WATER-RELATION BETWEEN PLANT AND SOIL. 



45 



exceptions may be related to special internal conditions in the Vicia 

 plants, which alone wilted in the experiments, or it may be connected 

 with the change in height of the irrigator water column, occurring 

 between the two tests. The general increase in the degree of the daily 

 fluctuation in these powers and rates is probably related to climatic con- 

 ditions, but the time is not yet ripe for even a tentative interpretation. 



TABLE 5. Daily fluctuations, ratios of maxima to mean values for night hours. 



It appears, in general, from our measurements, that the soil here 

 employed was dried out appreciably by root absorption, in the neigh- 

 borhood of the roots, and that this partial desiccation usually lagged 

 considerably behind its primary cause, rise in transpiration rate. This 

 lag rendered the attraction of the soil for water noticeably high for some 

 time after the transpiration rate had attained its low night value. Thus 

 there occurred a sort of after-effect of high transpiration rate, manifested 

 as resistance to root absorption with later hours in the day than those 

 in which the cause was primarily operative. 



To what degree the lag just considered occurred in the plant and to 

 what degree in the soil intervening between roots and the irrigator 

 cup can not of course be ascertained at present. That it is partly due 

 to increased saturation deficit in the leaves of the plant maybe regarded 

 as probable from what has been shown by earlier work. 



If the tendency of the whole environment to extract water from the 

 plant or to retard water entrance be considered as proportional to the 

 product of the evaporating power of the aerial surroundings and the 

 water-attracting power of the soil, then the lag before us operates to 

 maintain a comparatively high environmental aridity for several hours 

 later in the day than would be the case in the absence of the after-effect 

 in the soil. 



In the series of relative values given above we have derived com- 

 parable magnitudes by calculating all values of each series to the basis, 

 as unity, of the corresponding value for one of the night hours of the 

 earlier period of observations. We have considered, in each case, the 

 fluctuations in these relative magnitudes for two periods of 24 hours 

 or more, these two periods being a month apart. Had full records been 

 made from the beginning of the cultures to their discontinuance the 

 series of relative values which might thus have been obtained might 



