1770
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
1774. Priestley observed that a candle burning in
a closed space – a bell jar, soon gets extinguished
(Figure 13.1 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 (1730-1799)
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.