RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 103 



tions, were it not for the effect on the boiling point of the character 

 of the vessel itself, of gases in the liquid, and of solid particles which 

 form nuclei for steam bubbles. It is also self-evident that the boiling- 

 point method is not applicable to soils, and hardly more applicable 

 to plants unless the sap has been separated from the pulp, which, 

 under certain circumstances, as in the treatment of conifers, may be 

 impossible of attainment. 



The freezing-point method comes next in order, and has been con- 

 siderably used; though, at this stage, it is well to mention that the 

 foliage of coniferous trees frequently becomes so dry that a definite 

 freezing point can not be determined, probably because of the lack 

 of conductivity in the mass, which is such that each particle of the 

 pulp may freeze without affecting the rest of the mass quickly. 



The vapor-pressure method does not look so promising, because 

 of the technical difficulties in the way of any precise determination of 

 vapor pressure. However, the complicated apparatus necessary for 

 the direct determination of a vapor pressure may be done away 

 with if instead the determination of vapor pressure is made in a 

 vessel by means of a solution which is in equilibrium with that vapor. 

 This method especially commends itself in the treatment of soils 

 because of the possibility of preparing them and retaining them dur- 

 ing treatment in a state of compactness and granulation approaching 

 the natural. It does not seem so applicable to plant tissues because 

 of the danger of fermentation and enzymic action during the treat- 

 ment. 



The determination of osmotic pressures in plant cells by plasmolysis, 

 while evidently useful for the examination of restricted areas, such 

 as the epidermis, and possibly useful for any tissues which are ex- 

 ceedingly dry, does not recommend itself for general purposes be- 

 cause of the large amount of manipulation necessary and the experi- 

 mentation required to find the balancing solution. This method, of 

 course, necessitates the observation of individual cells under the 

 microscope, when placed in media of various osmotic concentrations. 



Osmotic Pressure of Plant Tissues. 



Dixon and Atkins (119), in 1913, were apparently the first to use 

 the then developing theoretical knowledge of the behavior of solu- 

 tions as a means toward looking into the internal conditions of plants. 

 In the citation given they deal at length with the method of extract- 

 ing sap from plant tissues for the purpose of freezing-point determi- 

 nations. 



Hibbard and Harrington (126), in 1916, and Harris, Lawrence, 

 and Gortner (123), in the same year, followed this work with further 



