178 TROPIC MOVEMENTS 



Klercker's l term of ' caloritropism ' to indicate curvatures produced by 

 conducted heat. 



SECTION 39. Chemotropism and Osmotropism. 



Chemical stimuli not only play an important part in the general vital 

 activity, but are often specially employed to produce tropic orienting 

 movements. This power of reaction has, however, been more especially 

 studied in connexion with freely motile organisms, and less is known in 

 regard to the production of chemotropic curvatures. Among these are 

 included all movements produced by a substance in virtue of its chemical 

 constitution and varying distribution. When the movement takes place 

 towards the source of the diffusing substance, or where it is more abundant, 

 we may speak of positive chemotropism, and of negative when the curvature 

 is in the opposite direction. Transverse chemotropism might be due to 

 the absence of any power of response, but could only be the result of 

 a definite chemotropic orienting stimulus when it was attempted in spite 

 of the action of other directive agencies. Reversal is possible as in the 

 case of heliotropism, for although a negative reaction may be produced by 

 some substances when in extreme dilution, frequently a positive reaction 

 becomes negative or transversal beyond a certain concentration. 



An increase of concentration also involves an enhanced osmotic action, 

 and when this acts as a tropic stimulus we have an osmotropic reaction 

 before us 2 . A special osmotropic irritability is often shown, although 

 comparatively high concentrations are required to excite it, and the response 

 hitherto observed has always been negative. It is, however, not impossible 

 that positive osmotropism may be detected in some cases 3 . 



Since osmotropic stimulation does not depend upon chemical quality 

 but upon osmotic action, all substances exercise the same osmotic stimulus 

 when in equivalent concentrations, so long as the power of perception or of 

 reaction remains unaffected *. On the other hand, chemotropic stimulation 

 is primarily dependent upon the chemical nature of the stimulating 

 substance, and hence isosmotic solutions of different materials exercise 

 widely dissimilar chemotropic actions. Furthermore, the chemotropic 

 sense, like the sense of smell and taste in animals, is developed to widely 

 dissimilar degrees in different plants. Hence a substance may be strongly 

 chemotropic for one organism but not for another, and while a power of 



1 Die caloritropischen Erscheinungen bei einigen Keimwurzeln, 1891, p. 767. 



1 Rothert (Flora, 1901, p. 408, footnote) suggests the terms 'osmotropism' and ' osmotaxis,' 



which are preferable to Massart's ' tonotaxis.' Since it is not merely a question of the attraction by 



food, and since all food-substances are not chemotropically active, the term ' trophotropism* 



suggested by Stahl (Bot. Ztg., 1884, p. 165) is highly unsuitable. 



3 Cf. Rothert, I.e., p. 403, footnote. 



* Id., p. 413. 



