34 AGRICULTURE 



phosphates and potash and furnish these freely to his crops, 

 and yet have them fail because one of the other elements 

 may be lacking. 



These precious elements and all other materials that go 

 from the soil into the plant must first be dissolved in water. 

 This solution of plant-food is then drawn into the cells of 

 the plant and carried up through the stem, the greater 

 part directly to the leaves, where it is used and mixed with 



the substances the leaves have 

 made from the carbon of the 

 air. The result of this diges- 

 tion and mixing is a liquid 

 called sap. This sap is slowly 

 distributed to all parts of the 

 FIG. 28. SHOWING HOW A CUT EDGE plant, to be used in enlarg- 



OF A LEAF LOOKS WHEN HIGHLY MAG- - and making new JeaVCS, 



NIFIED 



A little of the under surface is shown StemS > r ts > flowers, and 

 and in it the " gateways " or entrances seeds. 



Roots help to dissolve soil. 



Roots give off small amounts of a weak acid which dis- 

 solves more of the plant-food in the soil that the roots 

 touch than pure water can. This food dissolved by acid is 

 added to that already dissolved from the soil by water, and 

 the mixture is drawn into the plant and started towards 

 the leaves. The leaves prepare this solution still further 

 and make it into sap before it can nourish and build up 

 the plant. 



Plants need air. You have doubtless learned in study- 

 ing physiology how persons and animals breathe. They 

 take the air into the lungs and the oxygen gas which it 



