PLANT FOOD 



67 



the roots. Only soluble materials can pass through mem- 

 branes by osmosis. 



64. Importance of Water. Water not only constitutes 

 about nine-tenths of the growing plant, but it acts as the 



'\carrier of all the other food materials except the carbon. 



The enormous amount of water that passes through a 



plant in bringing the food from the soil was determined 



in Wisconsin by King, and is shown in the following 



table: 



AMOUNT OF WATER LOST BY TRANSPIRATION AND EVAPORATION FOR 

 EACH TON OF DRY MATTER IN THE CROP 



Corn 310 tons, equal to 2.64 inches rainfall 



Red clover 453 tons, equal to 4.03 inches rainfall 



Barley 393 tons, equal to 3.43 inches rainfall 



Oats 522 tons, equal to 4.76 inches rainfall 



Potatoes 423 tons, equal to 3.73 inches rainfall 



In producing a ton of clover hay, which is 85 per cent 

 dry matter, 375 tons of water are lost by transpiration 

 and evaporation. It will be seen that oats require more 

 water than any other 

 crop, a fact that is 

 observed by farmers. 



65. How the Plant 

 Gets Its Food from the 

 Air. Over half the 

 dry matter of a plant 



\a r>nrVnn Thp> small Fio. 40. Section of a leaf showing the 



breathing pores and intercellular spaces. The 



amount of carbon di- 8ma11 dota are chloro P h y' 1 *- 

 oxid in the air, about three parts in ten thousand, fur- 

 nishes all the carbon. With the air, the carbon dioxid 

 passes into the intercellular spaces through the breathing 

 pores, stomata, of the leaves. (Fig. 40.) When it is in 



