92 J. W. SHIVE AND B. E. LIVINGSTON 



in case of excessive wilting. To avoid such error as far as pos- 

 sible it was necessary to try always to consider plants as per- 

 manently wilted before excessive wilting had occurred, and this 

 attempt rendered it necessary to reject a number of determina- 

 tions because the plants in question revived later, without water- 

 ing, in the moist chamber test. Nevertheless, it seems probable 

 that the soil moisture contents here considered as the residues 

 at permanent wilting are more generally apt to be too low than 

 too high. It seems that this error can not be very large, however. 

 From the standpoint of the considerations just discussed it is 

 highly desirable that there be found some more easily determined 

 critical point than that of permanent wilting, for the carrying out 

 of such studies as these. It seems probable that the ingenious 

 balancing method devised and described by Briggs and Shantz 

 {loc. cit. (4), page 47) may become highly valuable in this connec- 

 tion. If the plants were all grown under the same environmental 

 conditions, it appears probable that the stage of incipient dry- 

 ing which is indicated by the balancing response might be as defi- 

 nite a critical point in the march of the soil-plant relation as 

 peimanent wilting has proved to be. If so, the employment of 

 this response might result in a much improved accuracy of ex- 

 perimentation, for there appears to be but little chance of error 

 in the use of the Briggs and Shantz device. It must be remem- 

 bered however, as was pointed out by Livingston and Brown, ^ 

 that the balancing response can not be expected to occur at the 

 same time as does permanent wilting, as appears to have been 

 assumed by the authors just mentioned; it should occur at an 

 earlier time. As has been pointed out by Caldwell {loc. cit., 

 page 9) , permanent wilting (or the corresponding stage of drying- 

 out met with in plants which do not actually show wilting on 

 the exterior) is an advanced stage of the incipient drying of 

 Livingston and Brown (loc. cit.). This in turn is an advanced 

 stage of Renner's^ saturation deficit, which it is safe to suppose 



8 Livingston, B. E., and Brown, W. H., Relation of the daily march of tran- 

 spiration to variations in the water content of foliage leaves. Bot. Gaz. 53: 

 309-30. 1912. 



^ Renner, O., Experimentelle Beitrage zur Kenntnis der Wasserbewegung. 

 Flora 103: 171-247. 1911. 



