STRUCTURE AND PROPERTIES OF PROTOPLASM 65 



SECTION 4. THE REACTIONS OF PROTOPLASM TO 

 EXTERNAL FORCES. 



Living Protoplasm is of soft and plastic consistency, con- 

 tains many complex and highly unstable constituents, and is 

 the seat of numerous and diverse energy changes. It is thus 

 brought into extremely delicate and unstable touch with the 

 innumerable forces acting upon it from the environment, and 

 the resultant relations are of the utmost physiological importance. 

 Theoretically it might seem best to study such relations by 

 direct microscopic observation of the effects of the forces upon 

 the living Protoplasm, but with rare exceptions this method 

 entails great or insuperable manipulative difficulties. Practi- 

 cally, therefore, it is best to infer the relations from the action 

 of the forces upon entire plants, and in this way the student 

 will gradually become acquainted with them during the present 

 course. It must suffice to consider under this section certain 

 typical cases in which the reaction of protoplasmic activity to 

 external forces can be directly observed. These forces are 

 essentially these six: Heat, Light, Electricity, Mechanical Im- 

 pact, Chemical Action, Gravitation. As to their action upon 

 the Protoplasm, it is plain, both upon theoretical grounds and also 

 from the results of experiments with which the student will later 

 become familiar, that they may operate in either one of three 

 ways, which ways are recognizable for practical convenience 

 as distinct, though ultimately they are no doubt related if not 

 identical. 



First of the three ways, the forces may act upon the Proto- 

 plasm purely mechanically, physically, or chemically, precisely 

 as they would upon any other substance of similar constitution, 

 quite regardless of its vital properties, or whether it be living or 

 dead. Thus heat may coagulate it or burn it to ash; pressure 

 may cruoh it; a chemical may dissolve it, and gravity may pull 

 its heavier down through its lighter parts. Here the force must 

 be present in considerable amount, its action is direct or imme- 



