CHAPTER XVII. 



THE MOVEMENTS OF ORGANISMS. 



The Movements of Lower Organisms. 



FREEDOM of the will is still a problem in philosophy, and even the 

 investigation of the purely reflex phenomena and actions of higher 

 organisms is still entirely in the stage of observation and measure- 

 ment. In any case, it is still impossible to connect the external 

 stimulus and the resultant action by a series of obvious physical and 

 chemical processes. 



It is otherwise in the case of the movements of certain portions of 

 plants and of the lowest organisms, especially certain amebae and 

 their relatives, our symbiotic blood fellows, the leucocytes. In this 

 case, opportunities are offered for an exact explanation of their 

 movements and actions; but even here analogies must frequently 

 carry us over gaps. 



It is customary to refer to such regulated movements of lower 

 organisms and portions of plants as tropisms. [In this connection 

 reference should be made to the discussion on " Animal Instincts and 

 Tropisms in the Organism as a Whole," by JACQUES LOEB. G. P. 

 PUTNAM'S SONS, 1916. JOHN HAYS HAMMOND, JR., has constructed 

 heliotropic machines which follow a lantern in the dark. The 

 " retina " consists of selenium wire which changes its galvanic resist- 

 ance when illuminated. Tr.] We speak of heliotropism when certain 

 plankton organisms swim toward the light or when a tree or a flower 

 grows toward the light. We speak of positive thermotropism if a root 

 grows in the direction of a heat stimulus, of negative thermotropism 

 when it grows away from it. Every fact in this connection is not 

 only valuable in explaining the subject but serves as well to enrich 

 the meaning of the term " stimulus." "Stimulus" is an expression 

 employed in biology wherever the more profound causes are not 

 evident. 



MARTIN H. FISCHER has already indicated how tropisms may be 

 explained in analogy to curling sheets of gelatin. 



TH. PARODKO contributed extremely valuable studies on plant 

 tropisms. He stimulated growing roots from one side and they 

 became crooked. The stimuli were chemicals, heat and traumata. 

 He concluded that all these tropisms might be explained by protein 



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