OSMOTIC PRESSURE AS AN INDEX OF HABITAT 1011 



Three noteworthy investigations concerning the freezing-point de- 

 pression (and consequently the osmotic pressure) of plant tissues in 

 relation to environment have appeared within the last ten months. 



Hibbard and Harrington,^ besides considerable work on methods of 

 determining the freezing-point depression, studied its relation to soil 

 moisture. They maintained, by means of Livingston auto-irrigators, 

 moisture contents of from ii to 31 per cent of dry weight. They found 

 that corn plants grown under these different moisture conditions had 

 freezing-point depressions of from 1.835° C. to 2.204° C. for the tops 

 and from .492° C. to .995° C. for the roots. This shows that soil 

 moisture has a marked effect on the osmotic pressure of the cell sap. 



Harris, Lawrance, and Gortner' determined the freezing-point de- 

 pressions and corresponding osmotic pressures of the leaves of plants 

 growing under dift'erent environmental conditions in the neighborhood 

 of the Desert Laboratory at Tucson, Arizona. They found that in 

 general the drier the habitat, the greater the osmotic pressure of the 

 cell sap. It is interesting to notice that the osmotic pressure of the 

 trees and shrubs was greater than that of the dwarf and half shrubs, 

 which in turn had higher pressure than the perennial herbs, and that 

 the winter annuals had the lowest of all. These investigators consider 

 that the freezing-point depression is just as important in ecology as the 

 structural characteristics of the plant upon which ecologists have 

 hitherto so largely depended. 



McCool and Millar* have greatly improved the method of determin- 

 ing the freezing-point depression and have thrown much light on the 

 relation between the osmotic pressure of cell sap and environmental 

 factors. 



Formerly it had been considered necessary to use the juice pressed 

 out of the plant tissue. This is inconvenient because : first, the actual 

 pressing is laborious ; second, the liquid has to be cleared as much as 

 possible by centrifugal force ; and third, with tough and comparatively 

 dry tissues it is necessary to collect and press large quantities in order 

 to secure enough juice. Furthermore, following Dixon and Atkins,^ it 



* Hibbard, R. P., and Harrington, O. E. : "Depression of the freezing point in 

 triturated plant tissues and the magnitude of this depression as related to soil 

 moisture." Physiological Researches, Vol. i, No. 10, pp. 441-454, 1916. 



' See note i for reference. 



* McCool, M. M., and Millar, C. E. : "The water content of the soil and the 

 composition and concentration of the soil solution as indicated by the freezing- 

 point lowerings of the roots and tops of plants." Soil Science, Vol. 3, No. 2, 

 pp. 113-138, 1917. 



° Dixon, H. H., and Atkins, W. R. G. : "Osmotic pressure in plants. I. Method 

 of extracting sap from plant organs." Sci. Proc. Roy. Dublin Soc.-N. S., 13, 

 422-433. 1913. 



