B. P. I.-695. 



THE WILTING COEFFICIENT FOR DIFFERENT 

 PLANTS AND ITS INDIRECT DETERMINATION. 



INTRODUCTION. 



The wide range in moisture content of different soils at the time of 

 wilting of the plant cover appears to have been first clearly recognized 

 by Sachs 1 in 1859. The differences which extreme types of soil 

 exhibit in this respect are truly remarkable, ranging, as we shall show 

 later, from 1 per cent in coarse dune sand to 30 per cent or more in 

 the heaviest types of clay. Sachs's experimental work in this field 

 was confined to a single plant. Later investigators in extending this 

 -work concluded that not only do soils show a wide range in moisture 

 retentiveness, but that different groups of plants differ widely in 

 their ability to reduce the moisture content of a given soil. Thus, the 

 experimental work of Gain (1895), 2 Heinrich (1894), 3 Hedgcock 

 (1902), 4 and Clements (1905) 5 all indicates considerable variation 

 in the moisture content of the soil at the time of wilting of different 

 plants, which has been interpreted to mean that some plants are 

 capable of reducing the moisture content of a given soil to a lower 

 point than others; in other words, that the nonavailable moisture 

 varies according to the kind of plant used as an indicator. In fact, 

 this view is the one usually presented in the standard works on plant 

 physiology and plant ecology. 



The difference exhibited by plants in this respect has also been 

 considered to be an important factor in drought resistance, the 

 additional supply of water thus made available to some plants being 

 supposed to be sufficient to carry them through a dry period when 

 other plants would succumb to drought. With this point of view in 

 mind the present writers have made an extensive series of determi- 

 nations with a number of plants, including native plants from semi- 



i Sachs, J. Bericht uber die physiologische Tbatigkeit an der Versuchsstation in Tharandt. Land- 

 wirthschaftlichen Versuchs-Stationen, 1859, vol. 1, p. 235. 



2 Gain, E. Action de l'Eau du Sol sur la Vegetation. Revue G6ne>ale de Botanique, vol. 7, 1895, p. 73. 



3 Heinrich, R. Zweiter Bericht uber die Verhaltnisse und Wirksamkeit der landwirtschaftlichen 

 Versuchs-Stationen zu Rostock, 1894, p. 29. 



4 Hedgcock, G. G. The Relation of the Water Content of the Soil to Certain Plants, Principally Meso- 

 phytes. Studies in the Vegetation of the State, pt. 2, 1902, pp. 5-79. In Botanical Survey of Nebraska, 

 vol. 6. 



s Clements, F. E. Research Methods in Ecology, Lincoln, Nebr., 1905, p. 30. 

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