THE LOSS OF WATER BY THE PLANT 



159 



they grow introduces a series of technical difficulties. The plants 

 either have to be grown in special containers impermeable to water 

 vapor (Fig. 66), or, at the time of the experiment at least, the pot 

 must be enclosed in some metal or rubber covering, in order to 

 prevent evaporation from the surface of the soil. For more exact 

 experiments special transpiration balances (Fig. 67) combining 

 high sensitivity with large capacity have to be used. 



Fig. 66. — Galvanized-iron pot with 



closely fitting cover for the exact 



determination of water used by a 



plant. 



Fig. 67. — Transpiration balances (after 

 Burgerstein). 



Experiments with cut shoots and leaves are more convenient 

 as they remove the necessity of weighing heavy pots of soil, which 

 must inevitably affect the sensitiveness of a balance, hence, this 

 method of studying transpiration is widely used. The shoot is 

 usually placed with its cut end in water, so that the water lost 

 may be continually replaced. Precautions must be taken to pre- 

 vent the evaporation of water otherwise than through the shoot. 

 This is accomplished by inserting the twig through a hole in the 



