Magnitude of Transpiration 49 



greatest economy is everywhere practiced in regard to the use of 

 water. 



If it were true that no water need be used by plants except 

 that which is assimilated during their growth and reproduction, and 

 in keeping the cells distended and turgid, so that wilting shall 

 not occur, then there would be little need for irrigation anywhere 

 except in the most arid of arid regions, for then even the hygro- 

 scopic moisture of a dry soil would be sufficient in quantity to 

 supply the demands of almost any land plant. 



The facts are, however, that during the hours of sunshine all 

 growing plants which feed directly upon soil and air must have 

 their assimilating chlorophyll-bearing cells continually in contact 

 with a changing volume of air, in order that the carbon, which 

 makes up so large a part of their dry weight, may be obtained in 

 sufficient quantity from the carbonic acid gas in the atmosphere. 

 But the more recent analyses of air show that on the average it 

 contains but one part of carbonic acid by weight in 2,000 parts. 

 Now, how much air must a field of clover breath in order that 

 it may produce two tons of hay per acre ? Let us see. 



Boussingault found by analysis that 4,500 pounds of clover 

 hay harvested from an acre of ground contained no less than 1,680 

 pounds of carbon, and as this was derived almost wholly from the 

 carbonic acid of the air, it must have decomposed 6,160 pounds 

 of carbonic acid in order to procure it. But as there is only 

 one pound of carbonic acid in 2,000 of air, it follows that 

 12,320,000 pounds of air must have yielded up the whole of its 

 carbonic acid gas in order to supply the needed amount of carbon. 

 Now, one cubic foot of air at a pressure of 29.922 inches and 

 at a temperature of 62 F. weighs .080728 pounds, and this being 

 true, not less than 152,600,000 cubic feet of air must have been 

 required to meet the demands of this clover field for carbonic 

 acid. This amount of air would cover the acre to a depth of 

 3,503 feet, having a uniform normal density. 



Of course, not all of the carbonic acid in the air which 

 passes across a clover field can be secured, nor indeed all of 

 that which enters the intercellular air passages of the green 

 parts of the plant, and hence it follows that very much larger 



