CH. IV] COBALT METHOD. 105 



commends 1 p.c. ; for the following we employ a 5 p.c. 

 solution. 



Strips of cobalt paper are placed on each side of a 

 hypostomatal leaf ^ and are covered by glass plates which 

 should project beyond the edges of the paper. Stahl 

 uses in some cases sheets of talc as being less heavy and 

 therefore more easily fixed in place than glass plates. 

 The glass or talc plates being gently clamped at the 

 edges the papers are confined in spaces in which the 

 dryness of the air will depend on the transpiration of 

 the two surfaces of the leaf. The paper on the lower 

 surface reddens rapidly, while that on the upper side 

 remains blue. 



A simple plan is to take a pair of similar leaves, 

 placing one, A, with the stomata upwards, the other, B, in 

 the reverse position on a dry folded cloth ; after covering 

 them with a strip of cobalt paper, place a sheet of glass 

 over them which makes a good contact with the yielding 

 cloth. After observing the rapid reddening of the paper 

 over A, the experiment should be repeated, reversing the 

 leaves, so that B has now its stomata upwards. 



To demonstrate the very small amount of transpira- 

 tion from the stomatal surface of some leaves we slightly 

 modify Stahl's method by cementing to the leaf surface 

 the glass plates covering the cobalt papers. We use 

 small glass plates about 2*5 cm. x 3"5 cm. made by cutting 

 ordinary microscopic slides in half and attach them by 

 running in melted wax-mixture at the point of junction 



1 That is with the stomata only on the lower surface ; Stahl recom- 

 mends Tradescantia zebriita, Fyriis communis, Po^yulus nigra, &c. 



