274 TRANSFER OF WATEE THROUGH THE PLANT, 



Ueheraln.^ 



drops. This metliod of demonstrating transpiration has been 

 used, when somewhat modified, l)y many investigators, notably 



' It is well adapted to class experiments, since very 



simple appliances'^ can be nsed : 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 tnmbler 

 is prevented from evaporating into the upper 

 one. The amount of water which collects on 

 the inside of the upper tumbler comes wliollj- 

 from the transpiration of the plant, and will be 

 found to vary according to the surroundings 

 (see page 275 et seq). 



736. If a weighed 

 amount of calcic chlo- 

 ride is placed with a 

 transpiring plant in a 

 confined atmosphere, the 

 salt will readilj' take up 

 the aqueons vapor, and 

 its increase in 

 weight gives 

 the amount of 

 water exhaled by the plant. This 

 method of measuring the amoiuit 

 of transpiration has been em- 

 ploj-ed by several experimenters, 

 who have obtained resnlts sub- 

 stantiality in accord. It must be 

 noted, however, that in this 

 method the air to which the 

 plant is exposed is rendered ab- 

 normally drj- bj- the presence of 

 the salt, and tlie 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 



147 



' Conrs lie Cliimie Agdcole, 1873, p. 180 et seq. 

 '■' Henslow. See Oliver's Botany (1864), \i. 15. 



Fig. 147. Apparatus for demonstration of transplnitioii. 

 ViQ. 148. Garreau's apparatus. 



