2 g SCIENTIFIC AGRICULTURE. 



ments enter into the structure of plants, and whether 

 they are all really necessary for plant-growth. The 

 results of these investigations, as given in the last 

 section, show what sort of food plants must have in 

 order to grow. Aluminium, an element found in all 

 soils and the base of all clay, does not enter into 

 plants. As animals derive their food from plants or 

 from each other, the same elements, except silicon, 

 enter into the composition of both. 



52. If a plant be heajted up to the boiling-point 

 of water, it loses a large part of its weight by the 

 escape of water which it contains, and becomes dry. 

 In turnips and cabbages, nine-tenths, and in potatoes 

 three-fourths of their weight is water which can be 

 driven off by heat. Even in cured hay and fodder, 

 from one-sixth to one-tenth is water. If the dried 

 plant be exposed to a red heat, it will take fire and 

 the greater part be consumed. The small portion 

 left is called the ash, and is generally a fine white 

 or gray powder (26). 



53. The portion of a plant that burns is chiefly 

 carbon combined with some hydrogen and oxygen, 

 and a little nitrogen. These four elements are some- 

 times called organic elements, because they form by 

 far the larger part of organic bodies. In the process 

 of combustion, while these elements disappear, it 

 must not be supposed that they are destroyed. They 

 merely enter into new combinations with the oxygen 

 of the air, forming chiefly carbon dioxide and watery 

 vapor, which float off unseen in the atmosphere. 



