142 AGRICULTURAL BOTANY 



water than is resupplied from the soil, the guard cells 

 close the openings till the cells of the plant again be- 

 come filled. 



Quantity of Water Transpired. --The amount of 

 water passing off by transpiration is enormous. To 

 ripen an ordinary crop of small grain there passes out 

 through the plants between three hundred and four hun- 

 dred tons of water an acre. An ordinary hard wood 

 tree will evaporate about as much water as is evapo- 

 rated from the surface of a body of water equal to one 

 third the total leaf surface of the tree. A grass plant 

 has been found to give off its own weight of water every 

 twenty-four hours in dry, hot, summer weather. 



These large amounts of water are absorbed by the 

 roots, carried through the plant and given off by the 

 leaves because the plant food in the soil water is so 

 diluted that the plant must take up great quantities of 

 the water in order to get food enough for sustenance and 

 growth. 



Leaves take in Plant Food (CO 2 ) from the Air. The 

 food which the plant takes in through the roots con- 

 sists entirely of mineral salts in solution. But this 

 is not all the food that goes to the upbuilding of plant 

 tissue. The minerals which go to make the plant 

 substance will be found, after combustion, in the form 

 of ashes. Nearly all of the plant tissue that passes off 

 into the air during burning came from the air while the 

 plant was growing. Air has mixed with it a substance 

 absolutely necessary to the life of the vegetable world. 

 This substance, known as carbon dioxide (CO 2 ), is a 

 compound of carbon and oxygen. 



The leaves of the plant are the great absorbents of 

 carbon dioxide. It enters through the stomata into 



