194 EXPElilMEXTS WITH PLANTS 



been deprived of starch in tlie way previously described. 

 Place the stopper in position and close all joints air- 

 tight with vaseline. Prepare a control in which the 

 lye is replaced by some indifferent substance, such as 

 pebbles or broken glass. After a day or so of expo- 

 sure to sunlight, test both leaves for starch. Has the 

 leaf which is deprived of carbon dioxide (but not of 

 the other constituents of the air) been able to make 

 starch ? 



The leaves, by their power of giving off oxygen, 

 "restore" foul air and make it fit for animals to 

 breathe; this is especially noticeable in aquaria where, 

 if a proper balance be struck between animal and 

 vegetable life, the air contained in the water does not 

 need to be renewed. Ordinary air contains about four- 

 hundredths of one per cent carbon dioxide. It is calcu- 

 lated that a square meter of ordinary leaf surface 

 decomposes every hour in sunlight as much carbon 

 dioxide as is contained in 1,000 liters of air. To offset 

 this, carbon dioxide is continually poured into the air 

 by combustion of all kinds as well as by the respira- 

 tion of plants and animals. We have learned (pages 

 34 and 142) that roots and germinating seeds give off 

 carbon dioxide just like animals. Leaves give off oxygen 

 only in the sunlight. Do they give off carbon dioxide at 

 other times, just as roots do ? We may easily investi- 

 gate this by partly filling a bottle or jar wdth water, 

 putting in as many leaves (with their stalks dipping in 



