CLIMATIC FACTORS : THE AIR 83 



soon become prohibitive if a comprehensive study is to be made. 



Relative rates of transpiration can be determined by the cobalt- 

 chloride method, which is rapid and permits numerous determina- 

 tions in a short time. Paper treated with cobalt-chloride is blue 

 when dry and turns pink as it takes up moisture. Small squares of 

 the dry paper can be attached to leaves between small glass plates 

 by means of a wire clip. The time required for the paper to turn 

 pink is taken as a basis of comparison. To get comparable values, 

 the paper must be absolutely dry and care must be taken that the 

 clip fits snuggly to the leaf. For increased accuracy, standard color 

 strips are usually attached to the glass to be used for comparison 

 with the sensitized strip. In spite of its simplicity, the method has 

 definite limitations. The close-fitting clips exclude all outside air 

 and thus eliminate air movement as a factor, while at the same 

 time diffusion into the air is practically stopped by the glass. Thus 

 the measurement is perhaps an indicator of the moisture in the in- 

 ternal atmosphere of the leaf. Rarely will two leaves on a plant 

 give identical readings, for their water loss varies with their posi- 

 tions on the plant and their ages. Thus several determinations must 

 be made simultaneously to evaluate a single plant, while to compare 

 this plant with others necessitates a considerable number of read- 

 ings. In spite of these limitations, it should not be assumed that the 

 method is ineffectual, for under certain conditions, it has been used 

 to great advantage. 



These methods have their greatest utility in intensive studies of 

 a few or of individual plants under experimental conditions in the 

 laboratory or field. In studies of communities, it is often desirable 

 to have a more generalized picture of transpiration conditions. 

 Under those conditions, the rate of evaporation may be more use- 

 ful than a limited number of measurements of transpiration. Per- 

 haps the most desirable information is obtained by using plants as 

 instruments (phytometers). Two or more habitats may be com- 

 pared by setting up potted plants of the same species in each of 

 these habitats and comparing their transpiration rates as indicated 

 by loss of weight over the same period of time. Again the work 

 involved is often prohibitive. As a result, ecologists have largely 

 come to depend upon mechanical devices that measure evaporation 

 over unit periods of time, and, since evaporation and transpiration 



