CLIMATIC FACTORS 131 



pheres. In plants like Umhih'cits pejidulinus the osmotic value rises to 

 13 atmospheres, at which point the leaves begin to wither. 



From investigations by Walter and by Walter and Braun-Blanquet 

 it appears that winter cold acts like desiccation in increasing osmotic 

 values. Walter found during the winter that species with Atlantic 

 affinities Hke Hedera helix, Ilex aquifolium, Helleborus foetidus, and 

 Sarothamnus were able to increase the density of their cell sap up to 20 

 atmospheres but died when the concentration reached 25 atmospheres, 

 while Buxus sempervirens was able to increase its concentration from 33 

 to 72.6 atmospheres without injury. 



It is known that concentrated solutions of sugars and of certain 

 salts very greatly increase the resistance of certain plants to cold. 

 Iljin has demonstrated experimentally that the accumulation of sugar 

 in the cell sap is also caused by dryness. He insists on the fact that 

 the action of salts on the resistance of cells depends upon the concentra- 

 tion of the sap as well as upon the specific constitution of the 

 protoplasm. Solutions of both sugar and salts possess the property 

 of plasmolyzing plant cells. Plasmolysis is the first sign, according 

 to Iljin, of the concentration of the cell sap. This explanation does 

 not contradict the observations of the osmotic values in plants. 



Osmotic Concentration of the Soil Water. — The osmotic concentra- 

 tion of the cell sap of plants is opposed to the osmotic concentration of 

 the soil water. High osmotic values in the soil prevent the free intake 

 of water by the roots of plants and tend to make the habitat barren of 

 vegetation. Stocker (1930) found that in the sodium soils of Hungary 

 an osmotic concentration equivalent to 28 atmospheres made the 

 acquisition of water by plants impossible. At concentrations equiva- 

 lent to 28 to 12 atmospheres only steppe plants were able to grow; 

 and only habitats with concentrations of less than 12 atmospheres were 

 open to colonization by non-steppe plants. The osmotic concentration 

 in roots usually exceeds that of the soil water by 2.7 atmospheres or 

 more. 



Measurement of Osmotic Concentration in Soil. — Strips of filter paper 

 saturated with a cane-sugar solution of known concentration placed in 

 contact with the soil solution must increase or decrease in weight 

 depending on whether the soil solution has a greater or a lesser con- 

 centration than the sugar solution. Strips 4 by 4 cm. are folded once, 

 attached to a small silver or copper wire, and dropped into the sugar 

 solution. They are then taken out, the surplus solution removed with 

 dry filter paper, are then quickly weighed, and hung by a wire in wide- 

 mouthed bottles which contain the soil samples. The wires may be 

 fastened to the corks with sealing wax and a grain of thymol added to 



