160 PLANT PHYSIOLOGY 



In comparing the structures of osmometers with the osmotic 

 apparatus of the roots, there appears one striking structural dif- 

 ference, namely, in the osmometer there is but one chamber in 

 which the solution works directly against the gauge, while in 

 the roots the solution is enclosed within cells and releases pure 

 water under pressure into the ducts. The physical conditions 

 whereby this water is thus released are not understood, but the 

 student should make sure he comprehends the osmotic problem 

 involved, the available evidence as to whether root absorption 

 is purely physical or in part physiological, and the attempts 

 which have been made to solve this problem. 



The study of the absorption of soil water by roots brings 

 us into contact with the structure and properties of the soil. 

 This subject is a vast one, of which our knowledge is still com- 

 paratively scant, but its scientific interest and economic impor- 

 tance are leading to its very active study at the present time. 

 Indeed, it already has methods and a technique of its own, making 

 it, like bacteriology, in practice a separate department of inves- 

 tigation. Its experimental study is hardly practicable to any, 

 extent in this course, but if the student is able thus to follow it, 

 he will be aided by the following suggestions: 



Experimental Study of Soil Physics. The principal topics include 

 (a) mechanical structure and its analysis, effected either by a sifting-and- 

 floating or by a centrifugal method; (b) aeration capacity and air move- 

 ments; (c) water absorbing and holding capacity, and movements of soil 

 water; (d) absorptive capacity for gases and special minerals; (e) tempera- 

 ture conditions and their relations to air temperatures; and (/) adsorption, 

 or the aggregation of materials in solution towards solid bodies. The sub- 

 ject approaches close to the chemistry of soils, on which some comments 

 will be found a little later. The various methods of study of soil physics 

 are given with the greatest fullness in various Bulletins of the Division of 

 Soils of the United States Department of Agriculture, where the subject 

 has been, and is being, studied with the greatest energy and success, and 

 some of the State Experiment Stations have also contributed much to the 

 subject. A full account of these matters is given also in the very excellent 

 work of Hilgard, "Soils" (New York, Macmillan Co., 1906), more briefly 

 in King's "The Soil" (New York, Macmillan Co., 1809), and in Waring- 

 ton's "Lectures on Some of the Physical Properties of Soil "(Oxford, Claren- 

 don Press, 1900). There are some simple methods of demonstrating the 

 most important of the above-named properties in the Year-took of the United 



