CH. IV] COBALT METHOD. 105 



vantage of this fact to demonstrate a number of points in 

 the physiology of the stomata. The sensitiveness of the 

 cobalt paper depends on the strength of the solution 

 employed, for delicate reactions Stahl recommends 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 1 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 there- 

 fore 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 small amount of transpiration 

 from the upper surface of hypostomatal leaves we make 

 use of Stahl's method, viz. 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 



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

 mends Tradescantia zebrina, Pyrus communis, Populus nigra, &c. 



