HOW A PLANT FEEDS. 
73 
6. And there is another reason why plants need 
air. They obtain oxygen from it as well as carbonic- 
acid gas, and they cannot live without oxygen any 
more than we can. That is to say, the plants — 
in addition to taking in carbonic-acid gas during 
sunlight, as already described — breathe oxygen in 
Enlarged views of Leaf-surfaces, showing the pores through which 
the plant breathes. 
and give out carbonic-acid gas; and this they do in 
the darkness as well as in the light. But the 
quantity of carbonic-acid gas thus given out is very 
much less than the amount taken in during sun- 
light. 
7 . How wonderful it is that from lifeless matter 
gathered from the air and soil the parts of a living 
plant can be formed! Plants are the world’s great 
manufacturers of the food upon which animals 
depend; for, as you know, we and all other animals 
