IRRITABILITY 237 



mentally the same causes, though there is no proof of this. 

 ( See discussion of heliotropism and heliotaxis on page 219. ) 

 If the chemical theory advanced in explanation of the influ- 

 ence of light upon the direction of growth and of locomotion 

 be true (p. 210), the influence of light upon the living 

 protoplasm is only through those compounds in the cell 

 which are acted upon by light. So. far, then, as the proto- 

 plasm only is concerned, it may be that heliotropic and 

 heliotactic phenomena are intrinsically chemotropic and 

 chemotactic. The sources of the influence upon the chemical 

 compounds within the cell are different, the influence which 

 directly affects the living protoplasm may be only that of 

 chemical compounds, not of light. 



INFLUENCE OF ELECTRICITY 



The electrical conditions in soil and air are constantly 

 changing, but except at rare intervals no great amount of 

 electrical force becomes evident or is concentrated in any 

 one place. Under natural conditions living organisms are 

 ordinarily exposed to feeble currents only, and to such they 

 are accustomed. Under the artificial conditions prevailing 

 in the planted streets and in the parks of cities, in the soil 

 of which powerful electric currents are induced or through 

 which they are transmitted in consequence of the very 

 general use of electricity^, the roots of plants are peri- 

 odically or constantly subjected to more violent electri- 

 cal influence. Not all electricity, however, comes from 

 without, for in the plant itself, under normal conditions, 

 electrical as well as other currents are constantly developed 

 and maintained. These are very likely due to the chemical 

 changes going on in the plant and in its different parts, and 

 just as these processes affect the balance of other forces— of 

 heat, osmosis, and chemical energy — so they may readily 

 alter the balance of electrical tensions in different parts of 

 the organism and thereby set up electrical currents. If, 

 by chemical and physical changes in the organism, elec- 

 trical currents develop in it, it is natural to suppose that 

 electrical influences brought to bear upon the organism 



