298 



PLANT PHYSIOLOGY 



by a plant per unit of time is several hundred times as great 

 as the increase in dry weight. The error resulting from not 

 taking into consideration assimilation and respiration is therefore 

 negligible. 



For the study of transpiration, whole plants rooted in soil, 

 or cut branches, or leaves may be used. Rooted plants permit 



Fig. 92. - Fig. 93. 



Fig. 92. — Galvanized-iron pot with closely fitting cover for the exact determi- 

 nation of water used by a plant. 



Fig. 93 .-^Transpiration balance (after Burg er stein) . 



the study of transpiration under more natural conditions, but the 

 necessity of weighing such plants together with the soil in which 

 they grow introduces a series of technical difficulties. The 

 plants either have to be grown in special containers impermeable 

 to water vapor (Fig. 92); 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. 93) 

 combining high sensitivity with large capacity have to be used. 



