HOW PLANTS FEED 



49 



CARBONIC ACID 



WA 



must be carried to all parts of the plant. Likewise, the 

 substances brought in from the roots must be taken up- 

 wards into the leaves. To perform the duties two cur- 

 rents are required : one to carry the product secured by 

 leaves downward, that 

 needed food may go to 

 the roots ; and another 

 to carry the root acqui- 

 sitions upward, that 

 leaves may not be neg- 

 lected. 



This explanation dis- 

 proves the old notion 

 that sap goes up in the 

 spring and down in the 

 autumn. This sap wa- 

 ter, when taken up by 

 the root hairs, is in a 

 very dilute state. The 

 minerals and nitrates 

 dissolved therein are 

 carried up into the 

 leaves, but left behind 

 when the water is evap- 

 orated into the air. 



When the summers 

 are dry and hot and 

 there is but little water 

 in the soil, the leaves 



shrink up. This is simply a way they have of keeping 

 the water from passing too rapidly off into the air. This 

 withering is a wise provision after all, for when the plant 

 closes the mouths or pores of the leaves, evaporation is 

 checked until the roots can secure a supply from the soil 



WATER AND 

 MINERALS 



HOW THE SAP CURRENT MOVES 



