CELLULAR 



piece of glass tubing. If we select a membrane 

 permeable to water but not to sugar, placing 

 inside of it a solution of sugar and outside of it 

 pure water, we find that as water is absorbed the 

 level rises in the tube 5 if we endeavor to pre- 

 vent this by means of a weighted piston placed 

 in the tube, we find that considerable force is 

 requisite. The pressure produced within the 

 apparatus is called osmotic pressure. 



It is supposed that pressure is produced in 

 much the same way in the living cell. Here it is 

 not the cell wall but the protoplasm (or its outer 

 layer) which corresponds to the membrane in 

 our experiment, and, like the membrane, it is 

 permeable to water but is more or less imperme- 

 able to sugar and certain other substances. Such a 

 membrane, permeable to some substances but 

 not to others, is called semipermeable. 



The discovery of these facts opened up a field 

 of great interest but no great progress was made 

 until quantitative methods began to be applied j 

 as soon as this happened developments of the 

 highest importance took place. The quantitative 

 era may be said to begin with some observations 



