26 ASSOCIATION 



when the species to be investigated is completely wilted, 

 and the amount of water present is determined as before. 

 This is the non-available water. The physiological water 

 content is then obtained by subtracting the amount of 

 non-available water from the physical water content. The 

 one drawback to this method is the difficulty of causing the 

 plant to wilt in nature. This can be done only by cutting 

 the soil in such a way as to destroy the capillarity, and by 

 covering it in such a way during times of precipitation that 

 there is no access of moisture. A more feasible method is 

 to transfer the plant in its own soil to the planthouse, where 

 the physical factors may be accurately controlled, but this 

 can be used only in the case of relatively small plants. As 

 the absorptive power varies widely in different individuals 

 of the same plant, it is imperative that water content ex- 

 periments be conducted only with normal healthy plants at 

 the time of maturity. Some preliminary work in the 

 accurate investigation of the water content of different 

 habitats has been done by two of the writer's pupils: Thorn- 

 ber (I90I) has investigated the comparative water content 

 values of woodland and prairies, while Hedgcock ( 1902) has 

 made a special study of the physioloeical water of various 

 soils. The present characterisation of many plants and 

 formations as hydrophytic, mesophytic or xerophytic must 

 be regarded as largely tentative, and the final classification 

 will be possible only after the thorough quantitative investi- 

 gation of their habitats. '^ 



The relation to water content association of the concept 

 underlying the groups, hylophytes, poophytes and aleto- 

 phytes, established by Pound and Clements in 1898, is a com- 

 plex one. This idea, which Schimper (J898:I73) recognized 

 later in the same year in the corresponding divisions, 

 Oehola, Orasflur and Wuste, is a valid one ; though involving 



•The methods and instruments employed in the exact quantitative 

 determination of water content, as well as humidity, temperature, 

 light, wind, etc., in the various formations of Nebraska and Colorado 

 are treated in detail in a paper entitled, "The Physical Factors in 

 Ecology." This paper was presented before the Botanical Society of 

 America at its Denver meeting in 1901, and will soon be published. 



