THE PROTOPLASMATIC MEMBRANE 



horny mass, hard, inflexible, and brittle. When it 

 is more or less saturated with water, it becomes 

 flexible, viscous, then gelatinous, and in the 

 course of imbibition with water it approaches the 

 liquid state. Many gels have the character of a 

 gelatinous mass. Some, as gum-arabic, finally 

 dissolve entirely. Others, as cherry gum, swell in 

 water to a jelly and never dissolve. Doubtless 

 gels are of great importance in plasmatic structure. 

 They are formed in plasmatic colloids by many 

 influences, such as surface tension, electrolytes, 

 and the mutual precipitating effects of colloids. 

 Wherever protoplasm sols meet precipitating in- 

 fluences, films must be formed, which separate the 

 different parts from each other. Such gel-mem- 

 branes, on the other hand, play the part of semi- 

 permeable filters. Some substances are soluble in 

 them, and consequently pass through, but other 

 substances being insoluble in the gel substance are 

 retained. There is still another retention of 

 substances in gels which is not a consequence of 

 their insolubility, but, on the contrary, must be 

 traced back to some affinity of the substance 

 retained with the gel colloid. We call this process 

 of retention Adsorption of Substances. There is 

 no doubt that adsorption is of the greatest im- 

 portance for chemical processes in life. We have 

 especially to consider that the resorption of dis- 

 solved substances by cell protoplasm from the 

 surrounding liquids must be connected with ad- 



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