THE MANIFESTATIONS OF LIFE 47 



Other poisons known to the chemists as toxins have an 

 exciting or depressing effect upon the living substance 

 by which life is eventually set aside without visible 

 chemical alteration. 



The effects produced by poisons bear a direct relation 

 to their concentration; for many substances which effect 

 rapid destructive influences in strong solutions are not 

 only rendered harmless, but also useful by sufficient 

 dilution, and many substances that are commonly util- 

 ized by the cells become injurious when presented to 

 them in excess. 



In dilutions so great that, recognizable, harmful effects 

 are no longer to be expected, chemical agents may excite 

 no irritable manifestations, or may provoke interesting 

 and important reactions that are described as positive 

 or negative chemotropism. 



The chemical nature of many of the substances by 

 which these reactions are excited is unknown, and in 

 some of the experiments by which chemotropic effects 

 resembling those seen in nature are developed, we can- 

 not be sure that the natural and experimental condi- 

 tions are identical because identical effects are observed. 



That chemotropic influences play an important func- 

 tion in determining many of the vital manifestations is 

 beyond question, though it is not always possible to 

 pursue the investigation because information concern- 

 ing the nature of the chemical stimuli is so defective. 



Pfeffer found that when motile spermatozoids of ferns 

 are suspended in water they are influenced by malic acid. 

 Thus if a capillary tube containing a dilution of this agent 

 be introduced into the water containing them, the cells 

 swim toward it and quickly enter in response to positive 

 chemotropic influences. From such an experiment it 

 seems justifiable to conclude that the spermatic ele- 

 ments of ferns as well as of other cryptogams find the 

 appropriate female elements to be fertilized by virtue 

 of chemotropic influences. Among the higher plants, 

 in which pollination is effected by currents of air which 



