300 LIQUID MANURE. 



preferable to fix ammonia ; it is more easily used, 

 and requires far less care and attention in mixing 

 with the solution containing ammonia. 



795. Animal substances must have undergone 

 decomposition before they can serve as the food 

 of plants. It has already been stated, that plants 

 appear only able to absorb nitrogen in a state of 

 combination (640) ; no form of organic matter is 

 suitable to the nourishment of a healthy plant. It 

 is the office of the leaves to convert carbonic acid, 

 ammonia, and water into organic matter, and hence 

 organic matter must be resolved by decomposition 

 into these substances, before it can be absorbed by 

 plants. 



796. There are some plants, however, which seem 

 to be exceptions to this rule, and which appear able 

 to feed on organic substances. Certain fungi and 

 parasitical plants, or those which grow upon others, 

 probably feed by directly absorbing organic matter. 

 The small fungi which constitute the various forms 

 of mouldiness are of this description ; they flourish 

 in the dark, and grow on any kind of dead organized 

 matter. All plants which have leaves, and require 

 the influence of light, feed on gaseous matter, and 

 never on organic compounds, except during the first 

 period of their growth (719). 



797. Liquid manure, consisting chiefly of urine 

 and other waste animal manures diluted with water, 

 is never so valuable in its fresh state as it becomes 

 after a time, when the organic matter is chiefly con- 



