Food from the air

Everyone has seen plants growing, but have you ever thought     1     they get their food? In the seventeenth century, a European scientist     2     (call) Van Helmot asked this question. Like most people, he thought that plants must get their food from soil. However, Van Helmot decided       3     (test) the theory with experiments.

First, he dried some soil, put it into a pot and weighed it. Next, he weighed a small tree, planted it in the pot and added rain water. Then, he watered it     4     (regular) with rain water.

After five years, he removed the tree from the pot and weighed it again. He found that the tree had gained     5     huge amount of weight. When he weighed the soil, however, it was almost exactly the same as it had been five years    6    . So Van Helmot drew the    7    (conclude) that the tree grew by drinking water. Though it turned out to be wrong, he showed the importance of the use of scientific evidence to support ideas.

We now know that plants and trees make their own food. Their leaves, when exposed     8     the air and sun, are like factories    9     can change the energy from the sun into chemical energy. During this process (过程), oxygen and sugar     10     (produce). The oxygen is released back into the air, and the sugar is used by the plant as food.