June, 1922] 



MUENSCHER — ABS6rPTION OF SALTS 



321 



gram of ash were reduced to approximately one half (table i). When trans- 

 piration was reduced to one half by shading, the total dry matter and ash 

 were reduced to considerably less than one half while the water requirement 

 and the water used per gram of ash were increased considerably (table 2). 



These results seem to indicate that growth, as limited by food supply, 

 rather than the rate of transpiration, determines the rate of absorption and 

 total absorption of salts by plants. This view is strengthened by the fact 

 that, in these experiments, the average percentage of ash expressed in terms 

 of dry weight in whole barley plants of the same age is about the same 

 regardless of whether transpiration was reduced by increasing atmospheric 

 humidity or by decreasing the light intensity by shading. Table 3 presents 

 a summary of the percentages, with probable errors,^ of ash based upon the 

 dry weights of tops, roots, and total plants, of the cultures grown under the 

 various conditions in the summer series. Table 4 presents a summary of 

 the percentages with probable errors of ash based upon green weights of the 

 same cultures. 



There is no good criterion for measuring growth under all conditions. 

 If total green weight is used, one must consider the variation in the 

 water content in the tissues of the plants growing under various conditions. 

 Data presented in tables i and 2 indicate that the water content of the 

 plants grown in the humid atmosphere or in the shade is much higher than 

 in plants grown in a dry atmosphere or in the open sunlight. This prob- 

 ably explains the variation in ash content expressed as percentage of total 

 green weight. Total dry weight seems to be a more satisfactory criterion 

 for measuring growth in plants where large quantities of storage products 

 are not formed. Table 3 shows a close relation between the weight of dry 

 matter and ash content in barley plants regardless of the quantity of trans- . 

 piration. The variation in the average percentage of ash for the cultures 

 grown under various conditions is only about five percent of the total 

 ash. This shows a remarkable constancy in the percentage of ash in dry 

 weight when compared with the great variation in the quantity of water 

 absorbed per unit of ash content of the plants under the various conditions 

 of this experiment. 



Table 3. Comparison of the Average Percentage of Ash Based upon the Dry Weight of 

 Tops, Roots, and Total Plants {Averages of cultures in tables i and 2) 



Set of Cultures 



Dry chamber 



Humid chamber 



Light 



Shade 



Percentage of 

 Ash in Tops 



20.74 ± -027 

 19.63 ± .026 

 18.91 ± .017 

 20.34 ± .027 



Percentage of 

 Ash in Roots 



20.02 ± .020 

 18.07 ± -030 

 30.55 ± .043 

 20.35 d= .048 



Percentage of 

 Ash in Plants 



20.63 ± -024 

 19.54 ± -013 

 21.13 ± -OiS 

 20.34 ± -024 



' For the calculation of probable errors Besspl's formula was used. 



