Jan. 2, 1920 Relation of Moisture in Substrata to Salt Balance 375 



It will be obsen'^ed that the cultures of the series having the lowest 

 moisture content throughout the experiment exhibit a much lower water 

 loss than do the corresponding cultures of the other two series. There 

 is no such clearly defined relation, however, between the actual amounts 

 of water lost and the highest and medium degrees of moisture employed 

 with the cultures of series B and series C, respectively. There appears 

 to be no general tendency for the transpiration to be either higher or 

 lower with the medium moisture content than with the highest. 



A comparison of the transpiration graphs with those representing 

 the yields of tops and of roots (fig. 2, 4), brings out several interesting 

 relations. lyow moisture content of the sand cultures denotes low 

 yields of tops and of roots and low transpiration, while a high moisture 

 content corresponds to low yields of tops and of roots but high transpira- 

 tion. Thus, while low soil (sand) moisture retards both the rate of growth 

 and transpiration, excessive moisture of the substratum, on the other 

 hand, markedly retards the growth rates but does not correspondingly 

 retard transpiration. The medium moisture content, 60 per cent of the 

 water-retaining capacity of the substratum, gave the highest yields of 

 both tops and roots and the highest transpiration. 



The water requirement of tops and of roots is graphically shown in 

 figures 6 and 7, respectively, in the same manner as are the yields of tops 

 and of roots and the actual amounts of water lost by transpiration (fig. 

 2, 4. 5)- 



Inspection of the graphs of water requirement of tops and of roots 

 shows at once that the values of the water requirement ratios increase 

 with each increase in the moisture content of the substratum. Thus, 

 from the positions of the graphs it is clear that the values of the water 

 requirement ratios are determined by the moisture conditions of the 

 cultures. High, medium, and low moisture content of the substratum 

 is correlated with high, medium, and low water requirement ratios, 

 respectively, of both tops and roots. This relation is perfectly definite 

 for the wheat tops of these tests and, in the main, also for the roots, 

 although several cultures with the lowest moisture content (series A) 

 show water requirement ratios which are slightly higher than are those 

 of the corresponding cultures with medium moisture content (series B), 

 as is indicated on the graphs of figure 7. 



As has previously been pointed out, both the highest and the lowest 

 moisture content here employed retarded the growth rates, the former 

 through some harmful influence (perhaps insufficient aeration) related 

 to excessive moisture, the latter undoubtedly through the resistance 

 ofifered to water absorption by the plant roots, which resulted in an 

 insufficient internal water supply necessary for good growth. The 

 transpiration graphs show, however, that with the highest moisture 

 content (series C) the transpiration rates were not correspondingly 

 retarded, so that under these conditions a comparatively large amount 



