OF AGRICULTURE. 159 



CHAPTER XX. 



RECAPITULATION OF SOME OF THE MOST IMPORTANT 

 FACTS. 



When a vegetable substance is burned the mass of 

 it disappears, taking the form of gases and escaping 

 into the air, and a small residue remains, termed ashes. 

 Now, when plants grow, they draw back again from 

 the air all those gases which escape into it by 

 combustion, and obtain from the soil only those 

 mineral solids which form ashes. The great bulk of 

 vegetable matter is derived from the air, and as the 

 atmosphere is uniform in composition, that portion 

 of the nutrition of plants which depends on this 

 source may go forward in all places with nearly 

 equal facility. 



The atmosphere contains an exhaustless store of 

 elements for the use of vegetation, and as far as it 

 alone is concerned, all plants may be grown with 

 equal success in all places. But other agencies step 

 in and say no. As, for instance, heat and light, which 

 radiate from the sun. In consequence of the almost 

 round shape of the earth, these rays fall unequally 

 upon its different parts. At the equator, where the 

 rays are perpendicular, the heat and light are more 

 intense, while we pass toward the poles the rays 

 strike the earth more obliquely, and the heat is not 

 as great. Now, to the variations of temperature 

 plants are adapted. Equatorial vegetation requiring 

 largp qualities of heat and light, cannot flourish in 

 temperate climates, for although the air and soil may 

 contain all the chemical elements necessary to its 



