340 PLANT PHYSIOLOGY 



ment of the maximum osmotic pressure contains not pure water 

 but the solution fiUing the osmometer. If additional liquid is 

 poured into the tube, the level in the latter will nevertheless very 

 soon return to its previous height. This is explained by the 

 fact that the excess of hydrostatic pressure, as a result of the 

 increase in height of the column in the tube not being counter- 

 balanced by the osmotic pressure, will force a part of the water 

 outward, through the walls of the osmometer. 



Now let us suppose two similar osmometers A and B (Fig. 109) 

 connected by means of a glass tube, through which the contents 

 may circulate. Osmometer A is filled with a concentrated solu- 

 tion; osmometer B, with a weaker 

 solution. For the purpose of 

 demonstration, suppose the con- 

 centrated solution is colored with 

 some dye. If both osmometers are 

 Fig. 109.— Diagram showing the immersed in One vessel with water 



movement of organic substances in ^ \^ ^^\[\ \^q observed that their 

 plants. A and B are permeable . i- , i i • j. 



membranes (osmometers) sub- contents mimediately begm to 



merged in water and connected by jxiOVe. Osmometer A wiU begin tO 

 a glass tube. The concentration i i j. x- 



in A is higher than in B. V, absorb Water; a hydrostatic pres- 

 vapor; w, liquid. g^re will arise inside the system 



that will be higher than the osmotic pressure in the osmometer B; 

 and as a result, water will be forced out of the latter into the 

 surrounding vessel. The solution from osmometer A will then 

 flow through the connecting tube V into osmometer B, replacing 

 the water that is being pressed out of it. This process will con- 

 tinue so long as the concentrations in A and B are not equal. 

 When . they are the same in both vessels, the osmotic pressures 

 will also become equal, and the whole system will be in 

 equilibrium. 



The same result of the leveling of the concentrations in the 

 reservoirs A and B may be obtained by means of diffusion of the 

 dissolved substance, with the following essential difference, 

 however. While with a flow, caused by pressure, equilibrium 

 is attained in several hours, in the case of equalization by means 

 of diffusion— for instance, if the reservoirs were made of glass- 

 it would take many years to reach equihbrium. According to 

 Munch's calculations, with the difference of the osmotic pressure 

 in the two reservoirs being only 1 atmosphere, the velocity of 



