IDEAL FERTILIZERS 57 



Carbonic Acid Gas -Carbon is the element which is 

 most abundant in the dry matter of vegetation, its 

 amount being about equal to all others combined. Though 

 carbon dioxid exists in great quantities in all fertile soils 

 and has a far-reaching influence on fertility, the carbon 

 used by plants is taken from the carbonic acid gas of the 

 air. This is absorbed by the leaves and other green tissues 

 and through the action of the green matter (chlorophyl) 

 under the influence of sunlight is combined with the 

 hydrogen and oxygen of water to make starch, sugar, 

 acids, oils, gums, etc., and cellulose the substance in cell 

 walls. This process of the formation of plant food from 

 carbon dioxid and water and the throwing off of the un- 

 used oxygen is called "photosynthesis." It is the reverse 

 of respiration, and since the plant's activities are much 

 greater in photosynthesis than in respiration, vegetation 

 removes from the air far more carbon dioxid than it 

 gives off, and gives off far more oxygen than it uses, 

 thereby purifying the air for animal life, which, on the 

 contrary, uses oxygen in great quantities and gives off 

 carbonic acid gas. 



Water 



It has been seen that the elements of water play an im- 

 portant part in the formal ion of different vegetable com- 

 pounds, but the uncombined water in plants is equally 

 important. The rigidity of tender vegelalion is due to 



