CH. IV] STIPA HYGfiOMETEE. 95 



SO that it is necessary to graduate the two hygrometers to 

 make their readings comparable. Take a filter paper and 

 damp it carefully, making sure that it is not wetter in one 

 part than the other, place it on a flat glass plate and 

 having marked the position of the index with pencil on 

 the paper rings in both hygrometers, place them side by 

 side on the wet paper. After from 4 to 8 minutes mark 

 the position of the index again. If, for instance, the 

 movement of hygrometer A is only | of that of B, it is 

 clear that the paper ring on A must be marked out in 

 divisions each of which is | of the unit 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.] 



(119) Stomata : connection with intercellular spaces. 

 For this experiment the choice of a suitable leaf 



' Ivy leaves are equally good, and if the apical part of the leaf is 

 used, the cork plates may be dispensed with. 



