292 THE INTERNAL WATER RELATIONS OF PLANTS 



simultaneously making an observation upon the volume of water vuhich had 

 been absorbed from the reservoir it was possible to make parallel determina- 

 tions of the rates of transpiration and absorption. Except for the fact that 

 the soil was irrigated the plants were grown under approximately "standard 

 day" conditions. 



The results of one of the experiments in which loblolly pine (Pinus taeda 

 L.) was used is shown in Fig. 76. As shown in this figure there was a distinct 

 lag in the rate of absorption as compared with the rate of transpiration during 

 the daylight hours — i.e. during the period of relatively high transpiration 



Fig. 76. Comparative daily periodicities of transpiration and absorption in the loblolly 

 pine {Pinus taeda). Data of Kramer (1937). 



rates. There was also a fairly well marked tendency, shown clearly on the 

 second day of the experiment, for the peak absorption rate to occur somewhat 

 later in the day than the peak transpiration rate. This effect was more pro- 

 nounced in experiments performed on other species by the same investigator. 

 During the night hours the rate of absorption was continuously higher than 

 the rate of transpiration. In other words the tissues of the plant were being 

 progressively depleted of water during the daylight hours, while their water 

 supply was being replenished at night. 



Since the rate of absorption of water in experiments such as that just 

 described is determined by measuring the rate at which water is removed 

 from the external reservoir it is possible that a part of the water deficit ob- 

 served may have occurred in the soil rather than in the plant. That this 

 source of error is not sufficiently serious to invalidate the conclusions just 

 drawn can be shown by similar experiments in which plants are grown with 

 their roots in potometers adapted to weighing. Even when the roots of plants 

 are immersed in water an internal water deficit develops during periods of 

 high transpiration due to a lag in the absorption rate as compared with the 

 transpiration rate. In all probability in soils in which the water content is at 

 the field capacity or lower, the rate of absorption often shows a more pro- 

 nounced lag as compared with transpiration than in soils in which capillary 



