PHTSIOLOGT 29 



ness a brief outline of some of them may not be 

 amiss. 



Consider first of all the astonishing difference 

 which may exist between the inside and the out- 

 side of the cell. Valonia macrofhysay which 

 grows in sea water, excludes certain substances 

 absolutely as long as it remains in a normal con- 

 dition j this is the case, for example, with mag- 

 nesium and with sulphate. If it admits calcium 

 at all it is only in traces. Sodium is admitted but 

 its concentration remains far below that in the 

 sea water. On the other hand, potassium is stored 

 up within the cell in much greater concentration 

 than is found in sea water. It is evident that the 

 cell possesses a trapping mechanism for accumu- 

 lating certain substances and also a means of 

 excluding certain other substances either wholly 

 or in part. 



As a consequence the inner and outer surfaces 

 of the protoplasm may be in contact with very 

 different solutions. Is it necessary for the life 

 of the cell that they should be different? What 

 will happen if they are made identical? To 

 answer this question sap was extracted and living 



