WATER CONTENT ASSOCIATION 25 



which the developmental series is established are determined 

 in the first degree by the physiological water content of the 

 habitat, while a careful study of the nutrient content groups 

 of Graebner will suffice to show that these depend in the 

 first place upon water, and only in a secondary degree upon 

 nutrition values. 



Up to the present time, the general character of the 

 habitat, together with the gross appearance of the plant 

 itself, has been thought sufficient to determine the proper 

 position of a plant or a formation in the water content 

 classification. Such a method is adequate, however, only 

 for plants and formations which bear a distinct impress. 

 For an accurate classification into the three categories, 

 hydrophytes, mesophytes and xerophytes, it is neces- 

 sary to make exact determinations of the normal 

 physical and physiological water of the habitat, and to sup- 

 plement this, in some degree at least, by histological studies. 

 Except in the case of saline, acid and frozen soils, the 

 physical water content alone will be a fairly accurate index, 

 especially in habitats of similar soil composition. For an 

 exact and comprehensive classification, however, and par- 

 ticularly in comparative work, the physiological water con- 

 tent must constitute the sole criterion. The uhysical water 

 content is readily obtained by taking soil samples from day 

 to day, or from time to time throughout the growing season. * 

 These samples are taken from the desired depth, usually 

 from the layers in which the root activities are greatest, in 

 so far as this may be determined, placed in air tight cans 

 and weighed as soon as possible. The cans are then opened 

 and oven -dried at a temperature of 40° C for three days, 

 though the time is much less for porous gravels and sands, 

 and is sometimes greater for the most compact clays. When 

 thoroughly dried, the samples are again weighed, the diff- 

 erence between the two weights giving the total amount of 

 water in the soil, i. e. , the physical water. Having deter- 

 mined the latter, a second soil sample is taken at a time 



'Pound and Clements I.e. 2 ed. 167 1900 



