Chemical Processes in Plants. 333 



carried to the various organs by the blood -stream, and 

 undergo chemical changes within the cells, the products of 

 these changes being afterwards removed and transported 

 away by the blood. Actions of this description involve a 

 knowledge of the conditions which render the cells per- 

 meable or otherwise to the substances with which the 

 organs come into contact ; again the investigator is faced 

 with problems dealing with the state of aggregation in a 

 heterogeneous system this time of the materials forming 

 the cell membranes. 



As examples of other problems which involve a know- 

 ledge of physical chemistry may be mentioned those con- 

 cerned with the processes of secretion, immunity, and the 

 pharmacological action of drugs. 



Nevertheless, as a preliminary to all such studies, it is 

 absolutely essential that acquaintance should be made with 

 the characters of the chief substances of which a living 

 organism is made up, and a review of these characters 

 constitutes a fitting "Introduction to the Study of Biological 

 Chemistry." 



