54 EXPERIMENTAL PLANT PHYSIOLOGY 



hydrogen, and oxygen. Hydrogen and oxygen, to- 

 gether, form water, which is taken in from the soil by 

 the roots of the plant. But it has already been shown, 

 by the use of culture-solutions, that the plant is not 

 dependent on the soil for its carbonaceous food (page 

 13). It follows, therefore, that a plant must obtam 

 carbon in some form from the air. 



The air, it will be remembered, is made up of oxygen, 

 nitrogen, carbon-dioxide, and water-vapour, together 

 with various rare gases and impurities in minute 

 quantities. The only possible source of carbon is, 

 therefore, the carbon-dioxide (a compound made up of 

 carbon and oxygen) which is present in the air. 



The importance of the element carbon is very great. 

 It is essential to all living bodies. 



It is not easy to determine practically by any direct 

 method whether the plant absorbs the carbon-dioxide 

 of the air. The reason for this will be understood 

 when the section on the breathing of the plant has 

 been studied. An indirect method will therefore be 

 adopted, and an experiment will be carried out to 

 ascertain if starch continues to be formed in an atmos- 

 phere that is deprived of carbon-dioxide. 



There are three substances that have the power of 

 absorbing carbon-dioxide. These are : 



1. Lime-water. 



2. Caustic potash solution. 



3. Soda-lime. 



In any particular case the substance that is most 

 suitable must be selected. For instance, if the aim is 

 to show that carbon-dioxide is being absorbed, then 

 lime-water should be used because the absorption of 

 carbon-dioxide is quickly demonstrated by the milky 

 reaction given. When, however, the aim is the com- 

 plete absorption of all carbon-dioxide present, then a 

 strong solution of caustic potash is more suitable. 

 Again, soda-lime can be used when a solid substance is 

 required. 



