34 PHYSICAL CHEMISTRY 



of unknown that has not changed in length found; its osmotic 

 pressure is between those of the graded solutions on each side 

 of it. One cc of the unknown is sufficient and hence the method 

 is useful in biological work. This method has the advantage that 

 it measures the osmotic pressure at any desired temperature. 

 The chief error is in the mixing of the drops, caused by the 

 adhesion of a surface film to the glass and the transfer of fluid 

 from one drop to another in this film. 



The way in which osmotic pressure may do work in the or- 

 ganism is best illustrated in plants. The plant cell is limited by 

 the plasma membrane which is impermeable to some dissolved 

 substances. Surrounding the plasma membrane is the cellulose 

 cell wall. If the cell is placed in water the osmotic pressure of 

 its interior causes it to swell, and the plasma membrane is pressed 

 against the cellulose cell wall and tightens it or makes the cell 

 turgid. This turgidity (turgor) is what gives stiffness to herba- 

 ceous plants, lacking which the plant is said to be wilted. The 

 osmotic pressure of many plant cells is a number o'f atmospheres, 

 and the cell wall has to be very strong in order to withstand the 

 pressure. The extent of this pressure is realized when the roots 

 of trees growing between rocks separate the rocks by the osmotic 

 pressure exerted. 



The cell wall of a young plant cell is very thin and is stretched 

 by the osmotic pressure as the cell grows. Some mechanism 

 must be provided for getting the osmotic substance into the cell, 

 before it can stretch the cell and cause growth. In the green 

 plant cell the osmotic substances are manufactured with the aid 

 of sunlight out of H 2 and C0 2 , to which the cell is freely 

 permeable. The colorless cells absorb certain organic substances 

 secreted by the green cells, transforming them into other sub- 

 stances to which the plasma membrane is impermeable, and which 

 therefore accumulate in the cells and cause the osmotic pressure. 

 Though these osmotic substances are not in all cases definitely 

 known, tannic and oxalic acid and their salts have been postu- 

 lated. The sugars are supposed by some investigators to be the 

 nutritive substances passed from the green to the colorless cells, 

 meaning that both kinds of cells are permeable to sugar. Over- 

 ton, however, claims that plant cells are impermeable to sugar 



