304 THE FOLIAGE LEAF 



into the leaf as it is used up by the chloroplasts of the 

 mesophyll cells, but also, of course, allow the water 

 vapour evaporated from the saturated mesophyll cells 

 into the intercellular spaces to diffuse out to the 

 (usually) drier air outside the leaf. This process is 

 called transpiration. 



Large quantities of water are lost by the plant in 

 this way. If a plant growing in a flower-pot is weighed, 

 and after a certain interval weighed again, the loss in 

 weight represents almost entirely transpired water, for 

 the gain or loss in weight from photosynthesis or respira- 

 tion is negligible by comparison with the loss by trans- 

 piration. The pot itself and the soil must be carefully 

 covered with a metal or rubber covering waxed to the 

 stem round the hole through which the stem passes, to 

 prevent loss of water by direct evaporation from pot 

 and soil. If desired, transpiration from the surface of 

 the stem through the stomata it bears can also be 

 prevented by waxing its surface. In this way it can be 

 shown that it is through the leaves that the loss of water 

 in transpiration mainly occurs. A sunflower plant whose 

 total leaf area was 5,616 square inches lost i| pints 

 of water in 12 hours ; and it has been calculated that 

 the trees in an acre of beech forest transpire about 

 i-,40O tons of water during the summer. 



This constant loss of water from the leaf is probably 

 useful to the plant because it helps to set up a current 

 (transpiration current) through the xylem vessels and 

 thus to bring up dissolved salts from the root more 

 rapidly. That active transpiration is not necessary to 

 the plant, or at least not to some plants, is shown by 

 the fact that growth continues in tobacco plants, for 

 instance, which are kept in air saturated with water 

 vapour, so that transpiration takes place very slowly 



