IRRITABILITY 227 



it may be purely physical or it may be chemical, involving 

 some change in composition. The same substance in differ- 

 ent concentrations, may produce favorable or unfavorable 

 effects or apparently none, and the effects may be first 

 physical and then chemical, or vice vei'sa. The effect pro- 

 duced is dependent upon, 1st, the acting substance, 2d, the 

 organism, 3d, the conditions prevailing at the time. The 

 organism will vary at different times in its activities, con- 

 tents, and composition, and will therefore vary in its reac- 

 tion or response to external influences. 



In studying the influence of gravity upon plants, w r e were 

 dealing with a constant force and a varying organism. In 

 studying the influence of light, we were dealing with a vary- 

 ing force as well as var} r ing organism, but the force pro- 

 duces its effects always by the same means. In studying the 

 influence of water, we were dealing with a single, simple 

 compound varying in quantity, but in its effects only 

 because of this fact and of the varying organism. In study- 

 ing the influence of other substances upon protoplasm 

 we have to deal with their relations to water and to each 

 other as well as to the organism. This materially compli- 

 cates the subject. The effects of common salt illustrate 

 these several points. A very dilute solution of common salt 

 (Nad) acts unfavorably upon plants the roots of which 

 are bathed in it. The sodium and chlorine atoms become 

 dissociated and act as independent poisons. Whether the 

 poisonous action of these atoms is due to their own chemi- 

 cal properties or to their electrical charges is not certain 

 and at all events need not now be discussed.* A solution of 

 such concentration that the atoms are not dissociated and 

 that molecules remain intact will have no effect. Still fur- 

 ther concentration will cause plasmolysis of the living cells. 



* Kahlenberg. L.., and True, R. H. On the toxic action of dissolved 

 salts and their electrolytic dissociation. Bot. Gazette, XXII., 1896. True, 

 R. H. The physiological action of certain plasmolyzing agents. Bot. Ga- 

 zette, XXVI., 1898. Heald. F. D. On the toxic effect of dilute solutions 

 of acids and salts upon plants. Bot. Gazette, XXII., 1896. Loeb. J. On 

 ion-proteid compounds and their role in the mechanics of life-phenomena. 

 I. The poisonous character of a pure NaCl solution. Amer. Journ. 

 Physiol., vol. III.. 1899-1900. 



