133- Salts change cell permeability. Often a pure salt affects 

 the permeability of the plasma membrane so that it cannot be used 

 to determine the osmotic pressure of the cell. A mixture of certain 

 salts (especially salts of Na, K, and Ca ) maintains the cell surface 

 in a normal condition. 



Place Spirogyra cells in the following solutions : 



1. Cane sugar: m 2, m 4, m/6, m/8. 



2. XaCl : m/2, 111/4, ni () . ni/8. 



3. XaCl (95 vols. ) -+- CaCl, (5 vols. ) : m/2, m/4, m/6, m/8. 

 Determine the plasmolytic limiting concentration in each case. 



The cane sugar will giye the true osmotic pressure. Note carefully 

 if at first plasmolysis begins, then disappears and then appears again 

 (false plasmolysis) in any of the XaCl solutions. Compare with 

 the mixtures of Xa and Ca. Explanation? 



134. Salts affect the penetration of NaOH. Place Spirogyra or 

 Elodea, stained in neutral red, in the following mixtures. 



1. 111/40 XaOH. 



2. 111/40 NaOH + m/8 XaCl. 



3. m/40 XaOH -f m/8 (95 vols. XaCl + 5 vols CaCl,). 

 Determine the time required for entrance of alkali in each case. 

 As a control kill the cells in chloroform water and place in the 

 above solutions. Conclusion ? 



C. SURFACE TENSION AND RELATED PHENOMENA 



Note in all the following observations and experiments that the 

 surface behaves in many ways as if it were a stretched elastic mem- 

 brane. 



A. THE SURFACE FILM. 



Experiments to give a concrete realization of the existence of 

 surface tension. 



135. Drop a needle carefully on the surface of still water, 

 so that the entire side of the needle strikes the surface at once. 

 \\~hy does it float ? Notice the depression on the surface film close 

 to the needle. 



136. Drop water slowly in drops from a pipette. Notice the 

 form of the drops, and how they become stretched or elongated 

 just before they drop. \Yhy do they take the form they do? 



137. Form a film on a circular wire frame with soap solution. 

 (This is really a double film.) Does the film pull? Test this by 

 forming a film on a circular wire frame, laying a loop of thread on 



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