HISTORICAL REVIEW II 



observations and the Ingenuity of their experiments. The 

 point of view of these men dominated the field of animal 

 behavior from the middle of the nineteenth century until 

 the appearance of Verworn and Loeb well on toward 1890. 

 As has been stated, they studied the behavior of animals 

 with the express purpose of demonstrating the evolution 

 of psychic phenomena in man. These investigators w^ere 

 therefore not primarily interested in a physico-chemical 

 explanation of animal behavior. 



The study of behavior in plants during this period was 

 however pursued with a very different aim. The question 

 as to the origin of mental phenomena influenced this study 

 but little, for it was generally conceded that plants were 

 devoid of all traces of psychic activity. There was con- 

 sequently nothing left but to attempt to account for their 

 behavior by means of physico-chemical analysis. Even 

 the vitalists realized that In the attempt of such analysis 

 lay the only hope of progress. 



6. Introduction of the Term '' Tropism^' and Development of 

 its Application to Different Reactions 



In 1806 De Candolle, a vltallst, succeeded in reversing 

 the daily periodic sleep movements of leaves by exposing 

 them to artificial light during the night and to darkness 

 during the day. The same year Knight showed by at- 

 taching developing seedlings to a rapidly revolving wheel 

 that the direction of growth of roots and stems Is regu- 

 lated by gravitation. He explained the directive action of 

 gravitation by assuming "that the root, being of a semi- 

 fluid consistence. Is bent downwards by its own weight, 

 while the nutrient sap in the stem moves to the underside 

 and causes stronger growth there, until by means of the 

 curvature so produced the stem assumes the upright posi- 

 tion." In 1828 Johnston found that roots In growing 

 downward can overcome considerable resistance and that 

 the direction of growth is therefore not due to their weight 



