HISTORICAL. 



27 



but the variation rises as high as 60 to 130 per cent in the last three. Part of 

 this may be due to a larger error in determining the low echard. The author 

 concludes that species not only wilt at different points, but also that this varies 

 for different stages of the development of the same species. 



Kihlmann (1890 : 105) was probably the first to perceive the ecological 

 significance of the echard, in connection with his studies of water relations in 

 the frozen bogs of Lapland. However, Schimper first recognized the universal 

 application of the concept and formulated it definitely as follows (1898 : 3; 

 1903:2): 



"It is necessary to distinguish between physical and physiological dryness 

 and wetness; the physiological water-content alone is important for plant-life 

 and hence for plant-geography." 



Neither Kihlmann nor Schimper appears to have made actual determina- 

 tions of the physiological water-content. Clements (Pound and Clements, 

 1900: 167; Clements, 1904:23; 1905:30; 1907: 13; 1916) developed methods 

 for determining the echard and chresard in the field as well as under control. 

 These were applied to various habitats in the prairie and woodland regions of 

 Nebraska, and on Pike's Peak in Colorado. The general results were in accord 

 with those of the earlier investigators, Sachs, Gain, and others, with respect to 

 the variation of the echard with different species as well as with different soils. 

 This led to a comprehensive investigation by Hedgcock (1902) of the echard 

 and chresard of some 130 species under control, and 25 in the field. These were 

 largely native and ruderal species, though a number of cultivated ones were 

 included also. The great majority were mesophytes, though they ranged from 

 xerophytic grasses, such as Bouteloua gracilis, to such hydrophytes as Sagittaria 

 and Potamogeton. The author reaches the general conclusion that " the ability 

 of plants to take water from the soil varies in an ascending scale from hydro- 

 phytes through mesophytes to xerophytes." 



Briggs and Shantz, 1912.— The most complete and thoroughgoing investiga- 

 tion of the echard has been made by Briggs and Shantz in connection with 

 crop-plants for the Great Plains. Their methods and results are perhaps too 

 well known to require comment, but it seems desirable to touch the latter 

 briefly for the sake of comparison. The term wilting coefficient is employed 

 for non-available water or echard, but it is an exact synonym of these. The 

 determinations of the echard for various soils are in essential accord with those 



