COBALT TEST FOR TRANSPIRATION 199 



saturated with cobalt nitrate placed on the leaf changes from a 

 bluish to a reddish color in the presence of watery vapor ; the 

 second method consists in the use of a hygrometer. Several 

 types of these instruments are in use in physiological laboratories. 

 In one the variations in length of a strand of human hair with the 

 changing humidity moves a lever carrying a pen which gives a 

 constant record of the proportion of wateiy vapor in the air. 

 This form has not been made suitable for testing the action of 

 leaves. Another hygrometer consists essentially of an awn of 

 some grass, like Stipa, which twists or untwists with the varia- 

 tions in humidity of the atmosphere. This type has been found 

 very useful in some forms of investigation. A third form con- 

 tains a thin strip of some material which curves and straightens 

 with the varying humidity, and the best example of this type is 

 the horn hygrometer of F. Darwin, in which the sensitive mate- 

 rial is made of a thin strip of pressed horn. The simpler forms 

 of hygrometer sold in the market for general use have a sensi- 

 tive strip composed of two layers of material of different hygro- 

 scopicity, and the one described below is based upon this 

 principle (Fig. 100). 



272. Cobalt Test for Transpiration. Cut a few squares of mica 

 2x2 cm., such as may be in use for slips or covers in mounting 

 algae. Saturate a few small pieces of good filter paper in a five 

 per-cent. solution of cobalt nitrate, and dry thoroughly in sun- 

 light, or in an oven. The transpiration of water from any given 

 surface may be determined as follows : Cut a piece one cm. 

 square from the prepared filter paper, dry for a moment over a 

 gas flame, and lay on the surface of the leaf or other organ ; 

 cover it with a piece of mica and seal the edges of the mica to 

 the leaf by means of a wax composed of equal parts of beeswax, 

 resin and vaseline. Open stomata and the excretion of watery 

 vapor will be denoted by the change of the color of the filter 

 paper from blue to reddish, which will take place in a few sec- 

 onds. On the other hand, the color of the test paper will remain 

 unchanged for days, perhaps, on surfaces free from stomata. 



