204 BOTANICAL GAZETTE [m.4rch 



VI), may be attributed to a temporary absence of the deep rooted 

 perennials or to their fewness. The factors just mentioned are 

 sufficient to account for the maintenance of a dry upper subsoil 

 through which no roots could develop into the moist zone. 



The question of whether the living roots are to be found in the 

 deeper subsoil only during each successive wet period, they follow- 

 ing the downward extension of the moist zone, continuing to 

 withdraw water until the ratio approximates i . o, and then dying 

 off, or whether they continue alive but withdrawing practically no 

 moisture throughout the dry periods of several years which inter- 

 vene between the successive wet periods, is to be answered only by 

 detailed field investigations, involving the use of pits or trenches 

 12-20 ft. deep. 



The present moisture conditions of the deeper subsoil of the 

 prairies, like their plant population, are to be regarded as the result 

 of a slowly established equilibrium, and any alteration of the plant 

 cover may greatly affect the subsoil moisture conditions. The 

 complete suppression of plant life over an acre or more, a condition 

 approached in young orchards and groves kept in clean cultivation, 

 might during a series of wet years raise the moisture content of the 

 deeper subsoil to its water-retaining capacity and maintain this 

 with little change during the ensuing dry period. If such a field 

 were neglected, however, it would soon be taken possession of by 

 many species, most of them shallow rooted annuals, but some 

 deeper rooting perennials, which, meeting little competition for 

 moisture in the deeper subsoil, could develop an extensive root 

 system there and gradually reduce the moisture to approximately 

 the hygroscopic coefficient. Then, as on the prairie, this dry con- 

 dition would be maintained except at such times as unusually 

 wet seasons extended the moist zone far below its normal limits. 



While it is evident from table VI that the lower limit of the dry 

 zone in the deep loessial soils in the semi-arid region is more than 

 12-15 ft. below the surface we have no data showing its maximum 

 depth. RoTMiSTROV (9), from his studies near Odessa, concluded 

 that there permanently moist subsoil in waste land occupied by 

 weeds, etc., is first encountered at 14-30 ft. The depth to which 

 the root systems of the deeper rooted prairie plants indicated by 



