196 Bureau of Farmers' Institutes. 



quantities in which they are present in plants. Thus, the air-de- 

 rived elements constitute, at least, 95 per cent, of the whole 

 vegetable kingdom, while the soil-derived elements occur in small 

 quantities, varying from a fraction of one per cent, up to 10 per 

 cent., or even more in some cases. Because the soil-derived ele- 

 ments occur in small quantities, it does not follow that their 

 presence is of less importance; in their absence, vegetation would 

 disappear. 



This fact has a most important application in enabling us to 

 control our yield of crops. Man cannot, to any appreciable ex- 

 tent, at least economically, control directly most of the air- 

 derived elements in feeding plants; but he can do so indirectly 

 through the soil-derived elements. In other words, by looking 

 after less than five per cent, of the materials used by plants as food, 

 he can, to a large extent, control the other 95 per cent., other 

 conditions being favorable. 



We will now consider some of the properties of the different 

 elements that enter into the composition of plants. 



Carbon. 



(a) Importance. — The element carbon may be called the cen- 

 tral element of all animal and vegetable substances; for there 

 is not a living thing, from the smallest cell to the giant tree, which 

 does not contain carbon as a necessary constituent. That all vege- 

 table and animal substances contain carbon can easily be shown by 

 simply heating them sufficiently, and thus causing them to blacken 

 or char. When, for example, wood is heated, the different ele- 

 ments of which it is composed are driven off in one form or an- 

 other, but the carbon is the last to go, and remains behind as a 

 black substance or charcoal, unless heated higher, when it dis- 

 appears or burns up. 



(b) Source. — Carbon usually occurs in nature united in com- 

 pounds with other elements. Thus, most products of .plant life 

 contain carbon combined with the elements hydrogen and oxygen; 

 such are starch, sugar and cellulose or woody fiber. Carbon com- 

 bined with oxygen occurs in the air in the form of carbon dioxide, 

 commonly called carbonic acid gas. Carbon, when combined with 

 oxygen and some elements such as calcium, occurs in the form of 

 carbonates; for example, marble, limestone and chalk are chemi- 

 cally known as calcium carbonate or carbonate of lime. 



Carbon by itself or in the free condition, that is, not united 



