104 STOMATA. [CH. IV 



used for hygrometer B. Our hygrometer scales are 

 usually divided into 60 — 100 divisions. 



To fit the hygrometers on to the leaf (we use laurel 

 leaves^) two plates of cork are wanted, each having a circular 

 opening slightly smaller than the hygrometer : one plate 

 has a groove running across the middle which receives the 

 midrib of the leaf, and allows it to lie flat between two 

 plates. One hygrometer is placed mouth upwards on the 

 table, then the leaf between the plates, then the other 

 hygrometer mouth downwards: the whole being kept 

 steady by a weight of 2 or 3 oz. placed on the top. For 

 laurel leaves 7 or 8 minutes is generally long enough to 

 wait before reading the hygrometers. 



[The general behaviour of stomata may be conveniently 

 studied here.] 



(118 a) Stahl's cobalt method-: 



It is well known that paper impregnated with a 

 solution of a cobalt salt, e.g. cobalt chloride, changes from 

 blue to red when it is placed in damp air and reassumes 

 the blue colour when dried. Stahl has been able by 

 taking advantage of this fact to demonstrate a number 

 of points in the physiology of the stomata. The sensi- 

 tiveness of the cobalt paper depends on the strength of 

 the solution employed, for delicate reactions Stahl re- 



1 Ivy [Hedera) leaves are equally good, and if the apical part of the 

 leaf is used, the cork plates may be dispensed with. Sparmannia leaves 

 give a good result. In some cases it will be found convenient to attach 

 the vessels to the surface of the leaf by melted wax-mixture instead of 

 using the arrangement above described. 



2 Stahl, Botan. Zeitung, 1894, p. 117. 



