CHAPTER III 



TROPIC 1 MOVEMENTS 



PART I 



INTRODUCTORY 

 SECTION 32. General. 



IN order that the plant and its organs may attain situations adapted for 

 the performance of their different functions they must possess special 

 tropic 1 irritabilities. These determine the primary orientation of the main 

 axis, upon which the lateral organs have definite positions assured to them 

 when they merely follow their inherent autotropic tendencies. This applies 

 to hairs and to the finer rootlets, whereas runners, leaves, and lateral roots 

 of the first order assume positions mainly determined by external tropic 

 stimuli. The latter induce movements which result in the organ placing 

 itself at a definite angle to the direction of the exciting stimulus, and 

 naturally such responses are best studied when the other external conditions 

 are kept constant and are diffusely applied. 



The terms geotropism l , heliotropism (phototropism), thermotropism, 

 chemotropism, osmotropism, hydrotropism, rheotropism, thigmotropism 

 (haptotropism), galvanotropism and autotropism, merely indicate the 

 exciting agency and say nothing as to the physiological response involved. 

 It was in this sense that the term heliotropism was used by de Candolle 

 and other early authors, so that Wiesner is neither historically correct nor 

 practically justified in restricting it to curvatures produced by growth 2 . 

 The curvatures may, in fact, either be produced by heterauxesis or by 

 variation movements, and the locomotory and orienting movements of free- 

 swimming organisms are produced in a variety of ways. In the latter case 

 it is permissible to use the terms phototaxis, chemotaxis, and the like, 

 although frequently no sharp line of demarcation can be drawn between 

 tropic and tactic movements 3 . An organism which passes through motile 

 and fixed stages may show at one time tropic and at another tactic 

 responses, while the movements of the chloroplastids of plant-cells, though 

 usually more tactic in character, simulate tropic movements in the case of 



1 Pronounced, tropic, tropism. 



2 'Die heliotropischen Erscheinungen, 1880, Ed. II, p. 22. 



3 Pfeffer, Druck- und Arbeitsleistungen, 1893, p. 414, footnote. 



