164 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



The annual summary of evaporation should be made on the 

 " Summary " form and should consider the total evaporation by 

 decades, months, the year, and the growing season. It may also be 

 desirable to record the maximum rate noted during each month or 

 decade. 



DIRECT TRANSPIRATION METHODS. 



In evaporation studies connected with plant life the chief purpose 

 is to obtain in as simple a term as possible a measure of the habitat 

 conditions, as they may affect transpiration. The response of the 

 plant to these conditions will be governed by physical facts that can 

 be fairly well comprehended, and by biological conditions which are 

 still more or less obscure. While, then, transpiration studies may 

 be made in lieu of evaporation studies, it will be far more profitable 

 to consider the one as supplementing the other, giving an insight 

 into plant functioning which can only be obtained as observations are 

 reduced to terms of well-known physical laws. One of the means 

 of determining the intensity or transpiration is by the aid of cobalt 

 chloride paper. 



Cobalt-chloride method. 



Although the actual amount of water transpired by a plant can 

 not be ascertained by means of the standardized cobalt-chloride 

 paper, this method is particularly adapted to field use where merely 

 relative rates of transpiration are desired, and for showing varia- 

 tions in rate with changes in environmental conditions. It is hardly 

 necessary to point out that it would be almost impossible to apply 

 to needle leaves. The method depends upon the fact that paper im- 

 pregnated with a weak solution of cobalt chloride is blue when dry, 

 but when exposed to moisture gradually turns pink. Specially pre- 

 pared " tripartite cobalt-chloride paper slips " may be purchased. 

 These slips are made up of three small strips of paper — a deep blue 

 standard, a light blue standard, and between them a strip of cobalt 

 chloride paper. In practice the strip is heated over a small flame 

 or heated surface until the cobalt chloride strip is of a more intense 

 blue than either standard. It is then applied to the surface of the 

 leaf under consideration and held by small glass clips. When the 

 cobalt chloride strip fades so that it matches the bluest standard, the 

 time is noted. When it fades further and matches the light blue 

 standard, the time is again noted and the elapsed time is recorded. 

 This process should be repeated several times to assure a good 

 average. A thermometer should also be hung amidst the foliage to 

 give the temperature of the leaves. 



The time taken for the cobalt-chloride paper to make the change in 

 color between the two standards is the measure of the rate of trans- 



