ROOTS AND THEIR FUNCTIONS 149 



story. Protoplasm is a colloid and, like many colloids, has the property 

 of soaking up (imbibing) water. Much of the water taken in by imbibition 

 is closely held to the colloidal particles by adsorption, but the remainder 

 is more loosely held, and can be withdrawn from the cell by osmosis or by 

 suction. 



Besides osmosis and imbibition, there is a third factor concerned in 

 water absorption. This is suction force, concerning which a few words of 

 explanation are required. The plant cell is contained within more or 

 less rigid walls against which the cell membrane is pressed. These walls 

 can resist varying amounts of pressure, depending upon their strength 

 and the amount of counterpressure exerted against them by adjoining 

 cells. The cell as a whole tends to maintain an unchanging volume. When 

 water enters the cell by osmosis, an internal pressure is created. The 

 actual pressure exerted by the cell contents against the cell wall is called 

 turgor pressure. It results from but is not the same as osmotic pressure, 

 which is the maximum pressure that can be developed in a given solu- 

 tion separated from pure water by a rigid semipermeable membrane. 

 Turgor pressure is determined in part by the osmotic pressure of the cell 

 contents and in part by the available water supply outside the cell 

 membrane. 



Let us consider a hypothetical cell in which the sum of the molar con- 

 centrations 1 of all the solutes present amounts to 0.3 M . This cell, im- 

 mersed in pure water, should attain a turgor pressure equal to the osmotic 

 pressure and equivalent to 0.3 M at 0°C, or 6.7 atmospheres. When the 

 cell is immersed in a 0.1 M solution, its turgor pressure should be equiva- 

 lent only to 0.2 M , and in a 0.3 M solution its turgor pressure must be 

 zero. The cell would lose water to any solution with a molar concentra- 

 tion greater than 0.3 M. 



A cell which has a capacity to reach a 0.3 M turgor pressure but ac- 

 tually has a pressure of only 0.2 M has a turgor deficit of 0.1 M. This 



1 A mole (gram-molecular weight) of any substance is the number of grams of that 

 substance that corresponds to its molecular weight. A mole of any substance there- 

 fore contains the same number of molecules as a mole of any other substance. A molar 

 solution is one that contains one mole of solute per liter of solution; such a solution is 

 said to have a molar concentration of 1.0 M, where M stands for molar. The osmotic 

 pressure is a function of the number of solute particles relative to the number of 

 solvent particles, regardless of what the solute particles are. Any dilute solution of a 

 nonelectrolytic substance contains as many solute particles as there are molecules of 

 solute; therefore a solution containing 0.1 M glucose, 0.1 M maltose, and 0.1 M lac- 

 tose has the same molar concentration (0.3 M) as one containing 0.3 M glucose. The 

 osmotic pressure of a 1.0 M solution is 22.4 atmospheres at 0°C; that of a 0.3 M 

 solution is 6.7 atmospheres. Electrolytes such as sodium chloride (NaCl) have higher 

 osmotic pressures than nonelectrolytes, since in solution some of their molecules are 

 dissociated into ions which act as independent solute particles. 



