274 TRANSFER OF WATER THROUGH THE PLANT. 



drops. This method of demonstrating transpiration has been 

 used, when somewhat modified, by many investigators, notably 

 Deherain. 1 It is well adapted to class experiments, since very 



simple appliances 2 can be used : for instance, 

 a leafy stem can be inserted in a piece of 

 pasteboard, and the cut end of the stem 

 placed in a tumbler of water ; another tum- 

 bler, inverted over the stem, rests on the 

 pasteboard. The water in the lower tumbler 

 is prevented from evaporating into the upper 

 one. The amount of water which collects on 

 the inside of the upper tumbler comes wholly 

 from the transpiration of the plant, and will be 

 found to vary according to the surroundings 

 (see page 275 et 



736. If a weighed 

 amount of calcic chlo- 

 ride is placed with a 

 transpiring plant in a 

 confined atmosphere, the 

 salt will readily take up 

 the aqueous vapor, and 

 its increase in 

 weight gives 



the amount of 



water exhaled b}* the plant. This 

 method of measuring the amount 

 of transpiration has been em- 

 ployed by several experimenters, 

 who have obtained results sub- 

 stantially in accord. It must be 

 noted, however, that in this 

 method the air to which the 

 plant is exposed is rendered ab- 

 normally dry by the presence of 

 the salt, and the plant is there- 

 fore subjected to an unusual draft upon its water-supply. 



737. Garreau's method of comparing the relative amounts of 

 transpiration on opposite sides of a leaf is based on that last 



1 Cours de Chimie Agricole, 1873, p. 180 et seq. 



2 Henslow. See Oliver's Botany (1864), p. 15. 



FIG. 147. Apparatus for demonstration of transpiration, 

 TIG. 148. Garreau's apparatus. 



