381] B. E. Livingston and E. E. Free 183 



the response of the root-system to deficiency of oxygen in 

 the soil atmosphere varies widely in different kinds of plants. 

 Some species are injured by a very slight deficiency below 

 the oxygen content of the general atmosphere. A swamp 

 willow, probably Salix nigra, endures successfully the com- 

 plete, or almost complete, exclusion of oxygen from its roots. 

 In the case of those plants which are injured by deficient 

 soil oxygen it is interesting physiologically that the first 

 effect of oxygen deprivation is an interference with the ab- 

 sorption of water by the roots. In the experiments the 

 apparatus for the supply of water is so arranged that the 

 amount of water taken up by the soil from the porous cups 

 of the auto-irrigator can be measured for periods as short 

 as one hour. The amount of water thus taken up depends 

 in part on the temperature. The surface tension of the 

 water films in the soil varies with temperature and this con- 

 trols the amount of water held in the water-film system. 

 However, this error disappears for periods the initial and 

 final temperatures of which are nearly the same (for in- 

 stance, the usual 24-hour period) and a correction can be 

 made for the error in the case of shorter periods or other 

 periods which do not satisfy this condition. When the tem- 

 perature error is thus eliminated, the absorption of water 

 from the auto-irrigator is closely parallel to the intake of 

 water by the plant roots. With the plants that are sensi- 

 tive to deficiency of oxygen in the soil air, it is found that 

 the replacement of the normal soil atmosphere by nitrogen 

 is followed within a few hours by nearly complete cessation 

 of water-intake by the roots. With the most sensitive species 

 tested, namely, Coleus blumei and ffeliotropium peruvianum, 

 this cessation of water-intake occurs always within 24 hours, 

 usually within 12 hours, after the soil oxygen is removed. 

 This time period varies with the individual plant, perhaps 

 because of differences in the root-system but probably also 

 because of differences in the completeness with which the 

 soil oxygen originally present is replaced by the nitrogen. 

 Since the oxygen must be displaced by washing out with 



