Exercise XV 



ORGANIZATION OF HIGHER PLANTS 75 



eucalyptus — may be over 300 feet high. How sap 

 is raised to such heights is a problem that has 

 plagued plant physiologists for generations. A 

 prevalent type of theory, entertained for a time, 

 was that as water evaporated from the leaves, it 

 left a vacuum in the ascending vessels (xylem), 

 which drew water upward. Even if one could 

 establish a perfect vacuum in the upper vessels 

 of a tree — a very unlikely possibility — this 

 would provide a pressure of only 1 atmosphere 

 to raise the sap. One atmosphere pressure 

 raises water about 34 feet. To bring sap to 

 the top of a 300-foot tree would require about 

 9 times this force, that is, about 9 atmospheres 

 pressure. 



OSMOTIC PRESSURE AND 

 PLASMOLYSIS 



There is little doubt that the major force for 

 the ascent of sap in plants is osmotic pressure. 

 Whenever two solutions are separated by a semi- 

 permeable membrane — a membrane that readily 

 passes water and small molecules, but blocks the 

 passage of larger molecules and some ions — water 

 tends to flow through the membrane from the 

 more dilute to the more concentrated solution. 

 It is easy enough to understand why. The more 

 concentrated side in terms of dissolved molecules 

 is the more dilute side in terms of water. Sup- 

 pose that there were pure water on one side of 

 such a semipermeable membrane, and a 10% 

 solution of molecules that could not go through 

 the membrane on the other side. At every instant 

 large numbers of molecules collide with the 

 membrane from both sides. On the side contain- 

 ing pure water, of every 100 molecules that hit 

 the membrane, 100 would go through. On the 

 other side, of every 100 molecules that hit the 

 membrane, only 90 would go through, that is, 

 only the molecules of water. The result is a tide 

 of water into the solution, exercising a water 

 pressure (the osmotic pressure) that tends to raise 

 its level to such a point that the added weight of 

 water pressing downward counterbalances the 

 further entrance of water. The height to which 



the level of the solution rises on the more con- 

 centrated side is a measure of its osmotic pres- 

 sure. 



A simple formula makes this relationship 

 quantitative. You know from last semester that 

 1 mole of any gas in a volume of 22.4 liters has 

 a pressure at 0°C of 1 atmosphere. In exactly 

 the same way, 1 mole of solute that cannot get 

 through a semipermeable membrane, distributed 

 in a volume of 22.4 liters of water, has an osmotic 

 pressure at 0°C of 1 atmosphere. That is, 1 mole 

 of such nondiffusing material dissolved in 22.4 

 liters of solution (a 0.045 M solution) exerts an 

 osmotic pressure that can raise water 34 feet. 

 To raise water 300 feet by this means would 

 require only about a 0.4 M solution. 



It is important to note that what one is con- 

 cerned with in accounting for osmotic pressure is 

 the total concentration of particles that do not 

 penetrate the membrane, whatever their nature. 

 The "particles" may be all alike, or greatly 

 mixed, small or macromolecules, or even molec- 

 ular aggregates, indeed, anything dispersed in 

 water that does not go through the membrane. 

 The essential factor is the degree to which such 

 particles dilute the water on both sides of the 

 membrane. 



The protoplasm of plant cells contains dis- 

 solved substances which do not readily diffuse 

 through the semipermeable plasma membrane. 

 Hence when a cell is immersed in water, more 

 water molecules diffuse into the cell than diffuse 

 out. The net tide of water into the cell inflates it, 

 producing a pressure against the cell wall. This 

 turgor pressure keeps the cell plump and relatively 

 rigid. Conversely, drying the cell or placing it in 

 a more concentrated solution, by withdrawing 

 water, decreases the turgor pressure, causing the 

 cell to soften or wilt. 



When a plant cell is placed in water, water 

 enters until the turgor pressure is large enough to 

 counteract its further (net) entrance. At this 

 point the turgor pressure, driving water out of 

 the cell, equals the osmotic pressure, drawing 

 water in. In this state of equilibrium, water has 

 not stopped moving in and out of the cell, but it 

 is moving in and out at equal rates. 



