THE SOURCES OF PLANT-FOOD. 4I 



as necessary as those elements that enter largely 

 into their composition. One ton of hay contains 

 about thirty pounds of nitrogen, a small quantity 

 proportionally, but in one hundred tons it amounts 

 to a great deal. The stimulating effect of guano and 

 similar manures is due mainly to the nitrogen they 

 contain in the form of ammonia and its salts. While 

 the atmosphere may contain ammonia enough to 

 meet the ordinary demands of vegetation, it has not 

 enough to supply the extraordinary demand of an 

 immense crop in a limited time. 



88. The four elements just mentioned are some- 

 times called organic elements, because they form 

 much the larger part of all organic bodies. The re- 

 maining constituents, called inorganic elements, are 

 found in the ash of plants, and are always taken 

 in as soluble matter through the roots. The question 

 arises, In what manner? Before answering this ques- 

 tion, it will be well to examine the structure and use 

 of the different parts of a plant. 



89. The main parts of a plant are the root, the 

 Stem, and the leaves (29). The root spreads through 

 the earth as the stem does through the air. The 

 seed contains an initial steml that is, the beginning 

 of a stem, with food enoughi^Kart it to growing. It 

 requires for germination or fir^growth, air, warmth, 

 and moisture. The moisture which is first absorbed 

 causes the seed to swell, oxygM^enters, a chemical 

 change takes place in the elenrcnts composing it 

 whereby the germ, or little plant, gradually enlarges, 



