94 STIPA HYGROMETER. [CH. IV 



of the two leaves. Now smear the lower surface of A and 

 the upper surface of B with vaseline, which should be 

 carefully rubbed on with a finger. Weigh the specimens 

 and leave them for 24 hours. It will be found that B loses 

 in weight something like 10 times as much as A. 



(118) Stomatal transpiration {observed by another 

 method). 



For demonstration purposes the well-known experi- 

 ment of Garreau' can be repeated in a very simple manner, 

 with a rough sort of hygrometer 

 represented in the sectional dia- 

 gram, fig. 18. It consists of a small 

 glass cylinder g across the mouth of 



which a glass tube is fixed : from the 



»^, ^ 1 . PQ,- Fia. 18. Exp. 118. 



centre of the tube a piece oi otipa-awn 



s projects at right angles and bears at its end an index i 



which may conveniently be made of thin iron wire. The awn 



is sensitive to hygrometric change, in damp air it untwists, 



in dry air it twists up again. If the vessel is therefore 



placed mouth downwards on damp blotting paper or on a 



transpiring leaf, the index i will rotate and its movement 



can be read off on a graduated ring of paper fastened to 



the bottom of the vessel. If two hygrometers are made, 



one may be placed on each surface of a leaf and the 



difference in the movement of their indices compared. 



Certain precautions are necessary : in the first place, it is 



difficult to get two pieces of awn", which behave similarly, 



1 Ann. Sc. Nat. 1850. 



^ The awn should be thoroughly ripe, brown in colour," not yellow, 

 and stiff not weedy in texture. 



