LOSS OF WATER 247 



who are interested in the evaporating power of the air, and who 

 wish to compare the evaporation from purely physical evaporating 

 surfaces with that from living membranes. Atmometers do not 

 respond to changes in the environment in the same manner as 

 plants, however, since the latter are alive and do not lose water 

 in the same manner as dead membranes. The atmometers are, 

 therefore, not exact indices to the behavior of plants under similar 

 conditions. 



Amount of Water Loss.— The amount of transpiration is 

 enormous. Haberlandt has estimated that a single corn plant 

 will transpire 30 pounds of water during the growing season. 

 A single sunflower plant during its growing season may transpire 

 150 pounds, and for this reason sunflowers have been grown in 

 swampy soils for reclamation purposes. It has been similarly 

 estimated that land covered by grass or clover might evaporate 

 500-750 tons of water from each acre, that an acre of 40 apple 

 trees 30 years old might lose during the same growing season 

 600 tons of water, and that an acre of corn requires four and a 

 half tons of water for photosynthesis in addition to the 1,700 tons 

 lost by transpiration. The average leaf under mesophytic condi- 

 tions transpires in 3 months an amount equal to its area and a 

 centimeter deep. The operators of greenhouses figure on 50 grams 

 per square meter per hour by day and 10 by night, or 720 grams 

 for each 24 hours. 



The water needs of plants are thus seen to be extremely high. 

 Various plants require different amounts of water because of 

 their differences in size, environmental conditions, etc., but the 

 actual amount of water transpired is not so important as the 

 efficiency with which the water is used. This efficiency is de- 

 termined by comparing the amount of total water transpired with 

 the weight of the dry plant material which the plant produces. The 

 quotient of these two is called th£ water requirement of the plant: 



weight of total water transpired ^^ requirement . 

 total dry weight 



Using this formula, durum wheats (grown in the dry-farming 

 sections of northwestern U. S. A.) have a water requirement of 

 about 400, while for common wheats the figure is about 600. 

 This means that the former can make a gram of dry matter (or 



