THE STEM 



169 



But such exudation of water in the form of drops is a 

 comparatively rare case. It generally happens under 

 the conditions just described, i.e. when the surrounding 

 air is saturated with water vapour ; but, as a rule, 

 plants give off water in great quantities in the form of 

 invisible vapour. 



We can realise the amount of water evaporated by 

 plants from the following approximate calculations : one 



Fig. 47. 



acre of oats evaporates during the summer from 150 

 to 250 tons of water ; an acre of mixed meadow grass 

 something like 750 tons. We can determine this amount 

 of evaporated water in many ways. Here is one of the 

 simplest and most exact methods. The pot in which a 

 plant is growing is placed in a glass or tin vessel and 

 covered with a glass or tin plate with a hole through it 

 for the stem (fig. 47) . In this way evaporation from the 

 surface of the soil and the pot is prevented, and if we 

 weigh the whole apparatus from time to time we know 



