326 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 14 



appearance of a globe studded with projections like the head of a 

 crusader's mace. 



That a substance may plasmolyze a cell it is obvious that it must 

 fulfill certain conditions. It must exert osmotic pressure; it must not 

 be able to penetrate into the cell, or if it does, it must do so very slowly, 

 that is, more slowly than water, for if it passes quickly into the cell the 

 osmotic pressure within and without will be very quickly equalized 

 and there will be no plasmolysis. In other words, the cell-membrane 

 must be semi-permeable to the plasmolyzing substance. A semi- 

 permeable membrane is one that permits only certain dissolved sub- 

 stances to pass through it. Now most cell-membranes are semi- 

 permeable to most salts of organic as well as inorganic bases, to the 

 sugars and to many other substances. They do not permit these 

 substances to pass through readily. When brought into solutions of 

 such substances, cells may be plasmolyzed. One group of organisms 

 seems to form an exception. A large number of bacteria do not seem 

 to be surrounded with membranes semi-permeable in this sense; 

 salts pass into them apparently unhindered. They all can be stained 

 according to the method of Gram, and are known as the Gram positive 

 bacteria. They cannot be plasmolyzed. 



Considerations of this sort help us to understand the action of 

 certain salts and other substances upon the intestines. You all know 

 that Glauber's salt (sodium sulfate) and Epsom salt (magnesium 

 sulfate) are laxatives. Now one of the characteristics that distinguish 

 such salts from others, say sodium chloride, is that they are not readily 

 absorbed through the intestinal wall. Not being absorbed readily 

 they cause the osmotic pressure of the intestinal contents to remain 

 great as the food passes down the alimentary canal. In order to with- 

 draw water from the intestinal contents the intestines would have to do 

 an amount of osmotic work which would be very great indeed. The 

 result is that water remains in the intestinal contents or is secreted into 

 it so that the contents remain bulky and peristalsis is stimulated. 

 This is one of the main factors, though not the only factor, in the 

 purgative action of these salts. It is a physical factor dependent upon 

 the slow absorption of these salts through colloid membranes, rather 

 than upon any strictly chemical reaction dependent upon the structure 

 of the molecule or its chemical reactivity. 



We have seen that the phenomenon of plasmolysis furnishes a method 

 to tell what substances do not enter cells. Conversely we can tell 

 what substances do pass readily into cells, for such substances will not 



