THE WATER OF SOILS. 



193 



be expected that in countries where the air is usually moist, the 

 evaporation will, other things being equal, be less than where it 

 is commonly far below the point of saturation. But the 

 " guardian cells " (stomata) of the leaf pores possess the power 

 of regulating, to a certain extent, the evaporation from the 

 leaf-surface in accordance with temporarily prevailing condi- 

 tions, so as to allow free evaporation in moist air, but to pre- 

 vent the wilting and drying-up of the leaf in hot and dry air, 

 save in extreme cases. Moreover, plants adapted to arid condi- 

 tions are usually provided with additional safeguards in the 

 form of thick, non-conducting layers of surface cells, or long 

 channels connecting the interior tissue with the breathing- 

 pores on the surface. Often hairy, scaly or viscous coverings 

 serve the same end. On the other hand, when the air is very 



-- 



moist, so as to check evaporation, water is sometimes found 

 secreted in minute droplets around the breathing-pores of the 

 leaves, since its ascent is a necessary condition of nutrition 

 and development. 



Relation between Evaporation and Plant -growth. There is 

 not in all cases any direct relation between the amount of evap- 

 oration and plant growth ; but experience, as well as numerous 

 rigorous experiments have shown that under ordinary condi- 

 tions of culture, and within limits varying for different soils 

 and crops, production is almost directly proportional to the 

 water supply during the period of active vegetation. 



On the basis of Hellriegel's results, showing that wheat uses 

 (in Germany) about 435 tons, or nearly four acre-inches of 

 water in the production of one ton of dry matter, and assuming 

 the ratio of grain to straw to be i 11.5, King calculates the 

 following table of probable production under different moisture 

 conditions (Physics of Agriculture, page 140) : 



YIELD PER ACRE. 



