20 PLANT PHYSIOLOGY 



chloride or other mmeral salts. In other plants, osmotic pressure 

 is induced chiefly by sugars and organic acids. 



Such immense magnitudes of the osmotic pressure in the cells 

 appear improbable at first sight and give rise to questions as to 

 how the thin cell walls are able to withstand the tension. A more 

 detailed study of the question shows, however, that the mag- 

 nitude of the osmotic pressure, calculated according to the' 

 concentration of the cell sap, considerably exceeds the actual 

 hydrostatic pressure on the cell walls. The solution absorbed 

 from saline soils, surrounding the cells of halophytes, shows a 

 very high concentration, and the cell wall is subjected only to 

 the excess of the inner over the outer pressure. If these plants 

 are placed in pure water, the abrupt increase of hydrostatic 

 pressure of the contents leads to a strong distention of the cell 

 walls and sometimes to their rupture. This takes place but 

 rarely, however, as with the distention of the cell its volume 

 rapidly increases, and simultaneously the concentration of the 

 cell sap and the osmotic pressure in the cell decreases. Osmotic 

 pressure varies continually, depending on the chemical processes 

 of the cell. Thus, when starch is hydrolyzed into sugar, the 

 pressure is increased considerably. It is decreased as a result 

 of the reverse process, the accumulation of starch at the expense 

 of sugar. The imperfect oxidation of sugar, accompanied by an 

 accumulation of organic acids of small molecular weight, e.g., 

 oxaUc acid, also leads to an increase of osmotic pressure. By 

 changing the chemistry of its internal processes, the plant is 

 able thereby to regulate to a certain degree its osmotic pressure. 



6. Laws Controlling the Penetration of Water into the Cell. 

 Suction Tension of Cells and Methods of Its Determination. — 

 The absorption of water by the cell from the surrounding medium 

 is determined by the colloidal and osmotic properties of the cell. 

 The phenomena taking place in dry seeds immersed in water or 

 placed in a moist soil will first be examined. 



First, the seeds will swell, considerably increasing in volume. 

 Swelling is conditioned by the fact that both the cell walls and 

 the protoplasm as well as the reserve food substances filling the 

 cells of the seeds represent dry colloidal gels that attract water 

 with great force. The limits of swelling are attained by various 

 substances at different percentages of water content The 

 greatest swelling is displayed by protein substances; starch 



