CHEMOTROP1SM AND OSMOTROPISM 179 



responding to oxygen is in many cases associated with a power of 

 responding to peptone and other substances, it need not always be so. 

 Furthermore, a particular organism may respond to one or a few substances, 

 whereas another may be chemotropically stimulated by a large number 

 of substances, though not all to the same extent. At the same time bodies 

 of similar constitution may exert widely dissimilar physiological actions, 

 while dissimilar substances may be comparatively alike from a chemotropic 

 point of view. Whenever the chemotropic action depends upon acid or 

 alkaline action it is only natural to expect that the influence of equi- 

 molecular solutions of neutral salts will partly depend upon the degree 

 of dissociation, as in the case of poisons. The dissociated ions as well as 

 the undissociated molecules may, quite apart from any acid or alkaline 

 character, exercise independent chemotropic actions l , 



Either or both of these forms of irritability may be developed in the 

 same organism, and in the latter case the two stimuli may act conjointly 

 when a chemotropic substance is applied in considerable concentration, or 

 when a dilute chemotropic solution has a large quantity of an indifferent 

 soluble substance added to it. Since the stimulating chemotropic action 

 is not directly proportional to the concentration, and since conjoint stimuli 

 may induce changes of tone, it is not always possible to say whether the 

 conversion of a positive into a negative response by increasing concentration 

 is of chemotropic or osmotropic origin. That the change is a chemotropic 

 one is, however, obvious in the case of organisms which have no osmotropic 

 irritability, and the same applies when the tropic reversal is shown in 

 a concentration at which isosmotic solutions of non-chemotropic salts 

 exert no osmotropic repulsion. When a chemotropic action is only shown 

 with high concentrations it is always accompanied by an osmotropic 

 excitation if the organ possesses this latter form of irritability. In this 

 way it arises that isosmotic solutions of different substances exert more 

 or less dissimilar stimulating effect. 



These forms of irritability are especially important in freely motile 

 organisms, and often serve to lead them to nutriment or to suitable 

 habitats, or aid them in avoiding injurious or unfavourable media. Of 

 equal advantage are the chemotropic and osmotropic curvatures performed 

 by the hyphae of mould and other fungi. Chemotropic stimuli also aid 

 in directing the pollen-tube to the ovule and in bringing the antheridial 



1 [Massart (Biol. Centralbl., 1902, Bd. xxn, p. 22) proposes the terms 'alcalio-' and 'oxy- 

 tropism ' for the chemotropism induced by alkalies and acids, while for the attraction exercised by 

 oxygen the term of ' oxygen otropism ' is suggested by Herbst, Biol. Centralbl., 1894, p. 694, and of 

 ' aerotropism ' by Molisch (Sitzungsb. d. Wiener Akad., 1884, Bd. xc, I, p. in). As a holiday 

 amusement the invention of special terms for detailed phenomena has its advantages, but for serious 

 scientific studies the unnecessary duplication of terms is strongly to be deprecated.] 



N 2 



