THE DISI'IAY OF ENERGY 367 



1. Thermal, that is, changes of temperature, as extremes of heat or 

 cold. 



2. Photic, Hght changes both in direction, intensity, and color. 



3. Chemical, changes that occur in the concentration of certain 

 substances which may come in contact with the organism. 

 Such changes might be the presence of salts, acids, alkalies, or 

 other substances in the soil, or various types of chemical sub- 

 stances such as are found in the food of animals. 



4. Electric, changes in the direction and strength of electric cur- 

 rents. Since the modern concept of matter is interpreted in 

 terms of electricity, it must be realized that these changes may 

 have a profound effect on living organisms. 



5. Mechanical stimuli, such as changes in osmotic pressure within 

 cells, the pull of gravity, changes in pressure of the medium. 

 Contact with various objects, and sound waves, are also impor- 

 tant. Many animals and plants respond definitely also to cur- 

 rents of air or water. 



In unicellular organisms responses are usually more predictable than 

 in higher organisms because the latter are complex structures in which 

 different parts may be differently affected by the same stimulus. For 

 example, gravity may act negatively on the stems of green plants and 

 positively on the roots of the same plant. While the stem of a plant 

 may be influenced to grow toward light the roots grow away from it. 

 These examples might be multiplied many times. 



Tropisms 



In 1918 Jacques Loeb, one of the foremost investigators in this 

 country, brought out a book entitled, Forced Movements, Tropisms, 

 and Animal Conduct. The author took for his thesis the mechanistic 

 point of view of life. To him, and to other members of his school, living 

 organisms are mechanisms whose activities are directly influenced by 

 the stimuli in their environment, the sum total of behavior being the 

 direct result of their reactions to various stimuli. In a series of con- 

 vincing experiments, Loeb showed that animals are forced to do certain 

 things because of a purely mechanical effect brought about by the 

 stimuli impinging upon them. If, for example, the common shrimp 

 (Palaemonetes) is placed in a trough through which an electric current 

 flows, with its head toward the anode pole, the tail at once becomes 

 stretched out . If it is placed with its head toward the cathode pole, the 

 tail is bent under the body. In the latter case the animal can only 



