III.] 



HYGROSCOPIC MOISTURE 



85 



i2«3 per cent of water retained by the soiL Heinrich 

 has further shown that wilting begins before the water 

 content of the soil has been reduced to the hygroscopic 

 water limit, as the following figures demonstrate — 



This is easily intelligible in view of the fact that the 

 root hairs cannot be in contact with all the soil particles, 

 nor can the water move from particle to particle when 

 / it has been reduced to so low a proportion. King has 

 made some observations of the amount of water still 

 present in soils in the field which had become so dry 

 that growth was at a standstill and the plants were 

 wilting. 



Under these conditions all the soils were about 

 equally dry, so far as any power to part with their 

 water went; if the estimates made previously of the 

 area of surface of the particles constituting various kinds 



