QUANTITATIVE DETERMINATION OF LIGHT REACTIONS 215 



color, and the results were interpretated in terras of human 

 experience. The behavior of an animal that was assumed to 

 have the freedom of choice was said to indicate its preference. 



The students of plant behavior — because the subjects of their 

 experiments were remote from human activities — had begun to 

 free themselves from the anthropomorphic point of view long 

 before the zoologists. With their earlier analytical outlook on 

 the natural phenomena, came an earlier seeking for mechanical 

 explanations. In the middle of thp last century, when animals 

 were still going toward light "because they liked it," or "because 

 it aroused their curiosity," the botanists were seeking to deter- 

 mine the nature of the mechanism involved in the reactions of 

 plants to light. 



As early as 1831 de Candolle had used the term heliotropism to 

 describe the bending of plants toward the sun. This he beheved 

 to be due to the direct effect of local difference of intensity, 

 growth being retarded on the more highly illuminated side. 

 The inadequacy of this explanation was shown by Charles Dar- 

 win and his son Francis in their work on "The power of movement 

 in plants;" these investigators demonstrated the difference be- 

 tween sensitive and reacting tissues and the transmission of 

 stimuli from one to the other. 



Sachs, however, did not accept the theories of the Darwins 

 and advanced the explanation that the turning was controlled 

 by the direction in which the rays of light penetrated the tissues 

 of the plant. The simpHcity of the theory of Sachs was very 

 alluring, but, like many "simple mechanical explanations," it 

 fitted only a few of the facts. 



Soon after the work of Sachs appeared, Loeb took up the study 

 of animal reactions with the purpose of analyzing the phenomena 

 in terms of physics and chemistry in opposition to the anthro- 

 pomorphic 'explanations' prevalent at that time among zoologists. 

 His attitude is well expressed by the following quotation from one 

 of his later works. He says ('05, p. ix) "I consider a complete 

 knowledge and control of these agencies (which determine be- 

 havior) the biological solution of the metaphysical problem of ani- 

 mal instinct and will." Whether or not future work bears out 



