116 TEXTBOOK OF PLANT PHYSIOLOGY 



rupture. Rupture rarely takes place, however, as with the dis- 

 tention of the cell its volume rapidly increases, and simultaneously 

 the concentration of the cell sap falls. 



Another instance of cells with very high concentration of the 

 cell sap is the pollen of many plants. According to Art. 35, the 

 pollen, as long as it is dry, shows no osmotic pressure. When 

 transferred to the stigma, it finds there a sugary liquid of rather 

 high concentration, balancing the internal concentration of the 

 pollen cells which in germinating increase their volume gradually. 

 In pure water, the pollen frequently bursts, owing to the excess of 

 inner pressure. This explains the harmful influence of rain and 

 fog on the setting of seed. From this, the various modes of pro- 

 tecting the pollen against the influence of atmospheric precipita- 

 tion, realized by very intricate arrangements in the structure of the 

 flower, become comprehensible. 



The osmotic pressure in the cell is not constant. It varies con- 

 tinually, depending on the chemical processes of the cell. Thus, 

 when starch is hydrolyzed into sugar, the pressure is increased 

 considerably. It is decreased as a result of the reverse process, 

 the accumulation of starch at the expense of sugar. The imperfect 

 oxidation of sugar, accompanied by an accumulation of organic 

 acids of small molecular weight, as for example, oxalic acid, also 

 leads to an increase of osmotic pressure. By changing the chem- 

 istry of its internal processes, the plant is able thereby to regulate 

 to a certain degree its osmotic pressure. 



The plasmolytic method, in which the pressure is calculated 

 from the concentration of the plasmolyzing solution, is most fre- 

 quently used for determination of the osmotic pressure. Very 

 often only the concentration isosmotic (i.e., causing an equal 

 pressure) with the cell sap is recorded. This magnitude has been 

 termed the "osmotic value." Following the example of DeVries, 

 we formerly used solutions of potassium nitrate for this purpose. 

 Now, cane sugar is preferred, as it is harmless to the protoplasm, 

 while strong salt solutions frequently induce pathological changes. 

 Besides, for solutions of cane sugar there now exists carefully cal- 

 culated tables showing the osmotic pressure for any concentration, 

 and translated into terms of atmospheric pressure. 



37. Substances Causing Temporary Plasmolysis. Perme- 

 ability of the Protoplasmic Membrane to Different Substances 

 and the Problem of Its Composition and Structure. — DeVries, 



