If you were asked to guess the number of all trees in the world, what would your answer be? 10 million? 250 million? 250 billion? No, none of them is the answer. According to the latest researches, there are a total of 3.1 trillion trees in our world. And now think about the number of leaves on all these trees. An enormous number comes up. The leaves in all these trees are working ceaselessly to maintain the oxygen - carbon dioxide balance in the atmosphere.
References:
Nature, 10 September 2015, Vol. 525, p. 201
National Geographic, 3 September 2016, http://phenomena.nationalgeographic.com/2016/03/09/the-earth-has-lungs-watch-them-breathe/