CHAPTER X 



TROPISMS 

 I. TROPISMS IN GENERAL 



The apparent lack of feeling or of responsiveness in plants 

 was the basis of the old distinction between plants and 

 animals: "Saxa crescunt, plantae crescunt et vivunt; ani- 

 malia crescunt, vi\iint et sentiunt." However, in tropistic 

 movements plants appear to exhibit a sort of intelligence; 

 their movement is of subsequent advantage to them. In 

 addition, tropisms are easily observable and lay themselves 

 open to obvious experiments, so that it is not surprising 

 that they have been a subject of investigation from the 

 earliest times of plant physiology. This sensitivity, other- 

 wise not noticeable in plants, explains why it is that in 

 regard to tropistic responses the parallelism between plants 

 and animals has been so much stressed. This had at first 

 the drawback that many complex concepts of zoology were 

 transferred to plants, but one of the great services which 

 the discovery and study of the auxins has performed is that 

 it has forced the development of new concepts independent 

 of the zoological heritage— concepts not only new, but also 

 more concrete and experimentally analyzable. It will be 

 among the aims of this chapter to elucidate this change in 

 thought, and with this in view, tropisms will be treated 

 only so far as they are demonstrably due to differential 

 growth and so far as their auxin relations have been investi- 

 gated. The material in this chapter could also be regarded 

 as an application of the knowledge of auxin which we have 

 gained in the preceding pages. However, this would not 

 be historically correct, because, as shown in Table I, p. 16, 

 the early evidence for the existence and role of auxin came 

 through the study of tropisms. In the recent development 

 of the field, the center of interest has shifted away from the 



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