84 PLANT PHYSIOLOGY 



shown that solutions of certain salts poisonous to plants 

 become innocuous upon the addition of certain other 

 salts which of themselves may also be poisonous. This 

 discovery has thrown doubt upon many of the con- 

 clusions of earlier botanists as to the functions of salts 

 that are supposed to be essential to plant life. 



122. So far we have merely considered what sub- 

 stances are required by the plant and something of the 

 form in which the plant takes them in. Before they can 

 be used they must undergo various decompositions and 

 recombinations; in other words after absorption there 

 must be assimilative processes. Perhaps the most funda- 

 mental of these processes is that by which the carbon 

 compounds are built up by green plants, a process called 

 photosynthesis. 



123. Photosynthesis. The green parts of all chloro- 

 phyll-bearing plants absorb carbon dioxide from the 

 surrounding water if aquatic plants, or from the air, which 

 contains about three parts of it to ten thousand. This 

 absorption goes on only when the plant is exposed to the 

 light. At the same time there is an increase in the 

 amount of carbohydrates often manifesting itself to the 

 eye by the formation of starch grains in the chloroplasts, 

 but also demonstrable chemically by the increased 

 amount of sugars (chiefly glucose C 6 Hi20 6 ) in the cell 

 sap. At the same time it can be demonstrated that 

 oxygen is given off by the plant. It is this process, the 

 manufacture of carbohydrates by green plants in the 

 presence of light, that has received the name photo- 

 synthesis (from the Greek meaning " putting together 

 by light")- 



124. Careful experiments have shown that this 

 process cannot occur in the absence of any one of the 

 factors mentioned in the preceding paragraph. Thus a 



