34 WILTIXG COEFFICIENT FOE DIFFEBEXT PLANTS. 



differences exhibited in plants on the wilting coefficient- is, then ; 

 very small as compared with the differences in the moisture re- 

 tentiveness of soils, the maximum values for plants being only 

 about 10 per cent greater than the minimum values, as compared 

 with 3,000 per cent in the case of extreme soil types. 



Slight differences in plants do, however, exist, and these differ- 

 ences as a rule hold for the different plants throughout the determina- 

 tions with the several soils employed. In order to make a more 

 extended comparison possible, all of the determinations which have 

 been made with a given plant have been brought together and 

 reduced in such a manner that the collected results for the different 

 plants can be directly compared. These results are given in the fol- 

 lowing section. 



RELATIVE WILTING COEFFICIENTS FOR DIFFERENT PLANTS. 



In making a final comparison of the relative ability of plants to 

 reduce the soil moisture before wilting it is necessary, when different 

 soils are used, to eliminate from the results the effect of the moisture 

 retentiveness of the particular soil used and thus reduce the de- 

 terminations for all the soils to a uniform basis for comparison. This 

 reduction has been accomplished by determining the ratio of each 

 individual determination to the mean of the whole series of de- 

 terminations made upon a particular soil, as given at the end of 

 each table. In other words, each determination is divided by the 

 mean of all the determinations for that soil, irrespective of the kind 

 of plant used as an indicator. If, then, the wilting coefficient of a 

 soil for a particular plant is higher than the average, the ratio of the 

 individual determinations for that plant to the mean of the series 

 will be greater than unity. If, on the other hand, a plant is capable 

 of reducing the moisture content of a particular soil somewhat below 

 the point attained by the other varieties of plants grown in that soil, 

 then the ratio of the individual determinations for this plant to the 

 mean of the whole series will be less than unity. With the aid 

 of these ratios, then, it is possible to determine for any individual 

 plant, no matter in what soil it may be grown, the extent to which 

 it is able to reduce the soil-moisture content below the point reached 

 by other plants before wilting occurs. 



The determinations in Table XI, together with those in Table 

 XIII, have been reduced in this way, and the results are presented 

 in Table XII, which contains under each crop and variety the total 

 number of wilting-co efficient determ in ations made with that variety 

 and the mean of all the individual ratios for that variety calculated 

 as described above. When the number of observations were suffi- 

 cient to justify the calculation of the probable error, such calculation 

 is also given. 



230 



