EFFECT OF BORDEAUX MIXTURE ON TRANSPIRATION 69 



Up to the present time, investigations dealing with the prob- 

 lem of the influence of surface films on rates of transpiration 

 have been confined almost etirely to experiments with detached 

 plant parts and with potted plants. Of the several methods 

 available for the determination of the rate of water loss from 

 plants or plant parts for any given set of conditions, the one 

 most frequently employed by workers in this field is that of 

 weighing the plants and their containers at stated intervals 

 and determining the water loss in weight for each interval. The 

 methods employed for the quantitative determination of water 

 loss by transpiration can not be used with plants rooted in the 

 soil under natural conditions. For such plants, therefore, abso- 

 lute transpiration quantities cannot be determined. 



The ability of plant leaves to give off moisture to their sur- 

 roundings has been called their transspiring power. ^ This foliar 

 characteristic can be measured directly through the power of 

 the leaf surface to give off water to a standard water-absorbing 

 surface. If the surrounding conditions remain the same and 

 the degrees of the transpiring power vary from time to time, 

 these variations must be due to internal conditions, and the 

 various degrees of the transpiring power should be directly 

 proportional to the rates of water loss. It has been pointed 

 out that the quotient of the transpiration rate for a given time 

 period divided by the evaporation rate from some standard 

 evaporating surface for the same period, should be a measure 

 of the effective internal conditions of the plant in question. The 

 quotient here referred to has been called by Li\dngston^ the rela- 

 tive transpiration ratio, and corresponds to the transpiring 

 power, which may be measured by m.eans of hygrometric methods. 



Two such methods have been devised, by means of which 

 the relative transpiring power of leaves may be measured. The 

 two methods are the same in principle, but differ in the kind 

 of materials composing the absorbing surfaces. The one em- 



^ Livingston, B. E., The resistance offered by leaves to transpirational water 

 loss. Plant World 16: 1-36. 1913. 



'^ Livingston, B. E., The relation of desert plants to soil moisture and to evapo- 

 ration. Carnegie Inst. Wash. Publ. 50. Washington. 1906. 



