(photosynthesis was still not known to scientists at that time). You may recall, Oxygen was discovered by Priestly in 1774. The name oxygen was coined later by Lavoisier in the year 1775. Priestly observed that a candle burning in a closed bell jar, soon gets extinguished. Similarly, a mouse would soon suffocate in a closed space of the bell jar. He concluded that a burning candle or an animal, both somehow, damage air. When he placed a mint plant in the same bell jars, he found that the mouse stayed alive and the candle when lighted from outside continued burning in the presence of the mint plant. Priestly hypothesized as follows: ‘Plants restore the air what breathing animals and burning candles remove’.

Do you find any relationship between candle, rat, mint plant ? Discuss.

Do similar experiment in your school. Use a candle, plant in a small

pot and a bell jar. Note down your observations and explain. Priestly’s experiment confirmed that gaseous exchange was going on and plants were giving out a gas that supported burning and was essential for the survival of animals.

But how do plants take in air and utilize carbon dioxide for photosynthesis and oxygen for respiration?

How do they make the choice? Massive amounts of gaseous exchange occur through the stomata (usually present in leaves) as long as they are open .While plants also carry on gaseous exchange through loose tissues in the stems, roots etc.

It is actually at the level of the organelles involved in the process of photosynthesis and respiration that the choice of the gas required is made.

Activity 4

Carbondioxide is necessary for Photosynthesis