36 AGRICULTURE 



themselves green plants use also the carbon dioxid of the 

 air, such as the animals give off. In fact, through the 

 pores or little gateways the air enters and comes in con- 

 tact with the inside cells of the leaves. These cells take 

 from the air the carbon it is holding in the form of a gas. 

 This gas is called carbon dioxid. Out of this carbon 

 dioxid gas and water the leaf cells form starch, sugar, and 

 other substances of which plants are made. 



Leaves need light. An interesting fact connected with 

 this latter use of air by plants is that the leaf cannot use 

 this carbon dioxid unless the leaf contains green coloring 

 matter. This green coloring matter usually forms only 

 when there is light. It disappears from plants when the 

 light is cut off. 



' Examine grass covered by a board or straw, or a 

 potato shoot grown in a dark room, and notice that the absence of light 

 has prevented the formation of the green color and left the leaves and 

 stems white. Such plants cannot grow to maturity because they cannot 

 feed on the carbon in the air. So if we accidentally throw dirt over all 

 the leaves of a little corn or cotton plant when cultivating the crop, we 

 cut off the light and stop the growth of the plant. This is one of the 

 ways by which the farmer kills weeds, namely, by covering them with 

 earth. 



EXERCISE. What is the color of young corn plants on wet spots 

 after a long period of wet weather ? This unhealthy color was caused 

 by lack of what ? 



In the middle of a hot, dry day what shape does a large green leaf 

 of corn take ? Do the edges roll upward or downward ? Do you 

 think this position increases or decreases the loss of water from the 

 leaves ? There are special cells in the corn whose work it is to roll 

 the leaf when water is scarce. They are different from the guard-cells, 

 but likewise help the plant to economize water. Observe whether 

 leaves are most abundant on the tips of the branches of trees or nearer 



