334 



THE AMERICAN NATURALIST [Vol. XLIV 



Darwin, in his presidential address before the British 

 Association for the Advancement of Science in 1908, 1 

 intimates that he does not understand Loeb's use of the 

 term "tropism." Botanists, probably quite generally, 

 are in doubt as to whether Jennings's " trial and error" 

 description can be applied to plant reactions. The exist- 

 ence of these two bodies of literature, and the uncertainty 

 as to the significance of terms, calls, to my mind, for an 

 examination of the phenomena of response in plants and 

 in the lower animals, to the end that doubt may be re- 

 moved as to the applicability of the same terms to both 

 plant and animal response. 



Proceeding to compare these two groups of organisms, 

 we may say that both have fixed and free-moving forms. 

 Most of the plants are fixed, and a group of their well- 

 known reactions which botanists call "tropisms" are 

 exactly simulated on the animal side by hydroids, as re- 

 corded by Loeb. 2 Geotropic and heliotropic bendings of 

 the so-called stems and the so-called roots of the hydroids 

 differ in no way from the manifestations of these phe- 

 nomena in the stems and roots of plants. 



Most of the lower animals are free-moving, and it is 

 upon the phenomena of movement in these forms that 

 the most of the animal studies have been made. Among 

 plants, corresponding studies have been made upon the 

 movements of bacteria, antlierozoids and the swarm-cells 

 of algae and fungi. A study of the literature describing 

 the behavior of the protozoa and of the free-moving 

 plants must convince one that the stimuli and the re- 

 sponses are the same in both divisions of organisms. 

 There are animals with amoeboid movements and plants 

 with amoeboid movements, both animals and plants with 

 cilia, and both with flagella. We may go a step farther, 

 and consider the stimuli that influence the movements of 

 these plants and animals. We have learned that both are 

 directed in their behavior by gravitation, light, chem- 

 icals and the electric current. The behavior in the plant 



1 Science, XXVIII, 353. 



