12 WATER-RELATION BETWEEN PLANT AND SOIL. 



evaporation removes water from the system. In the auto-irrigator 

 the "capillary" attraction of the surrounding soil brings about the 

 same result. As water passes from cup to soil it automatically dis- 

 tributes itself so as to tend toward equilibrium throughout the soil 

 mass. With a given soil mass and a given length of water column in the 

 tube, static equilibrium is attained with a certain, definite average 

 moisture content of this mass. A very thin layer of soil adjacent to the 

 cup maintains for a long time a water content somewhat higher than 

 that of the remaining soil, but this layer is only a millimeter or two in 

 thickness. Beyond this layer there is but slight fall in soil-moisture 

 content from the cup outward. Hawkins (1910) has shown that 

 the auto-irrigator furnishes to potted plants a soil-moisture content- 

 that never fluctuates very markedly, that plants thus grown develop 

 more rapidly and to a greater size than those spasmodically watered 

 in the ordinary way, and that the root systems of such plants are 

 much more uniformly distributed through the soil mass than is ordi- 

 narily the case. 



Preliminary experiments showed that the variation in soil-moisture 

 content of pot cultures provided with the auto-irrigator and sealed to 

 prevent water loss excepting through the plant, is exceedingly small, 

 after the system has once attained approximate equilibrium. (This 

 matter will receive special attention presently.) From this it appeared 

 highly probable that, with this apparatus, the rate of entrance of water 

 from porous cup to soil may vary approximately (but with more or 

 less pronounced lag) with the water-attracting power of the general 

 soil mass. This power should be proportional to the mean resistance 

 offered by the soil to root absorption, and this should depend, in any 

 given culture, upon the rate at which root absorption actually has 

 been occurring. As has been emphasized, the latter rate should vary 

 (a lag being here also to be expected) with that of transpirational water 

 loss from the aerially exposed plant surfaces. It was thus suggested 

 that measurements showing the daily march of the rate of removal of 

 water from the irrigator reservoir might throw light upon soil condi- 

 tions as related to air conditions, and upon the variations in transpi- 

 ration rate as related to the corresponding march of the rate of water 

 absorption by plant roots. 



The plants of these experiments were grown in a heavy loam soil, 

 in tinned sheet-iron cylinders 15 cm. high and 12.5 cm. in diameter, 

 the top of the soil mass being sealed over with plastiline after the first 

 few days. Each cylinder was provided with a single irrigator cup 

 joined by means of a rather long, flexible rubber tube to a bottle which 

 served as reservoir. Each rubber tube bore a mark in a suitable 

 position so that it might be fixed in a clamp support at that point, 

 always leaving the same length of tubing free and hanging loosely 

 between clamp and pot. Thus it was possible to place the cylinder 



