Roles of the Soil in Limiting Plant Activities. 51 



cally all derived from the soil. This liquid is of most funda- 

 mental importance in all life phenomena, being the single solvent 

 in which these phenomena are known to take place, and it is 

 absorbed throughout the whole active history of most plants. 

 Sometimes the presence of water-storage tissues renders it possi- 

 ble for growth to occur for a time in the absence of water absorp- 

 tion, but this is not the usual condition of affairs. 



A comparatively very small amount of the water which dif- 

 fuses into the root system is fixed in metabolism, in the formation 

 of carbohydrates, for example. But the greater portion of the 

 water entering the plant from the soil passes through it and is 

 lost, as transpiration water, from the surfaces of leaves and 

 stems. If the soil fails to supply water to the root system at a 

 rate equal to or surpassing that at which it is fixed by metabo- 

 lism and lost by transpiration, then growth is checked or partial 

 or total death ensues. Since the amount of water fixed by the 

 plant is so small in comparison with the amount lost, the former 

 may be neglected in this consideration, and we may say here 

 that the main limiting condition for ordinary growth is that it 

 must be possible for water to diffuse from soil to root system 

 somewhat faster than it is lost by the aerial organs. 



Since transpiration depends to such a degree upon the 

 evaporating power of the air, it follows that the last named 

 factor must be considered in connection with the rate of soil 

 diffusion; with a low evaporating power plants may thrive in 

 soils possessed of only a low rate of water supply, while if the 

 evaporating power of the air were to be greatly increased the 

 same plants, with the same soil conditions, might cease to grow 

 or succumb entirely. The real determining factor is here the 

 ratio of the possible rate of water diffusion from soil to roots 

 to the rate of transpiration. In order that ordinary growth may 

 occur, this ratio must be greater than unitv. 



The rate of water diffusion through the soil depends upon 

 its capillary power and upon the amount of water present in 

 neighboring regions. Capillary power, in its turn, depends 

 largely upon the size of the soil particles and upon their condi- 

 tion of aggregation. It is probable that soil temperature may 

 be found to play a role in this connection. 



The rate of diffusion of oxygen through the soil is dependent 

 largely upon the amount of water present, as well as upon the 



