66 FIRST STUDIES IN PLANT LIFE 



7. In making starch, the leaf does two good things 

 for us ; it uses up the carbon that was poisoning the 

 air, and it sets free the oxygen which is life-giving to 

 man. No one has eyes sharp enough to see the 

 oxygen that is passing continually into the air from 

 every leaf that is at work in the sunlight ; but you can 

 easily see the oxygen that passes off from a water- 

 plant. 



8. Experiment to shew that leaves give off 

 oxygen. Take a leafy water-plant in full growth and 

 place it in a glass bottle. The glass must be clear and 

 the water clean. Place the bottle where it will get 

 the sunlight, and notice how the bubbles of gas collect 

 on the leaves and how they grow in size till they 

 break away and rise to the surface. If you place one 

 of the plants upside down with the cut stem below the 

 surface of the water, you may see the bubbles of gas 

 coming freely from the cut end.* This gas has been 

 tested, and found to be oxygen. When the plant is 

 removed into a duller light, the bubbles do not rise so 

 fast. What does this mean ? On a cloudy day, too, 

 less gas is thrown off than on a bright day. When 

 the plant is put in a place where there is no light at 

 all, no bubbles are made. What does this prove ? 



9. We saw that, on the under-skki of every leaf, 

 there are thousands of little doors called breathing 

 pores. Through these there escapes all the water that 

 has passed through the plant. No eye is sharp enough 

 to see the water escaping ; for it passes out into the air 

 in the form of an invisible vapour. But by an easy 

 experiment you can make the water visible. 



* If the water be boiled, the carbonic acid gas in the water is driven off. 

 There is then no food for the leaves, and the bubbles of oxygen cease to 

 form. 



